The discovery of fullerenes - a new form of existence of one of the most common elements on Earth - carbon, is recognized as one of the most amazing and most important discoveries in science of the 20th century. Despite the long-known unique ability of carbon atoms to bind into complex, often branched and bulky molecular structures, which forms the basis of all organic chemistry, the actual possibility of forming stable framework molecules from only one carbon turned out to be unexpected. Experimental confirmation that molecules of this type, consisting of 60 or more atoms, can arise in the course of naturally occurring processes in nature occurred in 1985. And long before that, some authors assumed the stability of molecules with a closed carbon sphere. However, these assumptions were purely speculative, purely theoretical. It was rather difficult to imagine that such compounds could be obtained by chemical synthesis. Therefore, these works remained unnoticed, and attention was paid to them only after the fact, after the experimental discovery of fullerenes. A new stage began in 1990, when a method was found for obtaining new compounds in gram quantities, and a method for isolating fullerenes in pure form was described. Very soon after that, the most important structural and physicochemical characteristics of C 60 fullerene, the most easily formed compound among the known fullerenes, were determined. For their discovery - the discovery of carbon clusters of composition C 60 and C 70 - R. Kerl, R. Smalley and G. Kroto in 1996 were awarded the Nobel Prize in Chemistry. They also proposed the structure of fullerene C 60 , known to all football fans.

As you know, the shell of a soccer ball is made up of 12 pentagons and 20 hexagons. Theoretically, 12,500 arrangements of double and single bonds are possible. The most stable isomer (shown in the figure) has a truncated icosahedral structure that lacks double bonds in the pentagons. This isomer of C 60 was named "Buckminsterfullerene" in honor of the famous architect named R. Buckminster Fuller, who created structures, the domed frame of which is constructed from pentagons and hexagons. Soon a structure for the C 70 was proposed, resembling a rugby ball (with an elongated shape).

In the carbon framework, the C atoms are characterized by sp 2 hybridization, with each carbon atom bonded to three neighboring atoms. Valency 4 is realized through p-bonds between each carbon atom and one of its neighbors. Naturally, it is assumed that p-bonds can be delocalized, as in aromatic compounds. Such structures can be built for n≥20 for any even clusters. They must contain 12 pentagons and (n-20)/2 hexes. The lowest of the theoretically possible C 20 fullerenes is nothing more than a dodecahedron - one of the five regular polyhedra, in which there are 12 pentagonal faces, and there are no hexagonal faces at all. A molecule of such a shape would have an extremely strained structure, and therefore its existence is energetically unfavorable.

Thus, in terms of stability, fullerenes can be divided into two types. The border between them allows you to draw the so-called. the rule of isolated pentagons (Isolated Pentagon Rule, IPR). This rule states that the most stable fullerenes are those in which no pair of pentagons has adjacent edges. In other words, the pentagons do not touch each other, and each pentagon is surrounded by five hexes. If fullerenes are arranged in order of increasing number of carbon atoms n, then Buckminsterfullerene - C 60 is the first representative that satisfies the rule of isolated pentagons, and C 70 is the second. Among fullerene molecules with n>70 there is always an isomer subject to IPR, and the number of such isomers increases rapidly with the number of atoms. Found 5 isomers for C 78 , 24 - for C 84 and 40 - for C 90 . Isomers that have adjacent pentagons in their structure are significantly less stable.

Chemistry of fullerenes

Currently, the predominant part of scientific research is related to the chemistry of fullerenes. More than 3 thousand new compounds have already been synthesized based on fullerenes. Such a rapid development of the chemistry of fullerenes is associated with the structural features of this molecule and the presence of a large number of double conjugated bonds on a closed carbon sphere. The combination of fullerene with representatives of many known classes of substances has opened up the possibility for synthetic chemists to obtain numerous derivatives of this compound.

Unlike benzene, where the lengths of C-C bonds are the same, in fullerenes bonds of a more "double" and more "single" nature can be distinguished, and chemists often consider fullerenes as electron-deficient polyene systems, and not as aromatic molecules. If we turn to С60, then it contains two types of bonds: shorter (1.39 Å) bonds running along the common edges of neighboring hexagonal faces, and longer (1.45 Å) bonds located along the common edges of pentagonal and hexagonal faces. At the same time, neither six-membered nor, even more so, five-membered rings exhibit aromatic properties in the sense in which they are exhibited by benzene or other planar conjugated molecules obeying Hückel's rule. Therefore, usually shorter bonds in C 60 are considered double, while longer ones are single. One of the most important features of fullerenes is that they have an unusually large number of equivalent reaction centers, which often leads to a complex isomeric composition of the reaction products with their participation. As a result, most chemical reactions with fullerenes are not selective, and the synthesis of individual compounds is very difficult.

Among the reactions for obtaining inorganic fullerene derivatives, the most important are the processes of halogenation and the production of the simplest halogen derivatives, as well as hydrogenation reactions. Thus, these reactions were among the first ones carried out with fullerene C 60 in 1991. Let us consider the main types of reactions leading to the formation of these compounds.

Immediately after the discovery of fullerenes, the possibility of their hydrogenation with the formation of "fulleranes" aroused great interest. Initially, it seemed possible to add sixty hydrogen atoms to the fullerene. Subsequently, it was shown in theoretical works that in the C 60 H 60 molecule, some of the hydrogen atoms should be inside the fullerene sphere, since six-membered rings, like cyclohexane molecules, should take the “chair” or “bath” conformations. Therefore, currently known polyhydrofullerene molecules contain from 2 to 36 hydrogen atoms for fullerene C 60 and from 2 to 8 for fullerene C 70 .

During the fluorination of fullerenes, a complete set of C 60 F n compounds was found, where n takes even values ​​up to 60. Fluorine derivatives with n from 50 to 60 are called perfluorides and were found among the fluorination products by mass spectroscopy in extremely low concentrations. There are also hyperfluorides, that is, products of the composition C 60 F n , n>60, where the fullerene carbon cage is partially destroyed. It is assumed that this also takes place in perfluorides. The issues of the synthesis of fullerene fluorides of various compositions are an independent most interesting problem, the study of which is most actively studied at the Faculty of Chemistry of Moscow State University. M.V. Lomonosov.

Active study of the processes of chlorination of fullerenes under various conditions began already in 1991. In the first works, the authors tried to obtain C 60 chlorides by reacting chlorine and fullerene in various solvents. To date, several individual fullerene chlorides C 60 and C 70 obtained by using various chlorinating agents have been isolated and characterized.

The first attempts to brominate fullerene were made already in 1991. Fullerene C 60 , placed in pure bromine at a temperature of 20 and 50 o C, increased the mass by a value corresponding to the addition of 2-4 bromine atoms per fullerene molecule. Further studies of bromination showed that the interaction of C 60 fullerene with molecular bromine for several days produces a bright orange substance, the composition of which, as determined by elemental analysis, was C 60 Br 28 . Subsequently, several bromo derivatives of fullerenes were synthesized, which differ in a wide range of values ​​for the number of bromine atoms in a molecule. Many of them are characterized by the formation of clathrates with the inclusion of free bromine molecules.

The interest in perfluoroalkyl derivatives, in particular trifluoromethylated derivatives of fullerenes, is associated primarily with the expected kinetic stability of these compounds in comparison with halogen derivatives of fullerenes prone to nucleophilic S N 2'-substitution reactions. In addition, perfluoroalkylfullerenes may be of interest as compounds with a high electron affinity due to acceptor properties of perfluoroalkyl groups that are even stronger than those of fluorine atoms. To date, the number of isolated and characterized individual compounds of the composition C 60/70 (CF 3) n, n=2-20 exceeds 30, and intensive work is underway to modify the fullerene sphere by many other fluorine-containing groups - CF 2 , C 2 F 5 , C 3 F 7 .

The creation of biologically active fullerene derivatives, which could find application in biology and medicine, is associated with imparting hydrophilic properties to the fullerene molecule. One of the methods for the synthesis of hydrophilic fullerene derivatives is the introduction of hydroxyl groups and the formation of fullerenols or fullerols containing up to 26 OH groups, and also, probably, oxygen bridges similar to those observed in the case of oxides. Such compounds are highly soluble in water and can be used for the synthesis of new fullerene derivatives.

