Variation with the intermolecular distance of properties dependent on the electron density in hydrogen bond dimers

The variation with the intermolecular distance of features in hydrogen bond (HB) dimers dependent on the electron density rho (r) are studied in four complexes representative of weak/medium HB interactions. Topological properties, energy densities and integrated atomic properties are obtained with rho (r) of dimers at B3LYP/6-311++G(d,p) optimized structures obtained upon fully relaxing the geometry of monomers. The dependence of A-H . . .B bond properties on intermolecular R(H . . .B) distances allows to characterize the nature of the interaction as monomers move nearer from infinite separation. At long distances the interaction is only electrostatic while for separations about 1 Angstrom larger than the equilibrium distance R-eq, quantum effects arising from rho (r) begin to dominate. In the immediate neighborhood of R-eq the interaction is mainly led by the stabilization of the H-donor due in turn to energy lowerings in A and B atoms associated to polarization effects. The mutual penetration of electron densities of donor and acceptor monomers provokes a considerable reduction of atomic volumes for H and B atoms which reveals in the form of redistribution rather than transfer of charge. This range of distances exhibits noncovalent bond features but shortly after, when monomers approximate a few tenths of Angstrom below R-eq, characteristics typical of covalent interactions begin to appear while the rate of change of all the rho (r)-dependent properties increases rapidly. (C) 2001 American Institute of Physics.