Structural, Cohesive, Electronic, and Aromatic Properties of Selected Fully and Partially Hydrogenated Carbon Fullerenes

The structural, cohesive, electronic, and aromatic properties of some representative fully (CnHn, n = 8-180) and partially (C60H2,36-40, C70H36-40) hydrogenated carbon fullerenes are studied by accurate density functional theory, in an attempt to establish stability trends and correlate these properties with stability, and with respect to each other, seeking ways for improving stability and pinpointing best candidates for future design. To this end, comparisons are made with other isoelectronic and isovalent structures including open structures, such as fullerane fragments and prismatic ladderanes, as well as homologous silicon fulleranes. The main difficulty, among many, is to establish reliable general stability indices in analogy to aromaticity indices, although both concepts are rather nebulous. This becomes even more difficult if one has to compare structures of different size, bonding, and/or composition. Nevertheless, by restricting the detailed search to a judicially chosen representative set of structures, it is hoped that useful and apparently reliable trends could emerge. For both fully and partially hydrogenated fullerenes, it appears that aromaticity, as expressed by standard aromaticity indices, does not correlate well with stability, as expressed by standard cohesive and other criteria. This is largely due to kinetic reasons, as well as subtleties in the stability and aromaticity concepts and criteria. Neither different stability criteria, such as cohesive (cohesive and/or binding energies per C atom) and kinetic (HOMO-LUMO gaps), can correlate well with each other. On the basis of the table with the calculated (1) cohesive/binding energies (cohesive stability), (2) HOMO-LUMO gaps (kinetic stability), and (3) aromaticity-NICS(0) values (electron delocalization), the top four list for each of these criteria has been constructed. The three top four lists (in decreasing order) are [C20H20, C180H180, C80H80, C50H50], [C8H8, C20H20, C60H60, C50H50], and [C60H2, C50H50, C70H40, C70H36], respectively. All three of them include at least one hydrogenated cage which has been already synthesized. The rather unknown C50H50 fullerane is the only one common in all three lists. This is highly suggestive that it could be synthesized in the very near future, A rather obvious general trend is that stability decreases dramatically from fulleranes to fullerane fragments all the way to prismatic ladderanes. It is also shown here that one of the most efficient ways to improve stability, on the basis of cohesive criteria is puckering of the H-C-C-H bonds, induced by partial endohydrogenation, which facilitates optimization of the sp(3) bonding. Selective doping by appropriate atoms, such as phosphorus, can also improve cohesion and/or functionalization of the cages: Contrary to partially hydrogenated C-60 and C-70 cages which have been synthesized, puckered C H fulleranes have not as yet synthesized despite their energetic advantages in binding and cohesive energy. This is attributed to existing barriers for penetration of hydrogen into the cage. Overall, dodecahedrane appears to be the most stable of all fully and partially hydrogenated fullerenes. This s not true for the analogous hydrogenated silicon cage (Si20H20). Among the partially hydrogenated cages examined here; C70H36 and C70H40 appear to be the most stable due to the five linked equatorial benzene-like rings that form a highly conjugated, graphite-like region.