As for fullerene oxides, the compounds C 60 O and C 70 O are always present in the initial mixtures of fullerenes in the extract in small amounts. Probably, oxygen is present in the chamber during the electric arc discharge and some of the fullerenes are oxidized. Fullerene oxides are well separated on columns with various adsorbents, which makes it possible to control the purity of fullerene samples and the absence or presence of oxides in them. However, the low stability of fullerene oxides hinders their systematic study.

What can be noted about the organic chemistry of fullerenes is that, being an electron-deficient polyene, C 60 fullerene exhibits a tendency to radical, nucleophilic, and cycloaddition reactions. Particularly promising in terms of the functionalization of the fullerene sphere are various cycloaddition reactions. Due to its electronic nature, C 60 is able to take part in α-cycloaddition reactions, and the most characteristic are cases when n = 1, 2, 3 and 4.

The main problem solved by synthetic chemists working in the field of the synthesis of fullerene derivatives remains the selectivity of the reactions carried out to this day. Features of the stereochemistry of addition to fullerenes consist in a huge number of theoretically possible isomers. So, for example, the compound C 60 X 2 has 23 of them, C 60 X 4 already has 4368, among them 8 are addition products at two double bonds. The 29 C 60 X 4 isomers, however, will not have a chemical meaning, having a triplet ground state arising from the presence of an sp2-hybridized carbon atom surrounded by three sp 3-hybridized atoms forming C-X connection. The maximum number of theoretically possible isomers without taking into account the multiplicity of the ground state will be observed in the case of C 60 X 30 and will be 985538239868524 (1294362 of them are addition products at 15 double bonds), while the number of non-singlet isomers of the same nature as in the above example, does not lend itself to simple accounting, but from general considerations it should constantly increase with the growth of the number of affiliated groups. In any case, the number of theoretically admissible isomers in most cases is enormous, while going over to less symmetric C 70 and higher fullerenes, it additionally increases by several times or by orders of magnitude.

In fact, numerous data of quantum chemical calculations show that most of the reactions of halogenation and hydrogenation of fullerenes proceed with the formation, if not the most stable isomers, then at least slightly differing from them in energy. The greatest discrepancies are observed in the case of lower fullerene hydrides, whose isomeric composition, as shown above, can even slightly depend on the synthesis route. However, the stability of the resulting isomers still turns out to be extremely close. The study of these regularities in the formation of fullerene derivatives is an interesting problem, the solution of which leads to new achievements in the field of chemistry of fullerenes and their derivatives.

FULLERENES - A NEW ALLOTROPIC FORM OF CARBON

1. THEORETICAL SECTION

1.1. Known allotropic forms of carbon

Until recently, it was known that carbon forms three allotropic forms: diamond, graphite and carbine. Allotropy, from the Greek. Allos - different, tropos - turn, property, existence of the same element in the form of structures different in properties and structure. Currently, the fourth allotropic form of carbon is known, the so-called fullerene (polyatomic carbon molecules C n).

The origin of the term "fullerene" is associated with the name of the American architect Richard Buckminster Fuller, who designed hemispherical architectural structures consisting of hexagons and pentagons.

In the mid-1960s, David Jones constructed closed spheroidal cages from graphite layers folded in a peculiar way. It was shown that a pentagon can be a defect embedded in the hexagonal lattice of ordinary graphite and leading to the formation of a complex curved surface.

In the early 1970s, organic physicist E. Osawa suggested the existence of a hollow, highly symmetrical C 60 molecule with a structure in the form of a truncated icosahedron, similar to soccer ball. A little later (1973), Russian scientists D.A. Bochvar and E.G. Galperin made the first theoretical quantum-chemical calculations of such a molecule and proved its stability.

In 1985, a team of scientists: G. Kroto (England, University of Sussex), Heath, 0"Brien, R.F. Curl and R. Smalley (USA, Rice University) managed to detect a fullerene molecule in the study of mass spectra of graphite vapor after laser irradiation of a solid sample.

The first way to obtain and isolate solid crystalline fullerene was proposed in 1990 by W. Kretschmer and D. Huffman and colleagues at the Institute of Nuclear Physics in Heidelberg (Germany).

In 1991, the Japanese scientist Ijima for the first time observed various structures using a polar ion microscope, composed, as in the case of graphite, of six-membered carbon rings: nanotubes, cones, nanoparticles.

In 1992, natural fullerenes were discovered in a natural carbon mineral - shungite (this mineral got its name from the name of the village of Shunga in Karelia).

In 1997, R.E.

Let us consider the structure of allotropic forms of carbon: diamond, graphite and carbine.

Diamond - Each carbon atom in the diamond structure is located in the center of a tetrahedron, the vertices of which are the four nearest atoms. Neighboring atoms are interconnected by covalent bonds (sp 3 hybridization). This structure determines the properties of diamond as the hardest substance known on Earth.

Graphite finds wide application in a wide variety of areas of human activity, from the manufacture of pencil leads to neutron moderation units in nuclear reactors. Carbon atoms in the crystal structure of graphite are interconnected by strong covalent bonds (sp 2 - hybridization) and form hexagonal rings, which, in turn, form a strong and stable mesh similar to a honeycomb. Grids are arranged one above the other in layers. The distance between atoms located at the vertices of regular hexagons is 0.142 nm., between layers – 0.335 nm. The layers are loosely connected to each other. Such a structure - strong layers of carbon, weakly interconnected, determines the specific properties of graphite: low hardness and the ability to easily delaminate into tiny flakes.

Carbine condenses in the form of a white carbon deposit on the surface when the pyrographite is irradiated with a laser beam of light. The crystalline form of carbine consists of parallel oriented chains of carbon atoms with sp-hybridization of valence electrons in the form of straight macromolecules of polyyne (-С= С-С= С-...) or cumulene (=С=С=С=...) types .

Other forms of carbon are also known, such as amorphous carbon, white carbon (chaoite), etc. But all these forms are composites, that is, a mixture of small fragments of graphite and diamond.

1.2.Geometry of the fullerene molecule and the crystal lattice of fullerite

Fig.3 Fullerene C 6 molecule 0

In contrast to diamond, graphite and carbine, fullerene is essentially a new form of carbon. The C 60 molecule contains fragments with fivefold symmetry (pentagons), which are forbidden by nature for inorganic compounds. Therefore, it should be recognized that the fullerene molecule is an organic molecule, and the crystal formed by such molecules ( fullerite) – it is a molecular crystal that is a link between organic and inorganic matter.

A flat surface is easily laid out from regular hexagons, but a closed surface cannot be formed by them. To do this, it is necessary to cut part of the hexagonal rings and form pentagons from the cut parts. In fullerene, a flat grid of hexagons (graphite grid) is folded and stitched into a closed sphere. In this case, some of the hexagons are converted into pentagons. A structure is formed - a truncated icosahedron, which has 10 axes of symmetry of the third order, six axes of symmetry of the fifth order. Each vertex of this figure has three nearest neighbors. Each hexagon borders three hexagons and three pentagons, and each pentagon borders only hexagons. Each carbon atom in the C 60 molecule is located at the vertices of two hexagons and one pentagon and is fundamentally indistinguishable from other carbon atoms. The carbon atoms that form the sphere are bound together by a strong covalent bond. The thickness of the spherical shell is 0.1 nm, the radius of the C 60 molecule is 0.357 nm. The length of the C-C bond in the pentagon is 0.143 nm, in the hexagon - 0.139 nm.

Molecules of higher fullerenes C 70 C 74 , C 76 , C 84 , C 164 , C 192 , C 216 also have the form of a closed surface.

Fullerenes with n< 60 оказались неустойчивыми, оказались неустойчивыми, хотя из чисто топологических соображений наименьшим возможным фуллереном является правильный додекаэдр С 20 .

Crystalline fullerene, which was named fullerite, has a face-centered cubic lattice (fcc), space group (Fm3m). The cubic lattice parameter a 0 = 1.42 nm, the distance between nearest neighbors is 1 nm. The number of nearest neighbors in the fcc lattice of fullerite is –12.

There is a weak van der Waals bond between C 60 molecules in a fullerite crystal. Using the method of nuclear magnetic resonance, it was proved that at room temperature the C 60 molecules rotate around the equilibrium position with a frequency of 10 12 1/s. When the temperature drops, the rotation slows down. At 249K, a first-order phase transition is observed in fullerite, in which the fcc lattice (sp. gr. Fm3m) transforms into a simple cubic one (sp. gr. Pa3). In this case, the volume of fulderite increases by 1%. A fullerite crystal has a density of 1.7 g/cm 3 , which is much less than the density of graphite (2.3 g/cm 3 ) and diamond (3.5 g/cm 3 ).

