A Theoretical Investigation on the Hydrogen Bond

What is it about?

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 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.

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Photo by Neeqolah Creative Works on Unsplash

Photo by Neeqolah Creative Works on Unsplash

Why is it important?

Hydrogen bonds (HBs) are often introduced as simple noncovalent interactions, yet their true nature is remarkably rich and multifaceted. Rather than belonging to a single category of bonding, hydrogen bonds exist along a continuum of interaction types. At one end of this spectrum they behave as weak contacts dominated by dispersion and electrostatic interactions, while at the other they can acquire features typical of covalent bonding, involving significant polarization and even measurable charge transfer between donor and acceptor fragments. Despite its formal simplicity, the Hydrogen Bond is quite complex in nature, including binding components of different character and strength. Depending on the involved donor and acceptor, the interaction may range from a dispersive and electrostatic contact to a bond involving polarization and charge transfer. This change in bonding character typically mirrors an increase of the strength of the Hydrogen Bond, which changes from a weak noncovalent contact to a strong covalent bond. A distinct advantage of our approach is that it allows a direct visualization of the nature of the bond (noncovalent or with contribution of covalency), and, hence, of its strength, being informative in this regard the 3D representation of the H(r) = 0 isosurface. This is especially useful for systems stabilized by two or more HBs, whose in case different role can be eye-caught by looking at the graph of the H(0,ISO). Within this framework, our recently proposed GLED (Gradient-based Local Energy Decomposition) approach offers an intuitive and computationally efficient way to analyze hydrogen bonds across this spectrum. When applied to a representative set of hydrogen-bonded structures, the method confirmed the gradual variation in bonding character and enabled the proposal of a classification scheme based on the correlation between the interaction strength and the dominant bonding components.

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