THE NATURE OF MONOCYCLIC C-10 - A THEORETICAL INVESTIGATION USING COUPLED-CLUSTER METHODS

Three planar monocyclic geometries of C-10 (D5h cumulenic (1), D10h cumulenic (2), and D5h 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 4s3pld basis set. (2), however, is found to have an imaginary frequency for the B1u 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).