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

A computational study of 13C NMR chemical shifts of a series of α-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 13C NMR chemical shifts shows that second-order Moller–Plesset perturbation theory calculations [GIAO-MP2/tzp//MP2/6–31G(d,p)] perform adequately (deviation ≈ 3–4 ppm) for all carbons of cations 1–6, except for carbons in 6 involved in cyclopropyl hyperconjugation, which give some larger deviations (≈ 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 13C NMR spectra of these type of carbocations. Copyright © 2004 John Wiley & Sons, Ltd.