Electron correlation effects on the calculated C-13 NMR spectra of vinyl cations

A study of calculated C-13 NMR chemical shifts in vinyl cations is presented. The sensitivity of predicted isotropic shifts to correlation, basis set and geometry effects is explored. In order to obtain accurate estimates that are reasonably well converged with respect to further improvements in theory, it appears that the CCSD(T) method must be used with a basis of triple-zeta plus polarization quality on the carbon atoms. Second-order many-body perturbation theory performs adequately for all carbons except for that bearing the formal positive charge, while the self-consistent field approximation cannot be relied upon to predict even the correct qualitative ordering in the spectrum. Various extrapolation schemes that assume an approximate additivity of basis set and correlation effects are also explored. (C) 1997 Elsevier Science B.V.