The C 60 molecule remains stable in an inert argon atmosphere up to temperatures on the order of 1700 K. Significant oxidation is observed at 500 K in the presence of oxygen to form CO and CO 2 . At room temperature, oxidation occurs when irradiated with photons with an energy of 0.55 eV. which is much lower than the photon energy of visible light (1.54 eV). Therefore, pure fullerite must be stored in the dark. The process, which lasts for several hours, leads to the destruction of the fcc lattice of fullerite and the formation of a disordered structure in which there are 12 oxygen atoms per initial C6 molecule. In this case, the fullerenes completely lose their shape.

1.3. Obtaining fullerenes

The most efficient way to obtain fullerenes is based on the thermal decomposition of graphite. Both electrolytic heating of the graphite electrode and laser irradiation of the graphite surface are used. 4 shows a diagram of a plant for the production of fullerenes, which was used by W. Kretchmer. Graphite sputtering is carried out by passing a current with a frequency of 60 Hz through the electrodes, the current is from 100 to 200 A, the voltage is 10-20 V. By adjusting the tension of the spring, it is possible to ensure that the main part of the input power is released in the arc, and not in the graphite rod. The chamber is filled with helium, pressure 100 Torr. The evaporation rate of graphite in this installation can reach 10g/W. In this case, the surface of the copper casing, cooled by water, is covered with the graphite evaporation product, i.e. graphite soot. If the resulting powder is scraped off and kept for several hours in boiling toluene, a dark brown liquid is obtained. When it is evaporated in a rotating evaporator, a fine powder is obtained, its weight is not more than 10% of the weight of the original graphite soot. It contains up to 10% fullerenes C 60 (90%) and C 70 (10%). The described arc method for obtaining fullerenes was named "fullerene arc".

In the described method for obtaining fullerenes, helium plays the role of a buffer gas. Compared to other atoms, helium atoms most effectively "extinguish" the oscillatory motions of excited carbon fragments that prevent them from combining into stable structures. In addition, helium atoms carry away the energy released when carbon fragments combine. Experience shows that optimal pressure helium is in the 100 torr range. At higher pressures, the aggregation of carbon fragments is difficult.

Fig.4. Scheme of installation for obtaining fullerenes.

1 - graphite electrodes;

2 - cooled copper bus; 3 - copper casing,

4 - springs.

Changes in process parameters and plant design lead to changes in process efficiency and product composition. The quality of the product is confirmed both by mass spectrometric measurements and by other methods (nuclear magnetic resonance, electron paramagnetic resonance, IR spectroscopy, etc.)

An overview of currently existing methods for obtaining fullerenes and devices of installations in which various fullerenes are obtained is given in the work of G.N. Churilov.

Purification and detection methods

The most convenient and widespread method of extraction of fullerenes from the products of thermal decomposition of graphite (terms: fullerene-containing condensate, fullerene-containing soot), as well as subsequent separation and purification of fullerenes, is based on the use of solvents and sorbents.

This method includes several stages. At the first stage, fullerene-containing soot is treated with a non-polar solvent, which is benzene, toluene, and other substances. In this case, fullerenes, which have significant solubility in these solvents, are separated from the insoluble fraction, the content of which in the fullerene-containing phase is usually 70-80%. The typical value of the solubility of fullerenes in solutions used for their synthesis is several tenths of a mole percent. Evaporation of the fullerene solution obtained in this way leads to the formation of a black polycrystalline powder, which is a mixture of fullerenes various sorts. A typical mass spectrum of such a product shows that the fullerene extract is 80 - 90% C 60 and 10 -15% C 70 . In addition, there is no a large number of(at the level of fractions of a percent) of higher fullerenes, the isolation of which from the extract is a rather difficult technical problem. The fullerene extract dissolved in one of the solvents is passed through a sorbent, which can be aluminum, activated carbon, or oxides (Al 2 O 3 , SiO 2) with high sorption characteristics. Fullerenes are collected by this metal and then extracted from it with a pure solvent. The extraction efficiency is determined by the combination of sorbent-fullerene-solvent and usually, when using a certain sorbent and solvent, depends markedly on the type of fullerene. Therefore, the solvent passed through the sorbent with the fullerene adsorbed in it extracts fullerenes of various types in turn from the sorbent, which can thus be easily separated from each other. Further development of the described technology for obtaining the separation and purification of fullerenes, based on the electric arc synthesis of fullerene-containing soot and its subsequent separation using sorbents and solvents, led to the creation of installations that allow synthesizing C 60 in the amount of one gram per hour.

1.4 Properties of fullerenes

Crystalline fullerenes and films are semiconductors with a band gap of 1.2-1.9 eV and have photoconductivity. When irradiated with visible light, the electrical resistance of a fullerite crystal decreases. Photoconductivity is possessed not only by pure fullerite, but also by its various mixtures with other substances. It was found that the addition of potassium atoms to C 60 films leads to the appearance of superconductivity at 19 K.

Fullerene molecules, in which carbon atoms are linked to each other by both single and double bonds, are three-dimensional analogues of aromatic structures. Possessing high electronegativity, they act in chemical reactions as strong oxidizing agents. By attaching to themselves radicals of different chemical nature, fullerenes are able to form a wide class of chemical compounds with different physicochemical properties. For example, polyfullerene films have recently been obtained in which C 60 molecules are linked to each other not by van der Waals, as in a fullerite crystal, but by chemical interaction. These plastic films are a new type of polymer material. Interesting results have been achieved in the direction of the synthesis of polymers based on fullerenes. In this case, the fullerene C 60 serves as the basis of the polymer chain, and the connection between the molecules is carried out using benzene rings. This structure has received the figurative name "string of pearls".

The addition of radicals containing platinum group metals to C 60 makes it possible to obtain ferromagnetic materials based on fullerene. It is now known that more than a third of the elements of the periodic table can be placed inside a molecule. From 60 . There are reports of the introduction of atoms of lanthanum, nickel, sodium, potassium, rubidium, cesium, atoms of rare earth elements such as terbium, gadolinium and dysprosium.

The variety of physicochemical and structural properties of compounds based on fullerenes makes it possible to speak of fullerene chemistry as a new promising direction in organic chemistry.

1.5. Application of fullerenes

At present, the scientific literature discusses the use of fullerenes for the creation of photodetectors and optoelectronic devices, growth catalysts, diamond and diamond-like films, superconducting materials, and also as dyes for copiers. Fullerenes are used for the synthesis of metals and alloys with new properties.

Fullerenes are planned to be used as the basis for the production batteries. These batteries, the principle of which is based on the reaction of hydrogen addition, are in many respects similar to the widely used nickel batteries, however, unlike the latter, they have the ability to store about five times the specific amount of hydrogen. In addition, such batteries are characterized by higher efficiency, light weight, and environmental and health safety compared to the most advanced lithium-based batteries in terms of these qualities. Such batteries can be widely used to power personal computers and hearing aids.

Solutions of fullerenes in non-polar solvents (carbon disulfide, toluene, benzene, carbon tetrachloride, decane, hexane, pentane) are characterized by nonlinear optical properties, which manifests itself, in particular, in a sharp decrease in the transparency of the solution under certain conditions. This opens up the possibility of using fullerenes as the basis for optical shutters that limit the intensity of laser radiation.

There is a prospect of using fullerenes as a basis for creating a memory medium with an ultrahigh information density. Fullerenes can be used as additives for rocket fuels and lubricants.

Much attention is paid to the problem of using fullerenes in medicine and pharmacology. The idea of ​​creating anticancer medicines based on water-soluble endohedral compounds of fullerenes with radioactive isotopes is discussed. ( Endohedral compounds are fullerene molecules containing one or more atoms of an element). The conditions for the synthesis of antiviral and anticancer drugs based on fullerenes are found. One of the difficulties in solving these problems is the creation of water-soluble non-toxic fullerene compounds that could be introduced into the human body and delivered by blood to the organ subject to therapeutic action.

The use of fullerenes is constrained by their high cost, which consists of the laboriousness of obtaining a fullerene mixture and the isolation of individual components from it.

1.6 Carbon nanotubes

Structure of nanotubes

Along with spheroidal carbon structures, extended cylindrical structures, the so-called nanotubes, can also be formed, which are distinguished by a wide variety of physicochemical properties.

