Abstract
MP2/6‐311++G(d,p) and B3LYP/6‐311++G(d,p) quantum calculations are used to study the formamide–formic acid complex (FFAC), a system bound by two hydrogen bonds, NH···O and OH···O, that forms a bond ring at equilibrium. When the intermolecular separation between monomers R increases, this ring opens at a distance for which the weaker NH···O bond breaks remaining the stronger OH···O bond. The computational study characterizes that process addressing changes of interaction energy Δ__E__, structure and properties of the electron density ρ(r) as well as spatial distributions of ρ(r), the electrostatic potential U(r), and the electron localization function η(r). It is shown that the spatial derivatives of Δ__E__, the topology of ρ(r), and qualitative changes noticed in U(r) = 0 isocontours allow to identify a precise distance R for which one can say the NH···O hydrogen bond has broken. Both levels of theory predict essentially the same changes of structure and electron properties associated to the process of breaking and virtually identical distances at which it takes place. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2006