An ab initio study of (C---H...X)⁺ hydrogen bonds including biological systems

Abstract

Ionic hydrogen bonds are stronger than neutral hydrogen bonds. Ionic hydrogen bonds, with carbon as the donor, are investigated here at the ab initio level. The hydrogen bond energy for systems with sp³ carbon as donor is 9-15 kcal mol⁻¹ and the CX distance is ~2.75 Å, whereas the hydrogen bonds involving sp² and sp carbons are much stronger and shorter. Biological systems, such as protonated histidine and acetylcholine have been studied. The C-H· · · O hydrogen bond interaction in zwitterionic glycine was found to be substantial. In addition electrostatic potential studies on the donor systems were done to allow qualitative comparison of the hydrogen bond energies.