An ideal nanotube is a graphite plane rolled into a cylinder, i.e. a surface lined with regular hexagons, at the vertices of which carbon atoms are located ..).

The parameter indicating the coordinates of the hexagon, which, as a result of the folding of the plane, should coincide with the hexagon located at the origin of coordinates, is called the chirality of the nanotube and is denoted by the set of symbols (m, n). The chirality of a nanotube determines its electrical characteristics.

Electron microscope observations have shown that most nanotubes consist of several graphite layers, either nested one inside the other or wound around a common axis.

Single-walled nanotubes

On rice. 4 an idealized model of a single-walled nanotube is presented. Such a tube ends with hemispherical vertices containing along with

with regular hexagons, also six regular pentagons. The presence of pentagons at the ends of the tubes makes it possible to consider them as the limiting case of fullerene molecules, the length of the longitudinal axis of which considerably exceeds their diameter.

The structure of single-walled nanotubes observed experimentally differs in many respects from the idealized picture presented above. First of all, this concerns the tops of the nanotube, the shape of which, as follows from observations, is far from an ideal hemisphere.

Multilayer nanotubes

Multilayer nanotubes differ from single-layer nanotubes in a much wider variety of shapes and configurations both in the longitudinal and transverse directions. Possible varieties of the transverse structure of multilayer nanotubes are shown in rice. 5. Structure like "Russian dolls" (russian dolls) is a set of coaxially nested single-layer nanotubes (rice 5 a). Another variation of this structure, shown in rice. 5 b, is a set of nested coaxial prisms. Finally, the last of the above structures ( rice. 5 c), looks like a scroll. For all the above structures, the distance between adjacent graphite layers is close to 0.34 nm, i.e. the distance between adjacent planes of crystalline graphite. The realization of one structure or another in a specific experimental situation depends on the conditions of nanotube synthesis.

It should be borne in mind that the idealized transverse structure of nanotubes, in which the distance between adjacent layers is close to 0.34 nm and does not depend on the axial coordinate, is distorted in practice due to the perturbing effect of neighboring nanotubes.

The presence of defects also leads to a distortion of the rectilinear shape of the nanotube and gives it the shape of an accordion.

Another type of defects, often noted on the graphite surface of multilayer nanotubes, is associated with the introduction of a certain number of pentagons or heptagons into the surface, which consists mainly of regular hexagons. This leads to violation cylindrical shape, with the insertion of a pentagon causing a convex bend, while the insertion of a heptagon contributes to the appearance of a concave bend. Thus, such defects cause the appearance of bent and helical nanotubes.

Structure of nanoparticles

During the formation of fullerenes from graphite, nanoparticles are also formed. These are closed structures similar to fullerenes, but much larger than them. Unlike fullerenes, they, like nanotubes, can contain several layers. They have the structure of closed, nested graphite shells.

In nanoparticles, similarly to graphite, the atoms inside the shell are linked by chemical bonds, and there is a weak van der Waals interaction between the atoms of neighboring shells. Typically, nanoparticle shells have a shape close to a polyhedron. In the structure of each such shell, in addition to hexagons, as in the structure of graphite, there are 12 pentagons, additional pairs of five and heptagons are observed. An electron microscopic study of the shape and structure of carbon particles in a fullerene-containing condensate was recently carried out in the works of Jarkov S.M., Kashkin V.B.

Obtaining carbon nanotubes

Carbon nanotubes are formed by thermal sputtering of a graphite electrode in an arc discharge plasma burning in a helium atmosphere. This method, as well as the laser sputtering method, which underlies the efficient technology for obtaining fullerenes, makes it possible to obtain nanotubes in an amount sufficient for a detailed study of their physicochemical properties.

A nanotube can be obtained from extended graphite fragments, which are then twisted into a tube. For the formation of extended fragments, special conditions for heating graphite are required. Optimal conditions nanotubes are produced in an arc discharge using electrolytic graphite as electrodes.

Among the various products of thermal sputtering of graphite (fullerenes, nanoparticles, soot particles), a small part (several percent) is accounted for by multilayer nanotubes, which are partially attached to the cold surfaces of the installation, partially deposited on the surface along with soot.

Single-walled nanotubes are formed when a small admixture of Fe, Co, Ni, Cd is added to the anode (i.e., by adding catalysts). In addition, single-walled nanotubes are obtained by oxidizing multi-walled nanotubes. For the purpose of oxidation, multilayer nanotubes are treated with oxygen at moderate heating, or with boiling nitric acid, and in the latter case, five-membered graphite rings are removed, leading to the opening of the ends of the tubes. Oxidation allows you to remove the upper layers from the multilayer tube and open its ends. Since the reactivity of nanoparticles is higher than that of nanotubes, the fraction of nanotubes in the remaining part of it increases with significant destruction of the carbon product as a result of oxidation.

In the electric arc method of obtaining fullerenes, part of the material that is destroyed under the action of the graphite anode arc is deposited on the cathode. By the end of the process of destruction of the graphite rod, this formation grows so much that it covers the entire area of ​​the arc. This outgrowth has the shape of a bowl, into which the anode is introduced. physical characteristics cathode buildup are very different from the characteristics of graphite, of which the anode is composed. The build-up microhardness is 5.95 GPa (graphite -0.22 GPa), the build-up density is 1.32 g/cm 3 (graphite -2.3 g/cm 3), the build-up electrical resistivity is 1.4 * 10 -4 Ohm m, which is almost an order of magnitude greater than that of graphite (1.5 * 10 -5 ohm m). At 35 K, an anomalously high magnetic susceptibility of the build-up on the cathode was found, which made it possible to assume that the build-up consists mainly of nanotubes (Belov N.N.).

Properties of nanotubes

Broad prospects for the use of nanotubes in materials science open up when superconducting crystals (eg, TaC) are encapsulated inside carbon nanotubes. The following technology is described in the literature. We used a DC arc discharge of ~30 A at a voltage of 30 V in a helium atmosphere with electrodes that were a compressed mixture of thallium powder with a graphite pigment. The interelectrode distance was 2–3 mm. Using a tunneling electron microscope, a significant amount of TaC crystals encapsulated in nanotubes was found in the products of thermal decomposition of the electrode material.. X The typical transverse size of crystallites was about 7 nm, and the typical length of nanotubes was more than 200 nm. The nanotubes were multilayer cylinders with a distance between layers of 0.3481 ± 0.0009 nm, close to the corresponding parameter for graphite. Measurement of the temperature dependence of the magnetic susceptibility of the samples showed that the encapsulated nanocrystals transform intosuperconducting state at T=10 K.

The possibility of obtaining superconducting crystals encapsulated in nanotubes makes it possible to isolate them from the harmful effects of the external environment, for example, from oxidation, thereby opening the way to more efficient development of the corresponding nanotechnologies.

The large negative magnetic susceptibility of nanotubes indicates their diamagnetic properties. It is assumed that the diamagnetism of nanotubes is due to the flow of electron currents along their circumference. The value of the magnetic susceptibility does not depend on the orientation of the sample, which is associated with its disordered structure. The relatively large value of the magnetic susceptibility indicates that, at least in one of the directions, this value is comparable to the corresponding value for graphite. The difference between the temperature dependence of the magnetic susceptibility of nanotubes and the corresponding data for other forms of carbon indicates that carbon nanotubes are a separate independent form of carbon, the properties of which are fundamentally different from the properties of carbon in other states..

Applications of nanotubes

Many technological applications of nanotubes are based on their high specific surface area (in the case of a single-walled nanotube, about 600 sq. m. per 1/g), which opens up the possibility of using them as porous material in filters, etc.

The material of nanotubes can be successfully used as a carrier substrate for heterogeneous catalysis, and the catalytic activity of open nanotubes significantly exceeds the corresponding parameter for closed nanotubes.

It is possible to use nanotubes with a high specific surface as electrodes for electrolytic capacitors with a high specific power.

Carbon nanotubes have proven themselves well in experiments on their use as a coating that promotes the formation of a diamond film. As photographs taken with an electron microscope show, the diamond film deposited on the nanotube film differs in better side in relation to the density and uniformity of the nuclei from the film deposited on C 60 and C 70 .

