Unveiling two deuteration effects on hydrogen-bond breaking process of water isotopomers

Abstract

The quantum nature of hydrogen bonds in water manifests itself in peculiar physicochemical isotope effects: While deuteration often elongates and weakens the hydrogen bonds of typical hydrogen-bonded systems composed of bulky constituent molecules, it elongates but strengthens the hydrogen bonds of water molecular aggregates. The origin of this unique isotope effect of water molecules remains to be elucidated at the molecular level. By means of isotope-selective measurements on the sublimation of water ices with various H/D compositions, we disentangle two opposite deuteration effects on the hydrogen-bond breaking process of water molecules: (1) Deuterating a desorbing water molecule increases the energy needed for desorption ${E}_{d}$, while (2) deuterating water molecules neighboring a desorbing molecule reduces its ${E}_{d}$. The increase in ${E}_{d}$ originates from zero-point energy in the hindered rotation of the desorbing molecule, whereas the decrease in ${E}_{d}$ is caused by quantum anharmonic couplings between the inter- and intramolecular vibrational modes involved in the hydrogen-bonding interactions of desorbing water molecules. On the basis of these findings, we discuss the peculiar nature of hydrogen bonds of water molecules in comparison with bulky hydrogen-bonded molecules.