Transition from moderate to strong hydrogen bonds:: Its identification and physical bases in the case of O-H•••O intramolecular hydrogen bonds

A systematic investigation aimed at identifying the transition from moderate (M) to strong (S) hydrogen bonds (HBs) and the physical bases of the main geometry-based HB strength classifications reported in the literature has been undertaken using the quantum theory of atoms in molecules (QTAIM). Correlations between the Laplacian of the electron density (ρ) at the O⋯H hydrogen bond critical points (HBCPs), ∇2ρ hb, specifically between the more intuitive parameter Lhb = -∇2ρhb and other QTAIM parameters, have also been explored. The transition from MHBs to SHBs has been identified as the minimum (maximum) in the geometric dependence of Lhb (∇2ρhb)- For O-H⋯O intramolecular (IM) HBs (including resonance-assisted HBs), the transition is obtained, in a truly remarkable agreement with the existing geometry-based HB strength classifications, when the O⋯O (O⋯H) distance is ∼2.51 (∼1.55) Åand when the ratio of the potential energy density (|V hb|) to the kinetic energy density (Ghb) ≈ 1.3. Accordingly, the ranges of the |Vhb|/Ghb ratios are >2-1.3 and 1.3-1 for, respectively, SHBs and MHBs. When the O⋯O distance is not a genuine indicator of HB strength, the |Vhb|/G hb ratio and other parameters should be considered to characterize the strength of the HBs. Rationalizations have been provided by way of decoding the physical bases of the transition in terms of the properties of ρ and the mechanical characteristics of the interactions that created the HBCPs. L hb was found to correlate, with a very high degree of fidelity, with at least three parameters (in addition to O⋯O and O⋯H distances and the IMHB energy), Vhb/Ghb, Hhb/ ρhb (the ratio of the total energy density, Hhb, to the electron density, ρhb (the so-called bond degree parameter)), and δhb(O,H) (the delocalization index), demonstrating the importance and utility of Lhb (∇2ρhb) for the study of HB interactions. A new refined energetics-based classification of O-H⋯O IMHB strengths has been advanced. The approach taken in this investigation can be extended to other HB systems. © 2008 American Chemical Society.