Isotope effects in liquid water probed by x-ray Raman spectroscopy

Abstract

The isotope effect on the local structure of liquid water at room temperature is studied by x-ray Raman spectroscopy. The difference between the room-temperature spectra of liquid ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{D}}_{2}\mathrm{O}$ is compared to the difference spectrum between liquid ${\mathrm{H}}_{2}\mathrm{O}$ at 22 and $2\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The spectral changes between ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{D}}_{2}\mathrm{O}$ can partly be attributed to structural changes similar to a temperature change, in agreement with diffraction data. Additionally, we find that isotope substitution affects the local asymmetry in the hydrogen-bonded network: hydrogen-bonding configurations are more asymmetric on the donor side for ${\mathrm{H}}_{2}\mathrm{O}$ than for ${\mathrm{D}}_{2}\mathrm{O}$. A cluster model is used to computationally illustrate the spectral changes that arise due to the increased asymmetry, capturing all essential features in the difference spectrum. We infer from our study that quantum effects contribute to the formation of asymmetrical species in the liquid.