Comparison between proton transfers involving carbonyl and hydroxyl oxygens

Proton transfers between the carbonyl and hydroxyl groups of (H2CO-H-OH2)+ are studied by ab initio methods using a 4-31G* basis set. Also examined for purposes of comparison is (H2O-H-OH2)+ in which the carbonyl is replaced by a hydroxyl. Energetics of proton transfer are calculated for a range of fixed values of interoxygen distance R and with all other parameters fully optimized. The (H2COH--OH2)+ configuration lies 4-6 kcal/mol lower in energy than (H2CO--HOH2)+ for all values of R, although the energy barriers for proton transfer rise as the H bond is lengthened. The barrier for interhydroxyl transfer in (H2O-H-OH2)+ is consistently 1 kcal/mol higher than the hydroxyl to carbonyl barrier. Transfers are studied also with the angular orientation of one group held fixed with respect to the other. It is found that moving the hydroxyl group to lie along the C=O axis rather than along a carbonyl lone pair direction reverses the relative stabilities of (H2COH- -OH2)+ and (H2CO--HOH2)+. Similar reversals occur for (H2OH--OH2)+ and (H2O--HOH2)+ but are of smaller magnitude. In contrast to the above rotations within the plane of the oxygen lone pairs, out-of-plane distortions produce very little change in the relative stabilities of the two configurations. Although such distortions substantially raise the transfer barriers in (H2O-H-OH2)+, the barriers in the carbonyl case remain essentially unaffected. The underlying reasons for the above trends are analyzed in terms of deformation energies of individual configurations which are, in turn, rooted in H-bond strengths, intrinsic flexibilities, charge-dipole interactions, and angular dependence of electron densities.