Such properties of a nanotube as its small size, which varies considerably depending on the conditions of synthesis, electrical conductivity, mechanical strength and chemical stability make it possible to consider a nanotube as a basis for future elements of microelectronics. It has been proved by calculation that the introduction of a pentagon–heptagon pair into the ideal structure of a nanotube as a defect changes its electronic properties. A nanotube with an embedded defect can be considered as a metal-semiconductor heterojunction, which, in principle, can form the basis of a semiconductor element of record-breaking small dimensions.

Nanotubes can serve as the basis of the thinnest measuring tool used to control surface inhomogeneities of electronic circuits.

Interesting applications can be obtained by filling nanotubes with various materials. In this case, a nanotube can be used both as a carrier of the material filling it, and as an insulating shell that protects this material from electrical contact or from chemical interaction with surrounding objects.

CONCLUSION

Although fullerenes have a short history, this area of ​​science is rapidly developing, attracting more and more new researchers. This area of ​​science includes three areas: fullerene physics, fullerene chemistry and fullerene technology.

Physics of fullerenes is engaged in the study of structural, mechanical, electrical, magnetic, optical properties of fullerenes and their compounds in various phase states. This also includes the study of the nature of the interaction between carbon atoms in these compounds, the spectroscopy of fullerene molecules, the properties and structure of systems consisting of fullerene molecules. Fullerene physics is the most advanced branch in the field of fullerenes.

Chemistry of fullerenes associated with the creation and study of new chemical compounds, which are based on closed carbon molecules, and also studies the chemical processes in which they participate. It should be noted that in terms of concepts and research methods, this area of ​​chemistry is fundamentally different from traditional chemistry in many respects.

Fullerene technology includes both fullerene production methods and their various applications.

BIBLIOGRAPHY

1. Sokolov V. I., Stankevich I. V. Fullerenes - new allotropic forms of carbon: structure, electronic structure and chemical properties / / Advances in Chemistry, vol. 62 (5), p. 455, 1993.

2. New directions in fullerene research//UFN, v. 164 (9), p. 1007, 1994.

3. Eletsky A.V., Smirnov B.M. Fullerenes and structures of carbon//UFN, v. 165 (9), p. 977, 1995.

4. Zolotukhin I.V. Fullerite is a new form of carbon // SOZH No. 2, p. 51, 1996.

5. Masterov V.F. Physical properties of fullerenes / / SOZH No. 1, p. 92, 1997.

6. Lozovik Yu.V., Popov A.M. Formation and growth of carbon nanostructures - fullerenes, nanoparticles, nanotubes and cones//UFN, v. 167 (7), p. 151, 1997/

7. Eletsky A.V. .Carbon nanotubes//UFN, v.167(9), p.945, 1997.

8. Smalley R.E. Discovering fullerenes//UFN, v.168 (3), p.323, 1998.

9. Churilov G.N. Review of methods for obtaining fullerenes // Materials of the 2nd interregional conference with international participation "Ultrafine powders, nanostructures, materials", Krasnoyarsk, KSTU, October 5-7, 1999,. With. 77-87.

10. Belov N.N. et al. Structure of the surface of the cathode build-up formed during the synthesis of fullerenes // Aerosols vol. 4f, N1, 1998, pp. 25-29

11. S. M. Jarkov,. Titarenko Ya.N., Churilov G.N. Elektron microscopy studies off FCC carbon particles// Carbon, v. 36, No. 5-6, 1998, p. 595-597

12. Kashkin V.B., Rubleva T.V., Kashkina L.V., Mosin R.A. Digital processing of electron microscopic images of carbon particles in fullerene-containing soot // Proceedings of the 2nd interregional conference with international participation "Ultrafine powders, nanostructures, materials", Krasnoyarsk, KSTU, October 5-7, 1999. With. 91-92

Fullerene C 60

Fullerene C 540

Fullerenes, buckyballs or buckyballs- molecular compounds belonging to the class of allotropic forms of carbon (others are diamond, carbyne and graphite) and representing convex closed polyhedra, composed of an even number of three-coordinated carbon atoms. These connections owe their name to the engineer and designer Richard Buckminster Fuller, whose geodetic structures are built on this principle. Initially, this class of joints was limited to structures containing only pentagonal and hexagonal faces. Note that for the existence of such a closed polyhedron constructed from n vertices that form only pentagonal and hexagonal faces, according to Euler's theorem for polyhedra, which asserts the validity of the equality | n | − | e | + | f | = 2 (where | n | , | e| and | f| respectively, the number of vertices, edges and faces), a necessary condition is the presence of exactly 12 pentagonal faces and n/ 2 − 10 hexagonal faces. If the fullerene molecule, in addition to carbon atoms, includes atoms of other chemical elements, then if the atoms of other chemical elements are located inside the carbon cage, such fullerenes are called endohedral, if outside - exohedral.

The history of the discovery of fullerenes

Structural properties of fullerenes

In fullerene molecules, carbon atoms are located at the vertices of regular hexagons and pentagons, which form the surface of a sphere or ellipsoid. The most symmetrical and most fully studied representative of the fullerene family is fullerene (C 60), in which carbon atoms form a truncated icosahedron, consisting of 20 hexagons and 12 pentagons and resembling a soccer ball. Since each carbon atom of fullerene C 60 simultaneously belongs to two hexagons and one pentagon, then all atoms in C 60 are equivalent, which is confirmed by the nuclear magnetic resonance (NMR) spectrum of the 13 C isotope - it contains only one line. However, not all C-C bonds are the same length. The C=C bond, which is a common side for two hexagons, is 1.39, and C-C connection, common for a hexagon and a pentagon, is longer and equals 1.44 Å. In addition, the bond of the first type is double, and the second is single, which is essential for the chemistry of C 60 fullerene.

The next most common is the C 70 fullerene, which differs from the C 60 fullerene by inserting a belt of 10 carbon atoms into the C 60 equatorial region, as a result of which the C 70 molecule is elongated and resembles a rugby ball in its shape.

The so-called higher fullerenes containing more carbon atoms (up to 400), are formed in much smaller quantities and often have a rather complex isomeric composition. Among the most studied higher fullerenes, one can single out C n , n=74, 76, 78, 80, 82 and 84.

Synthesis of fullerenes

The first fullerenes were isolated from condensed graphite vapors obtained by laser irradiation of solid graphite samples. In fact, they were traces of the substance. The next important step was taken in 1990 by W. Kretchmer, Lamb, D. Huffman and others, who developed a method for obtaining gram amounts of fullerenes by burning graphite electrodes in an electric arc in a helium atmosphere at low pressures. . In the process of anode erosion, soot containing a certain amount of fullerenes settled on the chamber walls. Subsequently, it was possible to choose the optimal parameters of electrode evaporation (pressure, atmospheric composition, current, electrode diameter), at which the highest yield of fullerenes is achieved, averaging 3–12% of the anode material, which ultimately determines the high cost of fullerenes.

At first, all attempts by experimenters to find cheaper and more productive methods for obtaining gram quantities of fullerenes (combustion of hydrocarbons in a flame, chemical synthesis, etc.) did not lead to success, and the “arc” method remained the most productive for a long time (productivity is about 1 g / h) . Subsequently, Mitsubishi managed to establish industrial production fullerenes by burning hydrocarbons, but such fullerenes contain oxygen and therefore the arc method is still the only suitable method for obtaining pure fullerenes.

The mechanism of fullerene formation in the arc still remains unclear, since the processes occurring in the arc burning region are thermodynamically unstable, which greatly complicates their theoretical consideration. It was irrefutably established only that the fullerene is assembled from individual carbon atoms (or C 2 fragments). For proof, highly purified 13 C graphite was used as the anode electrode, the other electrode was made of ordinary 12 C graphite. After the extraction of fullerenes, it was shown by NMR that the 12 C and 13 C atoms are randomly located on the surface of the fullerene. This indicates the decay of the graphite material to individual atoms or fragments of the atomic level and their subsequent assembly into a fullerene molecule. This circumstance made it necessary to abandon the visual picture of the formation of fullerenes as a result of the folding of atomic graphite layers into closed spheres.

A relatively rapid increase in the total number of installations for the production of fullerenes and constant work to improve their purification methods have led to a significant reduction in the cost of C 60 over the past 17 years - from $ 10,000 to $ 10-15 per gram, which brought them to the boundary of their real industrial use.

