Influence of hydrogen bond geometry on quadrupole coupling parameters: A theoretical study of imidazole–water and imidazole–semiquinone complexes

Abstract

Density functional theory (DFT) was used to calculate electric field gradients that give rise to quadrupole couplings for nuclei with a nuclear spin I > 1/2. A systematic theoretical investigation of the influence of hydrogen bond geometry on ¹⁴N quadrupole coupling tensors of (methyl-)imidazole is presented for the hydrogen-bonded systems imidazole–water and methylimidazole–benzosemiquinone. It reveals a strong dependence of the corresponding interaction parameters on the intermolecular arrangement, especially for the asymmetry parameter η. For both systems the largest influence on the electric field gradients was found for a variation of the hydrogen bond length r(O–N), the effects of an in- or out-of-plane distortion of the bond geometry were found to be less pronounced. Furthermore, a comparison of the ¹⁴N quadrupole parameters of the hydrogen-bonded models with those of free (methyl-)imidazole allowed a characterization of specific hydrogen bonding interactions to the amino group of imidazole or histidine. Finally, the implications of the presented studies for the interpretation of experimental quadrupole coupling data of biologically relevant semiquinone–histidine systems, as e.g. found in bacterial reaction centres or photosystem II, are discussed.