FACTORS CONTRIBUTING TO DISTORTION ENERGIES IN BENT HYDROGEN-BONDS .2. IMINE, CARBONYL, CARBOXYL, AND CARBOXYLATE GROUPS

The source of the energy requirement for bending a hydrogen bond is sought through decomposition of the total ab initio interaction energy into a number of physically meaningful components. Systems studied include (H2CNH-H-NH3)+, (H2CO-H-OH2)+, (HCOOH-H-OH2)+, and (HCOO-H-OH)-. As in the earlier cases studied which had been restricted only to small hydride molecules, the Coulombic interaction very closely parallels the change in the total interaction energy as angular distortions are imposed. Despite the larger size of the current molecules and the arbitrariness in selecting an origin for expansion, a multipole series approximation truncated after the R-5 term mimics fairly well the full electrostatic component. The exception to this satisfactory reproduction is the inter-nitrogen H bond of (H2CNH-H-NH3)+. The leading term of the multipole series, encompassing the interaction between the charge of the ionic subunit and the dipole of the neutral, appears capable of providing qualitative guidance, even in these larger systems, as to the shift in proton equilibrium position which accompanies each angular distortion of the H bond. © 1990 American Chemical Society.