Intrinsic Raman scattering activity of OH and OD oscillators in liquid H2O, D2O, and HOD: Roles of zero-point energy and vibrational coupling

Liquid water is a strongly associated and vibrationally delocalized system in which nuclear quantum effects (NQEs), particularly zero-point energy and vibrational anharmonicity, influence the structure and dynamics. Here, we examine the roles of NQE and vibrational coupling on the intrinsic Raman scattering activity of the OH (σH) and OD (σD) oscillators in liquid H2O, D2O, and HOD using polarized Raman spectroscopy. For neat H2O and D2O, σHisoσDiso = 1.35 ± 0.02, and on isotopic dilution, it decreases to 0.9 ± 0.08. This arises from a symmetric 20% drop in σHiso and 20% rise in σDiso, revealing quantitative intensity redistribution via OH-OD vibrational coupling in isotopically diluted water. In other words, even in HOD, the OH and OD stretch modes are not fully decoupled vibrationally. The corresponding ratio for the anisotropic component is σHanisoσDaniso = 1.3 ± 0.05, and it remains unchanged upon isotopic dilution. The experimental σHanisoσDaniso = 1.3 ± 0.05 closely matches the quantum harmonic oscillator predicted ratio of 1.37 corresponding to comparable polarizability derivatives in H2O and D2O. Thus, ∼30% higher scattering activity of OH primarily arises from reduced mass dependent zero-point vibrational amplitude rather than anharmonicity-driven changes in polarizability.