Stereochemistry and transformations of NH---N hydrogen bonds - Part I. Structural preferences for the hydrogen site

Proton sites in bistable NH- - -N hydrogen bonds between chemically equivalent functional groups can be determined from their dimensions and interactions in the crystalline state. A directional criterion for intermolecular hydrogen bonds, minimizing the H- - -N distance and minimizing the deviation of the N-H- - -N angle from 180 degrees, is consistent with the energetically favoured hydrogen site. Similar criteria proved valid for the -OH- - -O= and -OH- - -O < hydrogen bonds. The criterion rationalizes the hydrogen site in the N-H- - -N hydrogen bond in the molecular crystal of imidazole solely from structural dimensions. The role of electrostatic forces for stabilizing the hydrogen site is illustrated for two ionic crystals, one with the proton ordered and one with the proton disordered in the hydrogen bonds of pyridinium-pyridine cations. (C) 1998 Elsevier Science B.V. All rights reserved.