Quantifying resonance through a Lewis valence bond approach: application to haloallyl and carbonyl cations

A recent Valence Bond scheme based on Lewis structures, the VBB method (M. Linares, B. Braida and S. Humbel, J. Phys. Chem. A, 2006, 110, 2505 2509) is applied to resonance effect quantification. An accurate evaluation of this effect is provided by targeting pi interactions only, while all other factors remain constant. Valence Bond theory allows us to circumvent two main shortcomings of other approaches, i.e. the lack of a quantitative aspect, and the difficulty to properly separate resonance from other effects. The pi effect of fluorine and chlorine atoms is found to be comparable and quite significant (approximately 20 kcal mol(-1)), in both haloallyl and protonated carbonyl cations. The validity of the resonance model for carbonyl compounds is confirmed. Resonance in protonated formamide is indeed found to be significantly larger than in formic acids, itself being more resonant than the formyl fluoride cation. Comparisons with other methods of resonance effect evaluation are also made.