An AB initio molecular orbital study of proton affinities and ion structures

Abstract

Ab initio SCF calculations have been performed in this study of the relative proton affinities of the carbonyl bases R2CO and the hydroxyl bases ROH, and of the structures of the ions R2CHO+ and RHO2+, where R is one of the isoelectronic saturated groups Ch3, NH2, OH, and F. For the bases R2CO, the calculated 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. A comparison of the proton affinities of corresponding mono and disubstituted carbonyl bases indicates that 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 bases R2CO are similar to the changes which occur upon protonation of the bases RCHO.

For the bases ROH, the predicted order of proton affinity with respect to R is CH3>H>NH2>OH>F, which is dramatically different from the order of proton affinity of mono- and disubstituted carbonyl compounds. Oxygen protonation leads to an increase in the length of the bond between the oxygen and a first row atom. This increase is larger when the protonated oxygen is a carbonyl rather than a hydroxyl oxygen. As in the protonated carbonyls, the O-H distance in the protonated species ROH2+ is relatively constant, independent of the substituent. These protonated species also show only a relatively small variation in the H-O-H angle and in the angle between the O-X bond and the bisector in the H-O-H angle. The study of the electron redistribution upon protonation of bases ROH suggests that the mechanism by which stabilization occurs is quite different from that in the ions R2CHO+ and RCHOH+.

Two computational methods to simplify the computation of proton affinities have been evaluated. Studies of the rigid monomer restriction applied to ions R2COH+ indicate that this is a severe approximation since it neglects the significant structural changes which occur in the relaxed ions, thereby altering the predicted order of proton affinity. The rigid monomer restriction is less severe structurally and energetically in ROH2+ than in R2COH+ and RCHOH+. Reasonable estimates of relative proton affinities without full geometry optimization at the 4-31G level can be obtained from calculations with the 4-31G basis set using optimized STO-3G geometries for bases and ions.