Partial atomic charge of oxygen and hydrogen-bonding ability: insights from mass-selective IR spectroscopy of jet-cooled hydrogen-bonded complexes

The hydrogen bond is a fundamental non-covalent interaction that underpins the structure and function of chemical and biological systems. While the nature of conventional hydrogen bonding is predominantly electrostatic, accurately describing the atomic charge distribution within molecules remains a significant challenge, as atomic charges are not physically observable quantities. In this work, we present a systematic experimental investigation of hydrogen bonding in the gas phase using mass-selective IR spectroscopy of jet-cooled <i>p</i>-cresol-acceptor dimers. The redshift of the <i>p</i>-cresol OH stretching frequency (Δ<i>ν</i>_OH) serves as a direct measure of hydrogen-bond strength. We analysed two series of acceptors, acyclic alcohols and ethers, which demonstrate increasing inductive effects, and cyclic ethers, which reveal the influence of resonance. The gas-phase spectroscopy results provide a dataset that serves as a benchmark for validating computational chemistry models used for atomic charge calculation. It was demonstrated that while standard hydrogen-bonding descriptors correlate well with the experimental data, many popular partial atomic charge models fail to reproduce the observed chemical trends. This failure could be due to the over-reliance of the models on the electronegativity of directly bonded atoms. By highlighting this discrepancy, this work serves as a valuable cautionary example regarding the use of atomic charges and underscores the need for better theoretical frameworks for atomic charge estimation.