A molecular orbital study of protonation. 5. Equilibrium structures and energies of ions R2COH+

Ab initio SCF calculations have been performed to determine the relative proton affinities of the carbonyl bases R₂CO and the structures of the ions R₂COH⁺, with R one of the isoelectronic saturated groups CH₃, NH₂, OH, and F. The predicted order of proton affinity with respect to R is NH₂ > CH₃ > OH > H > F, which is the same order predicted for the monosubstituted carbonyl bases RCHO. Replacement of the hydrogen atom in RCHO by a second R group causes a further change in the proton affinity of the base in the same direction as observed upon substitution of the first R group, although the effect of two substituents is less than additive except in F₂CO. Protonation of carbonyl bases leads to an increase in the C-O bond distance and a decrease in the bond distance between the carbonyl carbon and the substituent, the magnitude of which depends on the substituent. Protonation also causes changes in the bond angles about the carbonyl carbon which are essentially independent of the nature of the substituent, but strongly dependent on the position of the proton relative to the two substituents. Changes in bond lengths and bond angles and in the electron distribution upon protonation of the bases R₂CO are similar to the changes which occur upon protonation of the bases RCHO. From the computed results, a model for the protonation of carbonyl bases is proposed.