Testing the Validity of the Conventional Resonance Model for Protonated Carbonyl, Imine and Thiocarbonyl Compounds. An Ab Initio Valence Bond Study

The conventional resonance model describes protonated carbonyls, imines, and thiocarbonyls by a superposition of two structures, one π polar-covalent and the other of carbenium type. The validity of this model is clearly supported by high level valence bond calculations, giving a 32% weight for the carbenium form in protonated carbonyl, 19% in protonated formamine and thioformaldehyde. The carbenium form is further stabilized by π-donating substituents. Solvation effects do not fundamentally change the gas-phase picture.