Unfortunately, despite the optimization of the Huffman-Kretchmer (HK) method, it is not possible to increase the yield of fullerenes by more than 10-20% of the total mass of burnt graphite. Considering the relatively high cost of the initial product, graphite, it becomes clear that this method has fundamental limitations. Many researchers believe that it will not be possible to reduce the cost of fullerenes obtained by the XC method below a few dollars per gram. Therefore, the efforts of a number of research groups are aimed at finding alternative methods for obtaining fullerenes. The greatest success in this area was achieved by the Mitsubishi company, which, as mentioned above, managed to establish the industrial production of fullerenes by burning hydrocarbons in a flame. The cost of such fullerenes is about $5/gram (2005), which did not affect the cost of electric arc fullerenes.

It should be noted that the high cost of fullerenes is determined not only by their low yield during graphite combustion, but also by the difficulty of isolating, purifying, and separating fullerenes of various masses from carbon black. The usual approach is as follows: the soot obtained by burning graphite is mixed with toluene or another organic solvent (capable of effectively dissolving fullerenes), then the mixture is filtered or centrifuged, and the remaining solution is evaporated. After removing the solvent, a dark fine-crystalline precipitate remains - a mixture of fullerenes, usually called fullerite. The composition of fullerite includes various crystalline formations: small crystals of C 60 and C 70 molecules and C 60 /C 70 crystals are solid solutions. In addition, fullerite always contains a small amount of higher fullerenes (up to 3%). Separation of a mixture of fullerenes into individual molecular fractions is carried out using liquid chromatography on columns and high pressure liquid chromatography (HPLC). The latter is mainly used to analyze the purity of isolated fullerenes, since the analytical sensitivity of the HPLC method is very high (up to 0.01%). Finally, the last stage is the removal of solvent residues from the solid fullerene sample. It is carried out by keeping the sample at a temperature of 150-250 o C in a dynamic vacuum (about 0.1 Torr).

Physical properties and applied value of fullerenes

Fullerites

Condensed systems consisting of fullerene molecules are called fullerites. The most studied system of this kind is the C 60 crystal, less - the crystalline C 70 system. Studies of crystals of higher fullerenes are hampered by the complexity of their preparation. Carbon atoms in a fullerene molecule are linked by σ- and π-bonds, while there is no chemical bond (in the usual sense of the word) between individual fullerene molecules in a crystal. Therefore, in a condensed system, individual molecules retain their individuality (which is important when considering the electronic structure of a crystal). Molecules are held in the crystal by van der Waals forces, which largely determine the macroscopic properties of solid C 60 .

At room temperatures, the C 60 crystal has a face-centered cubic (fcc) lattice with a constant of 1.415 nm, but as the temperature decreases, a first-order phase transition occurs (T cr ≈ 260 K) and the C 60 crystal changes its structure to a simple cubic one (lattice constant 1.411 nm) . At a temperature T > Tcr, C 60 molecules rotate randomly around their center of equilibrium, and when it drops to a critical temperature, the two axes of rotation are frozen. Complete freezing of rotations occurs at 165 K. The crystal structure of C 70 at temperatures of the order of room temperature was studied in detail in the work. As follows from the results of this work, crystals of this type have a body-centered (bcc) lattice with a small admixture of the hexagonal phase.

Nonlinear optical properties of fullerenes

An analysis of the electronic structure of fullerenes shows the presence of π-electron systems, for which there are large values ​​of the nonlinear susceptibility. Fullerenes indeed have nonlinear optical properties. However, due to the high symmetry of the C 60 molecule, second harmonic generation is possible only when asymmetry is introduced into the system (for example, by external electric field). From a practical point of view, the high speed (~250 ps), which determines the suppression of the second harmonic generation, is attractive. In addition, C 60 fullerenes are also capable of generating the third harmonic.

Another possible area for the use of fullerenes and, first of all, C 60 is optical shutters. The possibility of using this material for a wavelength of 532 nm has been experimentally shown. The short response time makes it possible to use fullerenes as laser radiation limiters and Q-switches. However, for a number of reasons, it is difficult for fullerenes to compete here with traditional materials. High cost, difficulties in dispersing fullerenes in glasses, the ability to rapidly oxidize in air, non-record coefficients of nonlinear susceptibility, and a high threshold for limiting optical radiation (not suitable for eye protection) create serious difficulties in the fight against competing materials.

Quantum mechanics and fullerene

Hydrated fullerene (HyFn); (C 60 @ (H 2 O) n)

Aqueous solution C 60 HyFn

Hydrated C 60 - C 60 HyFn fullerene is a strong, hydrophilic supramolecular complex consisting of a C 60 fullerene molecule enclosed in the first hydration shell, which contains 24 water molecules: C 60 @(H 2 O) 24 . The hydration shell is formed due to the donor-acceptor interaction of lone pairs of oxygen oxygen molecules in water with electron-acceptor centers on the fullerene surface. At the same time, water molecules oriented near the fullerene surface are interconnected by a volumetric network of hydrogen bonds. The size of C 60 HyFn corresponds to 1.6-1.8 nm. At present, the maximum concentration of C 60 , in the form of C 60 HyFn, that has been created in water is equivalent to 4 mg/ml. Photo of an aqueous solution of C 60 HyFn with a concentration of C 60 0.22 mg/ml on the right.

Fullerene as a material for semiconductor technology

A molecular fullerene crystal is a semiconductor with a band gap of ~1.5 eV and its properties are largely similar to those of other semiconductors. Therefore, a number of studies have been related to the use of fullerenes as a new material for traditional applications in electronics: a diode, a transistor, a photocell, etc. Here, their advantage over traditional silicon is a short photoresponse time (units of ns). However, the effect of oxygen on the conductivity of fullerene films turned out to be a significant drawback and, consequently, a need arose for protective coatings. In this sense, it is more promising to use the fullerene molecule as an independent nanoscale device and, in particular, as an amplifying element.

Fullerene as a photoresist

Under the action of visible (> 2 eV), ultraviolet and shorter wavelength radiation, fullerenes polymerize and in this form are not dissolved by organic solvents. As an illustration of the use of a fullerene photoresist, one can give an example of obtaining submicron resolution (≈20 nm) by etching silicon with an electron beam using a mask of a polymerized C 60 film.

Fullerene Additives for the Growth of Diamond Films by the CVD Method

Another interesting possibility of practical application is the use of fullerene additives in the growth of diamond films by the CVD method (Chemical Vapor Deposition). The introduction of fullerenes into the gas phase is effective from two points of view: an increase in the rate of formation of diamond cores on the substrate and the supply of building blocks from the gas phase to the substrate. Fragments of C 2 act as building blocks, which turned out to be a suitable material for the growth of a diamond film. It has been experimentally shown that the growth rate of diamond films reaches 0.6 µm/h, which is 5 times higher than without the use of fullerenes. For real competition between diamonds and other semiconductors in microelectronics, it is necessary to develop a method of heteroepitaxy of diamond films, but the growth of single-crystal films on non-diamond substrates remains an unsolvable problem. One possible way to solve this problem is to use a fullerene buffer layer between the substrate and the diamond film. A prerequisite for research in this direction is the good adhesion of fullerenes to most materials. These provisions are especially relevant in connection with intensive research on diamonds for their use in next-generation microelectronics. High performance (high saturated drift speed); The highest thermal conductivity and chemical resistance of any known material make diamond a promising material for the next generation of electronics.

Superconducting compounds with C 60

Molecular fullerene crystals are semiconductors, however, in early 1991 it was found that doping solid C 60 with a small amount of alkali metal leads to the formation of a material with metallic conductivity, which at low temperatures passes into a superconductor. Doping with 60 is produced by treating crystals with metal vapor at temperatures of several hundred degrees Celsius. In this case, a structure of the type X 3 C 60 is formed (X is an alkali metal atom). The first intercalated metal was potassium. The transition of the K 3 C 60 compound to the superconducting state occurs at a temperature of 19 K. This is a record value for molecular superconductors. It was soon established that many fullerites doped with alkali metal atoms in the ratio of either X 3 C 60 or XY 2 C 60 (X, Y are alkali metal atoms) have superconductivity. The record holder among the high-temperature superconductors (HTSC) of these types was RbCs 2 C 60 - its T cr =33 K.

