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

A computational study of 13 C 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 13 C NMR chemical shifts shows that second‐order Møller–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 13 C NMR spectra of these type of carbocations. Copyright © 2004 John Wiley & Sons, Ltd.