Gas-Phase Isotopic Fractionation Study of Singly and Doubly Deuterated Isotopologues of Water in the H–D Exchange Reaction by Cavity Ring-Down Spectroscopy

Abstract

The underlying mechanisms of the triple-oxygen (16O, 17O, and 18O) isotopic content of deuterated (D) isotopologues of water in H–D exchange reactions in the gas phase remain elusive. Herein, we have demonstrated a high-resolution gas-phase spectral analysis of doubly (D2O) and singly (HDO) deuterated isotopologues of water in the region around 7.8 μm using quantum cascade laser-based cavity ring-down spectroscopy. Isotopic fractionations among doubly and singly deuterated species of water, D2 16O, HD16O, HD17O, and HD18O, in the gas phase were carried out by probing the fundamental and hot band transitions in the ν2 (bending) mode of D2O and the fundamental ν2 transitions for the other water isotopes. We subsequently investigated the fractionations of different D-enriched water isotopologues for the H–D exchange reaction using various mixtures of D2O in H2O. We explored the potential role of triple-oxygen isotopic contents through enrichments and depletions of HD16O, HD17O, and HD18O, involved in the H–D reaction. Our first clear, direct, and quantitative experimental evidence reveals a new picture of gas-phase isotopic fractionation chemistry in a mixture of light and heavy water (H2O–D2O).

Cavity Ring-Down Spectroscopy: Recent Technological Advancements, Techniques, and Applications

“…The acquired decay-rate spectrum is thereafter fitted with an appropriate line-shape function to obtain the probed sample concentration. Figure illustrates the experimental spectra of different mono and doubly deuterated isotopes of water molecule which are acquired by a CRDS system and subsequently fitted with Voigt line-shape function for the determination of concentrations of the individual isotopes . It is noteworthy to mention that several advanced line-shape functions such as Galatry, Rautian, speed-dependent Voigt, etc.…”

“…The absorption spectra were achieved by plotting the change of decay rates (Δ k ) against the scanning wavenumbers (cm –1 ) across the transitions, and the spectra were subsequently fitted with Voigt profile for absolute measurements of concentrations of the individual isotopes. Reproduced from Maithani, S.; Panda, B.; Maity, A.; Pradhan, M. Journal of Physical Chemistry A 2020 , 124 (6), 1104–1111 (ref ). Copyright 2020 American Chemical Society.…”

“…The group of M. Pradhan developed three independent cw-CRDS systems coupled with three external-cavity (EC) QCLs operating in the mid-IR spectral regions at around 5.2 μm, 6.2 μm, and 7.8 μm for spectroscopic investigations and trace sensing of several molecules like NO, , OCS, , NH 3 , CH 4 , N 2 O, C 2 H 2 , H 2 S, and D 2 O and their isotopologues. , The group subsequently exploited the cw-CRDS technique for the spectroscopic study of long-chain hydrocarbons like 1,3 butadiene where experimental results were simultaneously compared with the theoretical calculations from Gaussian 16 and PGOPHER simulation . In a recent study, Pal et al reported the natural abundances of 32 S, 33 S, and 34 S isotopes in a sulfur compound, Na 2 S, which was reacted with HCl to form the gaseous H 2 S to be eventually measured by the cw-CRDS system probing the transition lines of three sulfur isotopes in the spectral region of 7.8 μm.…”

“…In a recent study, Pal et al reported the natural abundances of 32 S, 33 S, and 34 S isotopes in a sulfur compound, Na 2 S, which was reacted with HCl to form the gaseous H 2 S to be eventually measured by the cw-CRDS system probing the transition lines of three sulfur isotopes in the spectral region of 7.8 μm. However, Maithani et al from the same group provided the first experimental measurements of the H-D exchange process by simultaneously measuring monodeuterated (HDO) and doubly deuterated (D 2 O) isotopes in the H 2 O–D 2 O reaction medium. They subsequently studied the isotopic fractionations of different oxygen isotopes (i.e., 16 O, 17 O, and 18 O) of HDO and D 2 O molecules during the H-D exchange reaction.…”

Probing the H–D isotopic exchange reaction in a liquid droplet via surface plasmon resonance

“…Studies in the past have explored isotopic fractionation of water in the liquid phase, 2,3 ice surfaces, 4-6 liquid microjets 7 and gas phase 8 to investigate the hydrogen-bonding interactions as well as temperature and composition dependence of the isotopic fractionation factor. 7,9,10 However, it would be interesting to explore the real-time kinetics of H-D isotopic exchange between D 2 O and H 2 O molecules at the liquid-vapour interface under ambient conditions where the underlying isotope reaction mechanisms remain intriguing.…”