Infrared spectra of partially deuterated water dimers in the fundamental O–D stretch region.

This article presents the first significant new gas-phase infrared spectroscopic study in over 25 years of nine of the ten possible mixed hydrogen/deuterium (H/D) water dimer isotopologues in the mid-infrared O–D stretch fundamental region (2630–2800 cm⁻¹). Using a pulsed supersonic slit jet and tunable optical parametric oscillator source, over 30 vibrational bands were observed and analyzed, providing detailed information on excited state tunneling splittings, vibrational shifts, and lifetime (predissociation) effects. The study highlights how isotopic substitution affects tunneling motions—acceptor switching, interchange, and bifurcation—and reports precise molecular parameters for each isotopologue, contributing valuable experimental benchmarks for ongoing theoretical calculations of water dimer dynamics. Except for the H₂O–HOD isotopologue and some donor free O–D stretch modes, nearly all species and vibrational modes were characterized, offering insights relevant to understanding microscopic water interactions in both liquid and solid phases.