CCSD calculations on C14, C18, and C22 carbon clusters

The structure and energetics of the ring isomers of C4n+2 (n=3-5) carbon clusters were studied by using coupled-cluster singles and doubles excitation theory to overcome the vast differences existing in the literature. The results obtained in the present study clearly indicate that C-14, C-18, and C-22 carbon rings have bond-length and bond-angle alternated acetylenic minimum energy structures. Contrarily, density functional theory calculations were unable to predict these acetylenic-type structures and they ended up with the cumulenic structures. It is found from the coupled-cluster studies that the lowest-energy ring isomer for the first two members of C4n+2 series is a bond-angle alternated cumulenic D(2n+1)h symmetry structure while the same for the remaining members is a bond-length and bond-angle alternated C(2n+1)h symmetry structure. In C4n+2 carbon rings, Peierls-type distortion, transformation from bond-angle alternated to bond-length alternated minimum energy structures, occurs at C-14 carbon ring. (c) 2008 American Institute of Physics.