CLASSICAL AND NONCLASSICAL FORMS OF THE VINYL CATION - A COUPLED CLUSTER STUDY

The energy difference between the classical and nonclassical forms of the vinyl cation is studied using the single and double excitation configuration interaction (CISD) and coupled cluster methods (CCSD and CCSDT-1). The basis sets employed range from double ζ to those including/ functions on carbon and d functions on hydrogen. It is shown that the theoretical energy differences are very sensitive to the inclusion of higher-order polarization functions and the inclusion of triple excitations in the correlated wave functions. The results are compared with other theoretical studies in the literature and with the very recent experimental work of Crofton, Jagod, Rehfuss, and Oka. The most reliable single theoretical treatment involves the CCSDT-1 method with basis set C(10s5p2d 1f/4s3p2d 1f), H(5s2p/3s2p), and predicts 3.3 kcal/mol for the classical/nonclassical energy difference. Basis set extension is expected to increase this separation to about 4.0 kcal/mol, still less than the experimental value, > 6 kcal/mol. © 1990 American Institute of Physics.