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Author(s):Arunan, Elangannan
Contributor(s):Das, Arijit
Subject(s):Mini-symposium: Large Amplitude Motions
Abstract:Water dimer is probably the most extensively studied hydrogen bonded system. The ground vibrational state of (H$_{2}$O)$_{2}$ is eight-fold degenerate and it splits into six levels (A$_{1}$, E$_{1}$, B$_{1}$, A$_{2}$, E$_{2}$, B$_{2}$). It has a large \textit{a}-dipole, which inverses on donor-acceptor interchange tunnelling. The result is that the E states of (H$_{2}$O)$_{2}$ give spectra of the rigid rotor type whereas the A and B states give rotational tunnelling spectra\footnote{ G. T. Fraser, International Review in Physical Chemistry, 1991, 10, 189–206.}. The incorporation of argon (Ar) introduces a new dipole in the system (along the trimer a-axis). The (H$_{2}$O)$_{2}$ (dimer) ‘‘a’’ axis is the ‘‘b’’ axis for the Ar(H$_{2}$O)$_{2}$ (trimer). In the trimer, the ‘\textit{a}-dipole’ transitions appear rigid rotor like for all three tunnelling states, whereas the ‘\textit{b}-dipole’ transitions show tunnelling splitting spectra. Due to the reduced barrier height in Ar(D$_{2}$O)$_{2}$, the three states namely A$_{1}$, E$_{1}$, B$_{1}$ could be observed previously. The splitting is measured to be only 106 MHz\footnote{E. Arunan, T. Emilsson, H. S. Gutowsky, The Journal of chemical physics, 2002, 116, 4886-4895} in Ar(D$_{2}$O)$_{2}$ compared to 1 GHz in (D$_{2}$O)$_{2}$. The tunnelling splitting in Ar(H$_{2}$O)$_{2}$ could not be observed early as the splitting was expected to be several GHz. Moreover, only the A$_{1}$ and E$_{1}$ states are allowed for the Ar(H$_{2}$O)$_{2}$ as the other B$_{1}$ state has zero statistical weight. The A$_{1}$ state could appear either above or below the E$_{1}$ states depending on the K quantum number. With the help of a fourfold periodic potential\footnote{J. D. Lewis, T. B. Malloy Jr, T. H. Chao, J. Laane, Journal of Molecular Structure, 1972, 427-449}, we have accurately predicted the fingerprints of \textit{b}-dipole A$_{1}$$^{+}$$\,\leftrightarrow\,$A$_{1}$$^{-}$ transitions and observed them using a pulsed nozzle Balle-Flygare Fourier transform microwave spectrometer. Measurement of these transitions enabled us to determine the donor-acceptor tunnelling splitting of 4257.41(4) MHz in Ar(H$_{2}$O)$_{2}$, compared to about 20 GHz in (H$_{2}$O)$_{2}$. Also the more detailed structural parameters of the Ar(H$_{2}$O)$_{2}$ have been evaluated in this work and critically compared with the (H$_{2}$O)$_{2}$.
Issue Date:24-Jun-20
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:APS
Date Available in IDEALS:2020-06-26

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