Accurate standard enthalpies of formation of 115 isolated pentagon rule (IPR) fullerenes with 60-180 carbon atoms have been derived from energies of isodesmic interconversion reactions computed at the B3LYP/6-31G* level of theory. The calculated values of ∆H°f, which may serve as benchmarks for both calorimetric measurements and less sophisticated theoretical studies, are reproduced within 3 kcal/mol by a simple scheme based upon counts of 30 distinct structural motifs composed of hexagons together with their first and second neighborhoods. The extremely low computational cost of such a scheme makes it ideally suited for a rapid prescreening for thermodynamically viable IPR fullerenes with cages composed of hundreds of carbon atoms. With the inclusion of a global curvature term, this scheme is expected to be equally successful for small and large carbon clusters.

Electronic structure calculations carried out at the BLYP/6-311G** level of theory accurately predict the dissociation energy of the C-H bond in benzene. The analogous energies of the homolytic C-H bond cleavage in the other nine polycyclic aromatic hydrocarbons (PAHs) are found to be governed almost entirely by steric factors, the hydrogens from congested regions of the PAHs being removed preferentially. The removal of hydrogens is accompanied by highly regular changes in the molecular geometries, namely a widening of the ipso bond angle by ca. 6.0°and a concomitant shortening of the adjacent C-C bonds by ca. 0.02 Å. These observations suggest an almost complete localization of the unpaired σ electrons on single carbon atoms and the separation of the local σ and π effects in the aryl radicals under study. This localization is confirmed by the computed charges and spin populations of atoms in the phenyl, 1-naphthalenyl, and 2-naphthalenyl radicals. In contrast with their UHF counterparts, the UBLYP electronic wave functions are only mildly spin contaminated.

Standard enthalpies of formation, ionization potentials, electron affinities, and band gaps of finite-length [5,5] armchair and [9,0] zigzag single-walled carbon nanotubes (SWNTs) capped with C(30) hemispheres obtained by halving the C(60) fullerene have been computed at the B3LYP/6-311G* level of theory. Properties of SWNTs are found to depend strongly on the tube length and, in the case of the [9,0] zigzag species, on the relative orientation of the caps. The metallic character of an uncapped infinite-length [5,5] armchair SWNT manifests itself in the oscillatory dependence of the properties of capped finite-length tubes on their size. An infinite-length [9,0] zigzag SWNT is predicted to be a semiconductor rather than a metal irrespective of the presence of caps. The present results underscore the slow convergence of SWNT properties with respect to the tube length and uncover small but significant radial distortions along the long axes of SWNTs.

A theory is presented in which the elimination of charging artefacts on uncoated insulating surfaces in the scanning electron microscope by a low-pressure ambient gas is taken to be due to ionisation of the gas. Curves of ion current against ionising field calculated from a model of gas neutralisation give good agreement with the variation of measured ion currents passing through metal specimens. This result permits a precise interpretation of the behavior occurring at the surface of insulating specimens.