The probability distribution function as a descriptor of hydrogen bond strength

Numerical ranges for structural and spectroscopic properties used to classify hydrogen bonds (HB) are somewhat imprecise and even overlap. This is a problem of weight in the identification of the much discussed low barrier hydrogen bond. In this study, we examine the use of the probability distribution function (pdf) as a more distinctive descriptor of HB strength. We use a previously developed theoretical model for the characterization of symmetric HBs to investigate the potential energy surfaces of four intramolecular O-H-O systems of increasing HB strength. Our computed interatomic distances, 1H chemical shifts and isotope effects upon deuteration are consistent with previous experimental and computational measurements. Our results indicate that δH values vary linearly with the size of the energy barrier; we discuss the relevance of this finding. We propose an alternative classification scheme and pictorial description that is particularly useful to distinguish between 'strong', 'low barrier' and 'very strong' HBs.