Structure and relative energetics of C2n+1 (n = 2−7) carbon clusters using coupled cluster and hybrid density functional methods

The geometry and harmonic frequencies of linear and cyclic structures of C5, C7, C9, C11, C13, and C15 have been calculated using a hybrid density functional method (B3LYP), and the linear-cyclic isomerism considered using coupled cluster methods. It was established unambiguously that, from C11 onwards, these molecules prefer cyclic structures. Structures with a blunt apical angle are considerably more stable than the elongated structures found at the MP2 level, which gives a qualitatively incorrect description of the potential surface. The B3PW91 (Becke 3-parameter-Perdew-Wang) functional appears to reproduce the CCSD(T) isomerization energies better than its B3LYP (Becke 3-parameter-Lee-Yang-Parr) counterpart. No indication appears to exist for the presence of either cyclic C7 and C9, or linear C11 and C13, in matrix-trapped graphite vapor.