Influence of small additives of fullerene soot on the antifriction and antiwear properties of PTFE

It should be noted that the presence of C 60 fullerene in mineral lubricants initiates the formation of a protective fullerene-full-dimensional film 100 nm thick on the counterbody surfaces. The formed film protects against thermal and oxidative degradation, increases the lifetime of friction units in emergency situations by 3-8 times, the thermal stability of lubricants up to 400-500ºС and the bearing capacity of friction units by 2-3 times, expands the working pressure range of friction units by 1.5-2 times, reduces the running-in time of counterbodies.

Other applications of fullerenes

Other interesting applications include accumulators and electric batteries, in which fullerene additives are used in one way or another. These batteries are based on lithium cathodes containing intercalated fullerenes. Fullerenes can also be used as additives for producing artificial diamonds using the high pressure method. In this case, the yield of diamonds increases by ≈30%. Fullerenes can also be used in pharmacy to create new drugs. In addition, fullerenes have found application as additives in intumescent (intumescent) fire-retardant paints. Due to the introduction of fullerenes, the paint swells under the influence of temperature during a fire, a rather dense foam-coke layer is formed, which several times increases the heating time to the critical temperature of the protected structures. Also, fullerenes and their various chemical derivatives are used in combination with polyconjugated semiconducting polymers for the manufacture of solar cells.

Chemical properties of fullerenes

Fullerenes, despite the absence of hydrogen atoms that can be substituted as in the case of conventional aromatic compounds, can still be functionalized by various chemical methods. For example, such reactions for the functionalization of fullerenes as

The molecular form of carbon or its allotropic modification, fullerene, is a long series of atomic clusters C n (n > 20), which are convex closed polyhedra built from carbon atoms and having pentagonal or hexagonal faces (there are very rare exceptions here). Carbon atoms in unsubstituted fullerenes tend to be in the sp 2 -hybrid state with a coordination number of 3. Thus, a spherical conjugated unsaturated system is formed according to the theory of valence bonds.

general description

The most thermodynamically stable form of carbon under normal conditions is graphite, which looks like a stack of graphene sheets barely connected to each other: flat lattices of hexagonal cells with carbon atoms at the top. Each of them is associated with three neighboring atoms, and the fourth valence electron forms a pi system. This means that fullerene is precisely such a molecular form, that is, the picture of the sp 2 hybrid state is obvious. If geometric defects are introduced into a graphene sheet, a closed structure is inevitably formed. For example, such defects are five-membered cycles (pentagonal faces), which are just as common along with hexagonal ones in carbon chemistry.

Nature and technology

Obtaining fullerenes in pure form is possible by artificial synthesis. These compounds continue to be intensively studied in different countries, establishing the conditions under which their formation occurs, and the structure of fullerenes and their properties are also considered. The scope of their application is expanding. It turned out that a significant amount of fullerenes is contained in soot, which is formed on graphite electrodes in an arc discharge. Previously, this fact simply no one saw.

When fullerenes were obtained in the laboratory, carbon molecules began to be found in nature. In Karelia, they were found in samples of shungites, in India and the USA - in furulgits. There are also many and frequent carbon molecules in meteorites and bottom sediments that are at least sixty-five million years old. On Earth, pure fullerenes can be formed during lightning discharges and during the combustion of natural gas. taken over the Mediterranean Sea were studied in 2011, and it turned out that in all the samples taken - from Istanbul to Barcelona - fullerene is present. The physical properties of this substance cause spontaneous formation. Also, huge amounts of it have been found in space - both in a gaseous state and in solid form.

Synthesis

The first experiments in the isolation of fullerenes took place through condensed vapors of graphite, which were obtained by laser irradiation of solid graphite samples. Only traces of fullerenes were obtained. Only in 1990, chemists Huffman, Lamb and Kretschmer developed new method extraction of fullerenes in gram quantities. It consisted in burning graphite electrodes with an electric arc in a helium atmosphere and at low pressure. The anode was eroded, and soot containing fullerenes appeared on the chamber walls.

Next, the soot was dissolved in toluene or benzene, and pure grams of C 70 and C 60 molecules were isolated in the resulting solution. The ratio is 1:3. In addition, the solution also contained two percent of higher-order heavy fullerenes. Now the matter was small: to select the optimal parameters for evaporation - the composition of the atmosphere, pressure, electrode diameter, current, and so on, in order to achieve the highest yield of fullerenes. They accounted for up to about twelve percent of the actual anode material. That is why fullerenes are so expensive.

Production

All attempts by experimental scientists were in vain at first: productive and cheap ways to obtain fullerenes were not found. Neither the burning of hydrocarbons in a flame nor chemical synthesis led to success. Method electric arc remained the most productive, allowing to obtain about one gram of fullerenes per hour. Mitsubishi has established industrial production by burning hydrocarbons, but their fullerenes are not pure - they contain oxygen molecules. And the very mechanism of formation of this substance still remains unclear, because the processes of arc burning are extremely unstable from a thermodynamic point of view, and this greatly slows down the consideration of the theory. Only the facts that fullerene collects individual carbon atoms, that is, C 2 fragments, are irrefutable. However, a clear picture of the formation of this substance has not been formed.

The high cost of fullerenes is determined not only by the low yield during combustion. Isolation, purification, separation of fullerenes different weight from soot - all these processes are quite complex. This is especially true for the separation of the mixture into individual molecular fractions, which are carried out by means of liquid chromatography on columns and at high pressure. On last step solvent residues are removed from the already solid fullerene. To do this, the sample is kept in a dynamic vacuum at temperatures up to two hundred and fifty degrees. But the plus is that during the development of C 60 fullerene and its production in macroquantities organic chemistry has grown into an independent branch - the chemistry of fullerenes, which has become incredibly popular.

Benefit

Fullerene derivatives are used in various fields of technology. Fullerene films and crystals are semiconductors that exhibit photoconductivity under optical irradiation. C 60 crystals, if doped with alkali metal atoms, pass into the state of superconductivity. Fullerene solutions have non-linear optical properties, therefore they can be used as the basis for optical shutters, which are necessary for protection against intense radiation. Fullerene is also used as a catalyst for the synthesis of diamonds. Fullerenes are widely used in biology and medicine. Three properties of these molecules work here: lipophilicity, which determines membranotropy, electron deficiency, which gives the ability to interact with free radicals, as well as the ability to transfer their own excited state to a molecule of ordinary oxygen and turn this oxygen into a singlet.

Such active forms of the substance attack biomolecules: nucleic acids, proteins, lipids. Reactive oxygen species are used in photodynamic therapy to treat cancer. Photosensitizers are injected into the patient's blood, generating reactive oxygen species - fullerenes themselves or their derivatives. The blood flow in the tumor is weaker than in healthy tissues, and therefore photosensitizers accumulate in it, and after directed irradiation, the molecules are excited, generating reactive oxygen species. cancer cells undergo apoptosis and the tumor is destroyed. Plus, fullerenes have antioxidant properties and trap reactive oxygen species.

Fullerene reduces the activity of HIV integrase, a protein that is responsible for embedding the virus into DNA, interacting with it, changing its conformation and depriving it of its main pest function. Some of the fullerene derivatives interact directly with DNA and prevent the action of restrictases.

More about medicine

In 2007, water-soluble fullerenes began to be used as anti-allergic agents. The studies were carried out on human cells and blood, which were exposed to fullerene derivatives - C60(NEt)x and C60(OH)x. In experiments on living organisms - mice - the results were positive.

Even now, this substance is used as a drug delivery vector, since water with fullerenes (remember the hydrophobicity of C 60) penetrates the cell membrane very easily. For example, erythropoietin, injected directly into the blood, is degraded in a significant amount, and if it is used together with fullerenes, the concentration more than doubles, and therefore it enters the cell.

According to www.fullwater.com.ua

"FULLEREN - THE MATRIX OF LIFE..."

So, unlike the well-known forms of carbon - diamond and graphite, fullerene is molecule made up of carbon atoms. The most important member of the C60 fullerene family consists of 60 carbon atoms. Indeed, we cannot say “diamond molecule” or graphite, these are just crystalline forms with a certain spatial arrangement of carbon atoms in the lattice. Fullerene is the only molecular form of carbon.

Nature has united many contradictory concepts in one object.

Fullerene is a link between organic and inorganic matter. This is a molecule, and a particle, and a cluster. The diameter of the C60 molecule is 1 nm, which corresponds to the fineness limit lying between the “true”, molecular and colloidal states of substances.

