Quantum chemical calculation of structures and NMR chemical shifts of substituted buta-1,3-dienyl-2-2-cations

A computational study of C-13 NMR chemical shifts of a series of a-vinyl substituted vinyl cations (1,3-dienyl-2-cations) 1-6 is presented. The sensitivity of the predicted isotropic shifts to electron correlation, basis set and geometry effects is explored. Comparison with experimental C-13 NMR chemical shifts shows that second-order Moller-Plesset perturbation theory calculations [GIAO-MP2/tzp//MP2/6-3 1G(d,p)] perform adequately (deviation approximate to 3-4 ppm) for all carbons of cations 1-6, except for carbons in 6 involved in cyclopropyl hyperconjugation, which give some larger deviations (approximate to 6-9 ppm). The Hartree-Fock self-consistent field (GIAO-HF/tzp) approximation as well as GIAO-DFT-methods together with hybrid functionals (B3LYP) give unsatisfactory results and cannot be relied upon to predict the sequence of signals in the C-13 NMR spectra of these type of carbocations. Copyright (C) 2004 John Wiley Sons, Ltd.