David L. Kepert

An analysis of the 63 possible isomers of C60H36 containing a three-fold axis. A new structure for C60H20

Abstract The structures and stabilities of all 63 isomers of C60H36, in which pairs of hydrogen atoms occupy the hex-hex edges of the truncated icosahedron and in which there is at least one three-fold axis, have been calculated. The two most stable structures, of Tand Dad symmetry, are based on C18H18 crowns and have been described previously. A statistical analysis of the heats of formation as a function of the 12 types of polyhedral face which may be present in each structure shows that C6H4 (or C6 which are equivalent in this case), 1,2,3-CA, 1,2-C5H2 and C5H rings are particularly stabilising, whereas C6H6 and C5H5 rings are particularly destabilising. Extension of this analysis shows enhanced stability is achieved by the edge sharing of C6 and 1,2,3-C5H3 rings. These indications of stabilising features have led to postulation of structures of other C60Hn molecules. One of these, a D5d belt structure of C60H20, is slightly more stable than any found previously.

Crystal structure of gadolinium(III) acetate tetrahydrate. Stereo-chemistry of the nine-co-ordinate [M(bidentate ligand)3(unidentate ligand)3]x± system

The crystal structure of the title compound has been determined by single-crystal X-ray diffraction at 295(1) K and refined by least squares to a residual of 0.034 for 2 874 ‘observed’ reflections. Crystals are triclinic, space group P1, with a= 10.790(2), b= 9.395(3), c= 8.941 (3)A, α= 60.98(2), β= 88.50(2), γ= 62.31 (2)°, and Z= 2. The compound is isostructural with its erbium analogue and comprises a dimeric species with the two rare-earth metal atoms bridged by acetate oxygen atoms; the co-ordination number of each rare-earth metal atom is nine, through the three bidentate acetate groups, two water molecules, and a bridging oxygen from one of the adjoining acetates. Bond lengths (Gd–O) range from 2.368(6) to 2.571(4)A. The co-ordination stereochemistry for the [M(bidentate)3(unidentate)3]x± is examined in terms of a repulsion model.

Structures and Stabilities of hydrofullerenes. Completion of Crown Structures at C60H18 and C60H24.

Abstract The calculation of the stabilities of different isomers of C60Hn has been extended from n = 12 to n = 24. Each structure in a first series of molecules was obtained by addition of a pair of hydrogen atoms onto the most stable structure for C60Hn-2. This series culminates in C60H24, consisting of a crown composed of 18 linked CH groups plus three CH-CH groups remaining from the original C60H12 structure. A second series of structures was similarly obtained based on C60H18, in which all hydrogen atoms are part of the C18H[in18 crown. The most stable molecules are C60H12 (first series) and C60H22 (second series).

Crystal structure of dicaesium octa-µ3-chloro-hexachloro-octahedro-hexa-tungstate(II) and -molybdate(II) complexes

The crystal structures of the title compounds have been determined from single-crystal photographic X-ray diffraction data by Patterson and Fourier techniques, and refined by block-diagonal least-squares methods. Cs2[(W6Cl8)Br6] : trigonal, P31c, a= 10·07 ± 0·03, c= 14·75 ± 0·01 A; Z= 2; R= 0·11 for 665 observed reflections. Cs2[(Mo6Cl8)Br6] : trigonal, P31c, a= 10·06 ± 0·02, c= 14·70 ± 0·01 A, Z= 2; R 0·12 for 598 observed reflections.The anion consists of an octahedral cluster of metal atoms with the chlorine atoms bridging the octahedral faces and the bromine atoms axially co-ordinated to the M6 core. The mean metal–metal distances are W–W 2·620 ± 0·007 and Mo–Mo 2·615 ± 0·006 A. Other bonding distances are as expected. Implications of the structure concerning the vibrational spectrum are discussed. Cell dimensions are given for the range of isomorphous complexes Cs2[(M6Cl8)Y6], M = Mo or W; Y = Cl, Br, or I.

Early stages in the addition to C60 to form C60Xn, X = H, F, Cl, Br, CH3, C4H9

Abstract The study of the early stages of addition to C60 forming C60Xn has been extended to X=H, F, Cl, Br, CH3, C4H9, using the AM1 Hamiltonian with the program mopac 6.0 and the density functional technique B3LYP/6-31G* with gaussian 98. For X=H, F, the density functional results show greater stability of structures based on addition to hex–hex edges than do the AM1 results. For X=Cl, Br, CH3, addition can either be on hex–hex edges or across the para positions of C6 rings for C60X2. A combination of hex–hex addition and para addition can occur for C60X4, C60X6 and C60X12. Isomers with only para addition are found for C60X4, C60X8, C60X18 and C60X24. For C60Bu2, there is greater separation of the tert-butyl groups.

Stereochemical patterns in bromofullerenes C60Br2 to C60Br12

Abstract The stabilities of different isomers of C 60 Br n have been calculated for n = 14–24. Two types of structure are found to be important. The first type involves the linking of alternating 1,4-C 6 Br 2 rings and 1,3-C 6 Br 2 rings, forming BrBr BrBr strings over the molecule. Additional stability is achieved if the string ends are eliminated by the formation of cyclic strings, and particularly stable structures incorporate the Br 12 cycle of S 6 symmetry. The strings may branch at 1,3,5-C 6 Br 3 rings with the formation of polycyclic string structures, culminating in the rhombicuboctahedral bromine pattern in C 60 Br 24 . These patterns are related to the conformation of the C 6 rings, which form shallow boats in 1,4-C 6 Br 2 and chairs in 1,3,5-C 6 Br 3 . A second important structural feature is the presence of discrete Br 6 skew pentagonal pyramids, one in C 60 Br 6 , two in C 60 Br 12 and three in C 60 Br 18 . These results indicate the possibility of forming C 60 Br 12 and C 60 Br 18 under the appropriate conditions.

