Factors Contributing to Distortion Energies of Bent H-Bonds. Implications for Proton Transfer Potentials

The ab initio interaction energy for the optimal arrangement of a number of H-bonded systems is decomposed qnd compared to a variety of geometries in which angular deformations are imposed. For cationic systems (H3NH-NH3)+ and (H2OH...OH2)+, the (HOH...OH)- anion, and the neutral dimer (HOH...OH2), the electrostatic term is the largest of the various components and provides a reasonable first approximation to the total interaction energy as a result of mutual cancellation between the remaining terms. Even though the multipole expansion of the electrostatic interaction is not rapidly convergent, its cumulative sum through R-5 offers a good approximation to the full electrostatic expression. Most of the distortion energy resulting from angular deformation of the H bond in (HOH...OH)- is concentrated in the dipole-ion term. The same is true of the cationic systems except that the ion-quadrupole term is of comparable magnitude and should be considered as well. In all these cases, a simple picture based on the preceding interactions is usually capable of predicting the sense of the asymmetry introduced into the proton-transfer potential by a given angular distortion