Extended ab initio studies of the vinylidene–acetylene rearrangement

The ground state vinylidene–acetylene isomerization was investigated by ab initio molecular electronic structure theory. The coupled-cluster method with single, double, and noniterative inclusion of triple excitations @CCSD~T!#; with single, double, and noniterative inclusion of triple and quadruple excitations @CCSD~TQ!#; and with full single, double, and triple excitations ~CCSDT! were used to treat the effect of electron correlation. Several correlation-consistent polarized valence basis sets, cc-pVXZ, were employed. Theoretical limiting values of the energetics of the reaction were then deduced from the series of computations. With zero-point energy correction, the energy of reaction is 242.95 kcal/mol and the reaction barrier is 1.5 kcal/mol. Both agree excellently with experimental values. © 1997 American Institute of Physics. @S0021-9606~97!03008-0#