STRUCTURE AND RELATIVE ENERGETICS OF C-2N-7) CARBON CLUSTERS USING COUPLED-CLUSTER AND HYBRID DENSITY-FUNCTIONAL METHODS(1 (N=2)

The geometry and harmonic frequencies of linear and cyclic structures of C-5, C-7, C-9, C-11, C-13, and C-15 have been calculated using a hy brid density functional method (B3LYP), and the linear-cyclic isomeris m considered using coupled cluster methods. It was established unambig uously that, from C-11 onwards, these molecules prefer cyclic structur es. Structures with a blunt apical angle are considerably more stable than the elongated structures found at the MP2 level, which gives a qu alitatively incorrect description of the potential surface. The B3PW9 1 (Becke 3-parameter-Perdew-Wang) functional appears to reproduce the CCSD(T) isomerization energies better than its B3LYP (Becke 3-paramete r-Lee-Yang-Parr) counterpart. No indication appears to exist for the p resence of either cyclic C-7 and C-9, or linear C-11 and C-13, in matr ix-trapped graphite vapor.