Isotope effects observed in diluted D 2 O/H 2 O mixtures identify HOD-induced low-density structures in D 2 O but not H 2 O

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 D 2 O/H 2 O since the hydrogen bond in H 2 O is slightly weaker than its counterpart (deuterium bond) in D 2 O. 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 D 2 O, 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 D 2 O and H 2 O were consistently confirmed with Raman spectroscopy and NMR diffusometry.