Unraveling the mysteries of an intramolecular hydrogen bonding defined by the phosphoryl group

Hydrogen bonds (HBs) drive many reactions, forming transition metal complexes, drug…protein interactions, and assembling biological structures. The functions of HBs follow their nature and strength. Recent studies revealed a mixed nature of HB, where not only electrostatics govern their stability and topology, but additional effects (i.e., charge transfer, dispersion, etc.) determine the utility of particular systems as molecular or supramolecular synthon. Our studies focus on phosphorylated hydroquinolinoles prone to forming intramolecular hydrogen bonds between the enol and phosphoryl groups. The formation of a planar quasi-ring by the hydrogen bond suggests resonance-assisted hydrogen bond (RAHB). However, the geometrical parameters of the HB shed new light on the nature of the observed interaction. Instead of the expected C-O bond shortening, we observed its elongation. The shortening may be an effect of the electronic structure of the strongly polar phosphoryl group described by negative hyperconjugation formalism, weakening the resonance effect directed from the phosphoryl to the enol group. The question guiding this proposal is related to the nature of intramolecular HBs as defined by the O-H…O=P synthon. A critical issue that helps answer this question is the experimental determination of this system's exact geometry, including the hydrogen atom's precise position.