Variation with the intermolecular distance of properties dependent on the electron density in cyclic dimers with two hydrogen bonds

Abstract

The variation with the intermolecular distance of geometries, energies, and other properties dependent on the electron density ρ(r) are studied in three cyclic dimers linked by two hydrogen bonds: formic acid and formamide homodimers and the heterodimer formamide/formic acid complex. Topological features, energy densities and integrated atomic properties provided by AIM theory are calculated with ρ(r) obtained at B3LYP/6-311++G(d,p) optimized geometries for a number of intermonomer distances covering large separations, equilibrium, and short distances. The variation with these distances of properties studied allows to characterize the nature of the interaction in A–H⋯B (A=N, O and B=O) hydrogen bonds. Whereas at large distances the attraction is purely electrostatic, quantum effects associated with redistributions of ρ(r) mainly around H and B atoms dominate the interaction in the neighborhood of equilibrium. Mutual penetration of the electron densities of these atoms leads to considerable reductions of their atomic volumes and associated polarization effects as well as energetic stabilization of atom A. Although the interaction in this range of intermonomer separations displays noncovalent features, when the dimers move at distances shorter than equilibrium, characteristics typical of covalent interactions begin to appear while the systems leave the planar structures presented until then. This work complements our previous study [O. Galvez, P. C. Gomez, and L. F. Pacios, J. Chem. Phys. 115, 11166 (2001)] of dimers with one single hydrogen bond.

Computational study of the process of hydrogen bond breaking: The case of the formamide–formic acid complex

“…In fact, while NÀ ÀHÁÁÁO has H c > 0 at its whole domain (as expected for closedshell interactions such as H-bonds), OÀ ÀHÁÁÁO shows H c < 0 at the two innermost R, a feature indicative of a partial degree of covalent bonding. 16,25,26,29,30,36,37,44,46 Taken together, the topological descriptors suggest thus a greater strength for OÀ ÀHÁÁÁO bond irrespective of the obvious fact that this H-bond must be stronger if it remains after the breaking of NÀ ÀHÁÁÁO. Although no particular features are noticed for the latter bond before its BCP vanishes at~4.7 Å , the behavior of OÀ ÀHÁÁÁO gives interesting information about the electron redistribution associated to the reorganization of hydrogen bonding.…”

“…Uncorrected MP2 and B3LYP binding energies are À14.5 and À14.8 kcal/mol, respectively, whereas the counterpoise correction sets these values at À14.3 kcal/mol. 30 Both methods predict an inflection point at nearly the same R (4.41 Å , MP2; 4.45 Å , B3LYP) and similar energies and derivatives from 5.0 Å outwards. As the evidence based on (r) suggests (see later), the NÀ ÀHÁÁÁO bond breaks at *4.7 Å .…”

“…AIM theory is firmly established and its applications to H-bond systems have been reviewed elsewhere. 4,5,16,[25][26][27][28][29][30] However, AIM domains do not supply explicit information in terms of bonds or electron pairs. This is achieved with the electron localization function (ELF), (r), originally devised to provide an orbital independent description of electron localization.…”

“…30, the formamide-formic acid complex (FFAC), was found to break one of its two H-bonds when the monomers moved outside equilibrium so that the complex became linked only by the stronger OÀ ÀHÁÁÁO bond while the weaker NÀ ÀHÁÁÁO bond apparently vanished. 30 However, since we then pursued an overall view of the spatial evolution of the interacting monomers, no particular attention was paid to that process. Here, we focus on properties associated with the H-bond breaking in an attempt to gain further insight into the nature of the interaction.…”

“…We started a research program to investigate the variation with the intermolecular distance of properties of the monomers as they move closer until a stable H-bond dimer is formed. 29,30,[35][36][37] Very recently, we have also explored the transition between covalent and hydrogen bonding in proton transfer associated to strong H-bond dimers. 37 The subtle electron redistribution in local environments of chemical groups involved in H-bonds is revealed in these studies (and in a report by other group 38 ) by the spatial changes of properties obtained from quantum correlated calculations.…”