Hydrogen-bond directionality at the donor H atom-analysis of interaction energies and database statistics

A thorough analysis has been performed on the effects of varying the angle at the donor hydrogen in hydrogen bonds using database studies and ab initio intermolecular interaction energy calculations. Non-activated C-H center dot center dot center dot acceptor interactions are seen to have a weak energy dependence on the D-H center dot center dot center dot A angle, but for all of the conventional hydrogen-bonds studied the D-H center dot center dot center dot A angle has a range of 40-65 degrees within an energy penalty of RT (2.5 kJ mol(-1)). Interactions with D-H center dot center dot center dot A angle in the range 120-140 degrees are seen to have substantially reduced stabilisation energies and angles below 120 degrees are generally unlikely to correspond to significant interactions.