Isotope effects observed in diluted DO/HO mixtures identify HOD-induced low-density structures in DO but not HO.

Normal and heavy water are solvents most commonly used to study the isotope effect. The isotope effect of a solvent significantly influences the behavior of a single molecule in a solution, especially when there are interactions between the solvent and the solute. The influence of the isotope effect becomes more significant in DO/HO since the hydrogen bond in HO is slightly weaker than its counterpart (deuterium bond) in DO. Herein, we characterize the isotope effect in a mixture of normal and heavy water on the solvation of a HOD molecule. We show that the HOD molecule affects the proximal solvent molecules, and these disturbances are much more significant in heavy water than in normal water. Moreover, in DO, we observe the formation of low-density structures indicative of an ordering of the solvent around the HOD molecule. The qualitative differences between HOD interaction with DO and HO were consistently confirmed with Raman spectroscopy and NMR diffusometry.