Delocalization does not always stabilize : a quantum chemical analysis of -substituent effects on 54 alkyl and vinyl cations

The effects of -substituents on alkyl and vinyl cations are studied using high-level ab initio calculations. The geometries, stabilities, and electronic properties of 27 alkyl cations and 27 vinyl cations with -substituents are computed at the B3LYP/6-311 G(d,p), MP2/6-311 G(d,p), and CBS-Q levels. The substituents studied vary from strongly destabilizing (e.g., -CN and -CF3) to strongly stabilizing (e.g., -OSi(CH3)3 and -NH2). The calculations show that in the case of vinyl cations the stabilization provided by the -substituents is larger by an average value of 4 kcal/mol than for the alkyl ones. This is the result of the intrinsically lower stability of vinyl cations (on average 17 kcal/mol). However, strong inductively donating or withdrawing -substituents show different behavior. Because of the high amount of s character in the carbon--substituent bond in the vinyl cations (sp hybridized), more pronounced effects are found than in the corresponding alkyl cations, leading to lower stabilization for inductively withdrawing -substituents and higher stabilization for inductively donating ones. Thus, distinct effects of -substituents on the stabilization of the cations are observed. However, no correlation is found between NBO-computed charge increases or bond-order increases at either the carbocationic center or at the -substituent of the molecule and the stability provided by an -substituent. This demonstrates the conceptual difference between stabilizing and electron-donating effects. Only for the C-H hyperconjugative effect in the vinyl systems is a correlation with the computed reaction enthalpies observed. Finally, the effect of leaving-group variation is studied. Changing the leaving group from H to Cl yields geminal effects ranging from 7 kcal/mol destabilization to 9 kcal/mol stabilization of the neutral precursor.