STRUCTURE AND VIBRATIONAL-SPECTRA OF CARBON CLUSTERS C-N (N=2-10, 12,14, 16, 18) USING DENSITY-FUNCTIONAL THEORY INCLUDING EXACT EXCHANGE CONTRIBUTIONS

Geometries and harmonic frequencies of linear C-n and cyclic C-2n (n = 2-9) clusters have been studied using the B3LYP (Becke 3-parameter-Le e-Yang-Parr) density functional method and compared with ab initio cou pled cluster calculations. For C-2 through C-10, results are of nearly the same quality as those obtained at the CCSD(T)/[3s2p1d] (coupled c luster with all single, double, and quasiperturbative triple substitut ions) level, except for relative energies. C-4n clusters (n = 2-4) are polyacetylenic rings with C-2nh symmetry, and C-4n+2 (n = 1-4) cluste rs are cumulenic rings with C((2n+1)h) symmetry. They have intense inf rared absorptions in the 1800-2000 cm(-1) region, in addition to inten se vibrations around 500 cm(-1) Tentative assignments of some bands in the 1900-1950 cm(-1) region to cyclic C-14 and C-18 are proposed.