Variation of geometries and electron properties along proton transfer in strong hydrogen-bond complexes

Proton transfer in hydrogen-bond systems formed by 4-methylimidazole in both neutral and protonated cationic forms and by acetate anion are studied by means of MP26-311++G(d,p) ab initio calculations. These two complexes model the histidine (neutral and protonated)-aspartate diad present in the active sites of enzymes the catalytic mechanism of which involves the formation of strong hydrogen bonds. We investigate the evolution of geometries, natural bond orbital populations of bonds and electron lone pairs, topological descriptors of the electron density, and spatial distributions of the electron localization function along the process N-H...O-->N...H...O-->N...H-O, which represents the stages of the H-transfer. Except for a sudden change in the population of electron lone pairs in N and O at the middle N...H...O stage, all the properties analyzed show a smooth continuous behavior along the covalent --> hydrogen bond transit inherent to the transfer, without any discontinuity that could identify a formation or breaking of the hydrogen bond. This way, the distinction between covalent or hydrogen-bonding features is associated to subtle electron rearrangement at the intermolecular space.