Structure and vibrational spectra of carbon clusters Cn (n = 2–10, 12, 14, 16, 18) using density functional theory including exact exchange contributions

Geometries and harmonic frequencies of linear Cn and cyclic C2n (n = 2–9) clusters have been studied using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional method and compared with ab initio coupled cluster calculations. For C2 through C10, results are of nearly the same quality as those obtained at the CCSD(T)/[3s2pld] (coupled cluster with all single, double, and quasiperturbative triple substitutions) level, except for relative energies. C4nn clusters (n = 2–4) are polyacetylenic rings with C2nh symmetry, and C4n+2 (n = 1–4) clusters are cumulenic rings with C(2n+1)h symmetry. They have intense infrared absorptions in the 1800–2000 cm−1 region, in addition to intense vibrations around 500 cm−1. Tentative assignments of some bands in the 1900–1950 cm−1 region to cyclic C14 and C18 are proposed.