Highresolution spectroscopic probing of ortho and para nuclear-spin isomers of heavy water in the gas phase

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Abstract Doubly deuterated water (D2O) exhibits two distinct nuclear spin-isomers, ortho and para, which have not much been studied and there is hardly any information about the spin-exchange processes of D2O. Here, we investigated the nuclear-spin quantum states of D2O in gas phase by evaluating the ortho-to-para ratio (OPR) using high-resolution cavity ring-down spectroscopy coupled with quantum cascade laser in the mid-IR spectral region at 7.8 μm. We experimentally achieved an OPR of (1.95 ± 0.16):1 for D2O spin-isomers in the isolated gaseous phase. We further analyzed the OPR of the residual D2O in a 1:1 mixture of D2O and H2O, and observed indications of potential alteration of thermodynamic probability of ortho and para nuclear-spin states of D2O associated with the D-H exchange reaction in the system. Our findings pave the way to probe directly the spin behaviour of D2O molecules in the gas phase and thus fostering exploration of spin-dependent chemistry in a wide range of chemical and biological systems.