α-Substituted Vinyl Cations:  Stabilities and Electronic Properties

Vinyl cations substituted at the α-position (H2CC(+)R), with R = H, CHCH2, CH3, F, and Cl, and their neutral precursors (H2CCHR) have been studied using various quantum chemical methods to analyze the influence of these substituents on the thermodynamic stability and electronic properties. B3LYP data obtained with various basis sets are compared to those of post-HF computations, including MP2, MP4(SDQ), and QCISD(T) computations and the CBS-Q model chemistry. The results of those calculations are benchmarked against experimental results. The NBO and AIM population analysis methods are used for analysis of the electronic properties. The geometry, stability, and electronic structure of the vinyl cations under study are already accurately described at the B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) and MP2/6-311+G(d,p)//MP2/6-311G(d,p) levels of theory. The results of the NBO calculations are shown to be preferable over AIM, since the latter, due to an artifact in that method, predicts counterintuitive charge distributions. Analysis of the reaction enthalpies and electronic properties shows the lack of correlation between the electron-donating ability of α-substituents and the stability provided by such substituents. Comparison of the results obtained for the vinyl cations with those of previously studied α-substituted ethyl cations shows a striking difference in stabilization of the two systems for R = F, destabilizing in vinyl systems, and stabilizing in ethyl systems, while the charge distributions are practically the same in these two systems. This outcome is discussed in detail.