Why are Hydrogen Bonds Directional?

Abstract

The recent IUPAC recommendation on the definition of hydrogen bonding points out that directionality is a defining characteristic of a hydrogen bond and the angle ∠X-H-Y is generally linear or 180 •. It also suggests that the X-H• • • Y angle be greater than 110 • for an interaction to be characterized as a hydrogen bond but does not provide any rationale for the same. This article reports a rationale for limiting the angle, based on the electron density topology using the quantum theory of atoms in molecules. Electron density topology for common hydrogen bond donors HF, HCl, HBr, HNC, HCN and HCCH are reported in this work. These calculations lead to an interesting observation that the atomic basins of H atom in all these donor molecules are limited justifying the restriction of hydrogen bond angle. Moreover, similar analysis on some hydrogen bonded complexes confirms that beyond this angle the acceptor atom Y starts interacting with the atomic basin on X. However, conclusions based on bond lengths and angles have to be treated with care and as the IUPAC recommendation points out that independent 'evidence for bond formation' in every case is important.

Charting Hydrogen Bond Anisotropy

“…Results confirmed that all donors present rather similar directional propensity, with minor variations across the set (Supplementary Fig. S5) and in agreement with previous reports 35,50,51 . Acceptors, on the other hand, were found to be anywhere from nearly isotropic to very directional (Fig.…”

“…Overall, these results challenge the established consensus according to which strength and directionality are correlated, with the latter depending on the former. Therefore, strong acceptors are expected to be more directional 35,45,[52][53][54] , and weak ones less so 35,45,54,55 . When quantifying directionality of acceptors and donors separately, we found no such correlation with acceptor strength For example, in the same molecular context, sulfur shows more directional character than oxygen, despite the former being a weaker acceptor (Fig.…”

“…Furthermore, structural patterns of HBs can be smeared by factors such as steric hindrance 8 , packing forces 26 , or be biased by the refinement programs used (which often include specific constraints for HB) 27 . Quantum mechanics (QM) calculations, while requiring more resources than statistical surveys, have been used to characterize various HB properties 26,[28][29][30][31][32][33][34][35] , usually with good agreement with observations 13 . Among them, very few calculated interaction energies at non-equilibrium geometries (the ones that did are summarized in Supplementary Table S1).…”

Electrostatics for probing lone pairs and their interactions

“…The aim here is to find an interpretation of the directionality and strength of the hydrogen bond by correlating the orientation of the lone pair and the corresponding MESP minimum value. According to the recent IUPAC recommendation, directional preference is a defining characteristic of the hydrogen bond …”

“…According to the recent IUPAC recommendation, directional preference is a defining characteristic of the hydrogen bond. [28,[34][35][36] The lone-pair-bearing molecules such as amines and carbenes show electron donating tendency toward the CO 2 molecule and significant research has been conducted on using polyamines as potential CO 2 capturing agents. [37][38][39][40][41][42] Keeping this in view, we also study the intermolecular complexes of lone-pair-bearing molecules with CO 2 and investigate the structural and energetic features of these complexes in relation with the features of the lone pairs.…”