If we look inside the fullerene, we will find only a void penetrated by electromagnetic fields. In other words, we will see some kind of hollow space, about 0.4 nm in diameter, containing “ nothing" - vacuum, enclosed in a carbon shell, as in a kind of container. Moreover, the walls of this container do not allow any material particles (ions, atoms, molecules) to penetrate into it. But the hollow space itself, as if part of the cosmos, is rather something than nothing is capable of participating in subtle, informational interactions with the external material environment. A fullerene molecule can be called a "vacuum bubble", for which the well-known thesis that nature does not tolerate emptiness does not fit. Vacuum and matter- the two foundations of the universe harmoniously united in one molecule.

Another remarkable property of fullerenes is their interaction with water. The crystalline form is known to be insoluble in water. Many attempts to obtain aqueous solutions of fullerenes lead to the formation of colloidal or coarsely dispersed fullerene-water systems, in which the particles contain a large number of molecules in crystalline form. Obtaining aqueous molecular solutions seems impossible. And to have such a solution is very important, and first of all for their use in biology and medicine. Ever since the discovery of fullerenes, its high biological activity has been predicted. However, the generally accepted opinion about the hydrophobicity of fullerenes has directed the efforts of many scientists to the creation of water-soluble derivatives or solubilized forms. In this case, various hydrophilic radicals are sewn to the fullerene molecule or surrounded by water-soluble polymers and surfactants, thanks to which the fullerene molecules are “forced” to remain in the aqueous medium. In many works, their high biological activity. However, any changes in the outer carbon shell lead to a violation of the electronic structure and symmetry of the fullerene molecule, which, in turn, changes the specificity of its interaction with the environment. Therefore, the biological effect of artificially transformed fullerene molecules largely depends on the nature of the attached radicals and the solubilizers and impurities contained. The most striking individuality of fullerene molecules is shown in unmodified form and, in particular, their molecular solutions in water.

The resulting aqueous solutions of fullerenes are stable over time (more than 2 years), have unchanged physical and chemical properties and a constant composition. These solutions do not contain any toxic impurities. Ideally, it is only water and fullerene. Moreover, the fullerene is embedded in the natural multilayer structure of water, where the first layer of water is firmly bound to the fullerene surface due to donor-acceptor interactions between water oxygen and acceptor centers on the fullerene surface.

The complex of such a large molecule with water also has a significant buffer capacity. Near its surface, the pH value = 7.2–7.6 is preserved, the same pH value is found near the surface of the membranes of the main part of healthy cells of the body. Many cell “disease” processes are accompanied by a change in the pH value near the surface of its membrane. At the same time, a diseased cell not only creates uncomfortable conditions for itself, but also negatively affects its neighbors. Hydrated fullerene, being near the cell surface, is able to maintain its healthy pH value. Thus, favorable conditions are created for the cell itself to cope with its illness.

And the most remarkable property of hydrated fullerene is its ability to neutralize active radicals. The antioxidant activity of fullerene is 100-1000 times higher than the action of known antioxidants (for example, vitamin E, dibunol, b-carotene). Moreover, hydrated fullerene does not suppress the natural level of free radicals in the body, but becomes active only when their concentration increases. And the more free radicals are formed in the body, the more actively hydrated fullerene neutralizes them. The mechanism of antioxidant action of fullerene is fundamentally different from the action of known antioxidants used in practice. Thus, one molecule of a traditional antioxidant is needed to neutralize one radical. And one molecule of hydrated fullerene is capable of neutralizing an unlimited number of active radicals. It is a kind of antioxidant catalyst. Moreover, the fullerene molecule itself does not participate in the reaction, but is only a structure-forming element of the water cluster. ...

Even at the beginning of the last century, Academician Vernadsky noticed that living matter is characterized by high symmetry. Unlike the inorganic world, many organisms have a fivefold symmetry axis. Fullerene C60 has 6 axes of the fifth order, it is the only molecule in nature with such a unique symmetry. Even before the discovery of fullerenes, the molecular structures of some proteins were known in the form of a fullerene, and some viruses and other vital biological structures (for example) have similar structures. It is interesting to match the fullerene molecule and its minimal cluster secondary structure of DNA. So the size of the C60 molecule corresponds to the distance between three pairs of complementary bases in DNA, the so-called. codon, which specifies information for the formation of one amino acid of the synthesized protein. The distance between the turns of the DNA helix is ​​3.4 nm. The first spherical C60 cluster, consisting of 13 fullerene molecules, has the same size.

It is known that carbon, and especially graphite and amorphous carbon, have the ability to adsorb on their surface the simplest molecules, including those that could be a material for the formation of more complex biologically important molecules in the process of forming the foundations of living matter. Fullerene, due to its acceptor properties, is able to selectively interact with other molecules, and in the conditions of an aqueous environment, transfer these properties to ordered layers of water at a considerable distance from its surface.

There are many theories of the origin of life from inorganic matter, and their main conditions are factors such as

1. Concentration of simple molecules (CO, NO, NH3, HCN, H2O, etc.) near active sites on which reactions occur with the participation of external energy sources.
2. Complication of the formed organic molecules to polymeric and primary ordered structures.
3. Formation of high order structures.
4. Formation of self-reproducing systems.

Experimentally, when creating the conditions that existed on earth in the prebiological period, the possibility of observing the first factor was proved. The formation of vital and unimportant amino acids and some nucleic bases under these conditions is quite real. However, the probability of fulfilling all the conditions for the emergence of life is practically zero. This means that there must be some other condition that allows purposefully implementing the mechanism of assembling simple elements, complicating and ordering the resulting organic compounds to the level of the appearance of living matter. And this condition, in our opinion, is the presence of a matrix. This matrix should have a constant composition, have a high symmetry, interact (but not strongly) with water, create around itself a symmetrical environment of other molecules at a considerable distance, capable of concentrating active radicals near its surface and contributing to their neutralization with the formation of complex organic molecules, in at the same time to protect neutral forms from the attacks of active radicals, to form structures similar to themselves and similar structures of the water environment. And most importantly, carbon must be the matrix of carbon life. And fullerene in its hydrated state satisfies all these requirements. And, most likely, the main and most stable representative of the C60 fullerene family. It is quite possible that the emergence of life is not a primary act, but this process occurs continuously and somehow affects the development of life, the testing of the existing one and the formation of its new forms.

Fullerenes exist in nature wherever there is carbon and high energies. They exist near carbon stars, in interstellar space, in places where lightning strikes, or near volcano craters, even when gas is burned in a home gas stove. Fullerenes are also found in places of accumulation of carbon rocks. A special place here belongs to the Karelian shungite rocks. These rocks containing up to 90% pure carbon are about 2 billion years old. The nature of their origin is still not clear. One of the assumptions is the fall of a large carbon meteorite. IN shungite natural fullerenes were discovered for the first time. We also succeeded in extracting and identifying C60 fullerene in shungite.

Since the time of Peter I, there was a healing spring in Karelia " Martial waters". For many years, no one could finally explain the reason for the healing properties of this source. It was assumed that the increased iron content is the cause of the healing effect. However, there are many iron-containing sources on earth, and, as a rule, no therapeutic effect. Only after the discovery of fullerenes in the shungite rocks through which the spring flows, the assumption arose that the fullerene is the ultimate healing effect of the Martial waters. However medicinal properties This water, like melted water, does not last very long. It cannot be bottled and used as needed. The very next day it loses its properties. Martial water, having passed through the rock containing fullerenes and fullerene-like structures, only “saturates” with the structure that the rock gives it. And during storage, these life-giving clusters disintegrate. Fullerene does not spontaneously enter water and, therefore, there is no structure-forming element capable of maintaining ordered water clusters for a long time, and, therefore, such water quickly acquires the properties of ordinary water. In addition, the ions present in it themselves rearrange the native structure of water, creating their own hydrate clusters.

Having once received molecular-colloidal solutions of fullerenes in water, we tried to reproduce the essence of Martial waters in the laboratory. But for this, they took high-purity water and added an aqueous solution of fullerenes in a homeopathic dose. After that, they began to conduct biological tests on various models. The results were amazing. In almost any model of pathology, we find a positive biological effect. Experiments have been going on for more than 10 years. With a well-placed experiment, any pathological changes in a living organism almost always try to return to normal. And it's not a drug. purposeful action and not a foreign chemical compound, but simply a ball of carbon dissolved in water. Moreover, one gets the impression that the hydrated fullerene tends to bring into " normal condition"all changes in the body, to the structures that it generated as a matrix in the process of the birth of life.