An analysis of the 94 possible isomers of C60F48 containing a three-fold axis

Abstract The structures and stabilities of all 94 isomers of C60F48 in which there is at least a three-fold axis have been calculated using the AM1 Hamiltonian and the program MOPAC 6.0. Two structures, of D3 and S6 symmetry, are substantially more stable than all the other structures. These two predicted isomers are the only two that have been experimentally observed. An analysis of all 94 isomers reveals that stability is enhanced if the six carbon-carbon double bonds are localised along six pent-hex edges of the truncated icosahedral structure of C60, rather than along hex-hex edges, and if the double bonds are separated from each other as much as possible.

Structures, stabilities and isomerism in C60H36 and C60F36. A comparison of the AM1 Hamiltonian and density functional techniques

Abstract The study of the structures and stabilities of the more stable isomers of C60H36 and C60F36, previously found in broad surveys using the AM1 Hamiltonian and the program mopac 6.0, has been extended with the density functional technique B3LYP/6-31G∗ using gaussian 98. The density functional calculations favour structures with unhydrogenated and unfluorinated C6 rings whereas the AM1 calculations favour structures with isolated carbon–carbon double bonds. The density functional calculations reveal a remarkable diversity in the types of structure for the more stable isomers. The most stable isomer for both C60H36 and C60F36 has four bare C6 rings as far apart as possible. The second most stable isomer has three bare C6 rings with three isolated CC units on pent–hex edges. The next three isomers have two bare C6 rings which may be as far apart as possible or both be on the same side of the molecule, while the CC bonds on pent–hex edges may be isolated from each other or form C4 units.

Opening of carbon nanotubes by addition of oxygen

Abstract The opening of carbon nanotubes by the addition of oxygen to form C n O 6 has been modelled using the AM1 Hamiltonian and the program mopac 6.0. Three series of nanotubes based on C 60 were considered. The first series was based on pulling two hemispherical halves of C 60 apart along a fivefold axis and inserting additional carbon atoms between the hemispheres to form C 70 , C 80 , C 90 and C 100 . The second series was obtained similarly by pulling C 60 apart along a threefold axis to form the D 3 h and D 3 isomers of C 78 and the D 3 d and D 3 isomers of C 96 . The third series was based on the extension of C 60 along a twofold axis to form C 76 and C 92 . There is a wide diversity in the bonding patterns in these nanotubes. In general, the hemispherical ends resemble C 60 with single bonds on pent–hex edges and double bonds on hex–hex edges, with the exception of C 80 in which there is significant double bond character on the pent–hex edges. The six-membered rings on the hemispherical ends have the normal Kekule pattern of single and double bonds. In general, the distinction between single and double bonds is continued into the tube part of the structure but again there are some exceptions. For example, in C 70 , D 3 -C 78 and C 76 there are some six-membered rings with delocalised bonding. In C 90 , D 3 h -C 78 and D 3 d -C 96 there are six-membered rings composed of six single bonds with six double bonds radiating out from the ring and these rings show significantly different chemical properties. Addition of oxygen occurs as near as possible to the end of the nanotube with the opening of a large hole leading into the interior of the structure. In the most favourable structures, a pair of oxygen atoms adds onto a hex–hex edge replacing the CC double bond by two CO ketone groups and the coalescence of two six-membered rings. Multiple additions may be of the same type leading to tris(diketone) structures with the coalescence of four six-membered rings into two different types of 18-membered rings. Alternatively, additional oxygen atoms may add onto pent–hex edges replacing the CC single bond with a COC ether linkage forming the mixed ether/ketones or the mixed ether/lactones which are generally the most stable types of structure.

Hatch opening and closing on oxygenation and deoxygenation of C60 bathysphere

Abstract The structures and stabilities of C 60 O n , where n =1–6, 9, have been calculated using the AM1 Hamiltonian and the program mopac 6.0, and by the density functional technique B3LYP/6-31G* at the AM1 geometry using Gaussian 98. Modes of oxygen addition considered were ethers, epoxides, ketones and ketenes. It is confirmed that in C 60 O a carbon–carbon bond on a pent–hex edge is replaced by an ether linkage. For higher levels of oxygen addition the replacement of a carbon–carbon bond by two ketone groups becomes important. Particularly stable structures are formed if the additions are on the same, or adjacent, C 6 rings of the C 60 structure where the oxygen atoms cooperate in opening up large holes in the molecule. Molecules of greatest stability have a mixture of ether oxygen atoms on pent–hex edges and diketone additions on hex–hex edges. A particularly important stable structure is the mixed ether/ketone isomer of C 60 O 6 in which a hinged C 5 O 2 hatch lid containing two ketone oxygen atoms is opened revealing a hatch 4–5 A in diameter. An even larger hole of approximately 6 A in diameter is opened up in a particularly stable isomer of C 60 O 9 which contains three ether and six ketone groups. Removal of the oxygen atoms from these structures and reoptimisation leads to hatch closing and C 60 is reformed.