Explicit Solvation Effects on the Conventional Resonance Model for Protonated Imine, Carbonyl, and Thiocarbonyl Compounds

摘要

The conventional resonance model describes carbonyls, imines, and thiocarbonyls, as well as their protonated analogues, by a superposition of two valence bond structures. Ab initio Breathing-Orbital Valence Bond computations on formaldehyde, formimine, and thioformaldehyde as well as their protonated forms are performed to directly quantify the weights of their valence bond structures. Following a gas phase study that showed that protonation significantly increases the weight of the carbenic form relative to the polar-covalent bonded form (Braida, et al., Org Lett, 2008, 10, 1951), the present work estimates the influence of a polar protic solvent, modelized by water. Solvation effects are modeled explicitly by performing VB calculations on supersystems made of the organic substrate surrounded by four water molecules. It is shown that protonation significantly increases the polarity of the COX bond in all three cases (X O, NH, S) in solvated phase, in line with the known acceleration of nucleophilic additions on these compounds by acidic catalysis. Moreover, solvation significantly enhances the polarity of the COX bond in the protonated forms of formaldehyde and thioformaldehyde, but has practically no effect on the COX bond of protonated formimine.