Effect of intermolecular hydrogen bonding on the molecular structures of imidazole and 1,2,4-triazole: A study by ab initio molecular orbital calculations

The effect of intermolecular hydrogen bonding in the solid state on the molecular structures of imidazole and 1,2,4-triazole has been studied by SCF ab initio molecular orbital calculations at the HF/6-31G^* level. The crystals of these species contain endless chains of molecules, connected by unusually strong N-H N hydrogen bonds. Our simulation of the crystal field, based on two simple models, unequivocally shows that hydrogen bond formation not only lengthens the N-H bond but also causes a concerted change in the length of two N-C bonds. The change indicates that the contribution of a polar canonical form to the structure of the molecule increases in going from the gaseous phase to the crystal. This provides a rationale for the strong intermolecular hydrogen bond occurring in the solid state. We have also optimized the geometry of the free molecules at the MP2/6-31G^* level, to investigate the effect that correcting for electron correlation has on the equilibrium structure of these systems.