CARBON CLUSTERS - THE STRUCTURE OF C-10 STUDIED WITH CONFIGURATION-INTERACTION METHODS

The structure of the C-10 carbon cluster is studied by ab initio quantum mechanical methods using self-consistent-field and configuration interaction theories. Three monocyclic and two linear structures are considered. It is found that all the monocyclic structures are considerably lower in energy than the linear ones, in agreement with previous theoretical studies. At the highest level of theory, the lowest energy cyclic structure is predicted to lie 67 kcal/mole below the lowest energy linear structure. Correction for entropic contributions to the free energy at 3700 K reduces this DELTA-H value by about 40 kcal/mole. It is also concluded with confidence that the delocalized ring structure is more stable than the localized one with alternating single-triple bonds. Although the D5h symmetric monocyclic structure with cumulated double bonds is predicted to be the lowest state at the highest theoretical level (CISD with a DZP basis set) employed here, it is cautioned that the fully symmetric D10h monocyclic form might be more stable. Among the two linear structures, the triplet acetylenic form is almost isoenergetic with the triplet cumulenic one at CISD/DZP, and Davidson's correction places the former about 5 kcal/mole above the latter. The possibility of the bending of linear C-10 was also investigated. Qualitative theoretical arguments are employed to rationalize the ab initio results obtained.