The nature of monocyclic C 10. A theoretical investigation using coupled-cluster methods

Abstract

Three planar monocyclic geometries of C 10 (D 5h cumulenic ( 1), D 10h cumulenic ( 2), and D 5h acetylenic ( 3)) have been investigated using coupled-cluster methods. Geometries have been optimized with the coupled-cluster singles-and-doubles (CCSD) method, effects of triple excitations on the relative energies have been estimated using the CCSD+T(CCSD) and CCSD(T) methods, and some distortions of ( 2) to lower symmetry have been investigated. With all basis sets considered, ( 1) is the lowest energy structure, although ( 2) and ( 3) are respectively 2.8 and 5.8 kcal mol −1 higher in energy with a polarized valence double-zeta basis set, and ( 2) is less than 1 kcal mol −1 higher in energy than ( 1) with a 4s3p1d basis set. ( 2), however, is found to have an imaginary frequency for the B 1u mode, which leads to ( 1). It is suggested that monocyclic C 10 is highly fluxional, ( 2) being the low energy transition state for automerization of ( 1).