The Low-Barrier Hydrogen Bond of Deuterated Benzoylacetone Probed by Very Low Temperature Neutron and X-ray Diffraction Studies and Theoretical Calculations

Abstract

Low-temperature neutron diffraction data is used to show that the enol deuteron in deuterated benzoylacetone has a bimodal probability density distribution. In a previous study of normal benzoylacetone we showed that the enol hydrogen atom has a broad unimodal probability density distribution. Deuteration gives hardly any change to the molecular framework other than a small lengthening of the O...O distance. We solve the vibrational Hamiltonian for a series of model hydrogen potentials and show how the proton/deuteron probability density function can become strongly perturbed by small changes to the potential. The potentials are used to show that the deuteron in benzoylacetone can be interpreted as an atom tunnelling between two possible localised positions, while the proton in benzoylacetone can be viewed as having sufficient energy to shuttle over the low-energy barrier, even at 15 K. We underline the necessity of accounting for the dynamic nature of the hydrogen bond and not relying on a description of the proton/deuteron by a single atomic coordinate, and compare our results to structure correlation studies.