A Theoretical Investigation on the Hydrogen Bond Based on the GLED Method of Bonding Analysis

We propose here a description and classification of the hydrogen bond (HB) that is based on the Graphic representation of the Local electron Energy Density H(r) (GLED). A peculiar aspect of the GLED method, proposed by us in a recent study [Journal of Chemical Physics 163 (2025): 034107], is that the major character of the bond (covalent or noncovalent) can be inferred simply by the visual inspection of the plotted H(r), particularly the 3D H(r) = 0 isosurface. The analysis of the hydrogen-bonded complexes unraveled, in particular, that their bonding character is strictly related to their dissociation energy (DE), so that the GLED assignment can be used to estimate the strength of the interaction. We also found that increasing values of DE mirror, in particular, an increased degree of covalency of the interaction. We could thus propose a classification of the HB that is based on the combined use of bonding character and stability. The HB was, in particular, assigned as weak (0.5-4.5 kcal mol^-1), medium (3.5-5.5 kcal mol^-1), and strong (4.5-15.0 kcal mol^-1) for the neutral species, and medium (8.5-13.0 kcal mol^-1), strong (15.0-32.0 kcal mol^-1), and very strong (30.0-70.0 kcal mol^-1) for the ionic ones, respectively. For systems stabilized by more than one HB, the method allows to eye-catch in case different role of the various interactions.