PROTON INTERCHANGE TUNNELING AND FREE INTERNAL ROTATION IN $H_{3}N-HSH$
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Date
1994
Journal Title
Journal ISSN
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Publisher
Ohio State University
Abstract
An electric-resonance optothermal spectrometer and phase-locked backward-wave oscillators are used to investigate the {b}-type, $\Delta K=1, \Delta m=0$ spectrum of $H_{3}N-HSH$ and $H_{3}N-H^{34}SH$ near 300 GHz. The spectrum is characterized by nearly free internal rotation of the $NH_{3}$ against the $H_{2}S$, as initially concluded from Stark-effect measurements by Herbine {et al.}$^{1}$, Transitions are observed for the $K= 1 \leftarrow 0, m=0$, A symmetry and the $K=0 \leftarrow 1$ and $2 \leftarrow 1$, $m=\pm 1;Km>0$, E-symmetry subbands. The transitions are split into doublets with a 3:1 relative intensity ratio indicative of interchange tunneling of the two $H_{2}S$ protons. From the observed selection rules, symmetric $\longleftrightarrow$ antisymmetric in the tunneling state, it follows that the tunneling pathway must reverse the sign of the $\mu_{b}$ component of the molecular electric dipole moment. The most likely interchange motion consists of a partial internal rotation of the $H_{2}S$ unit about its c inertial axis, through a bifurcated, doubly hydrogen-bonded transition state. The proton interchange tunneling splittings range from 859 - 864 MHz, indicating that the interchange motion is only weakly coupled to the internal rotation. The barrier to proton interchange is calculated to be $510(3) cm^{-1}$ which can be compared to the $\sim 700 cm^{-1}$ barrier determined from the 57 MHz tunneling splittings associated with $H_{2O}$ proton interchange in the related $H_{3}N-HOH$ $complex^{2}$.
Description
$^{1}$ P. Herbine, T. A. Hu, G. Johnson, And T. R. Dyke, J. Chem. Phys. 93, 5485 (1990). $^{2}$ P. A. Stockman, R. E. Bumgarner, S. Suzuki, And G. A. Blake. J. Chem. Phys. 96, 2496 (1992).
Author Institution: Institut f\""{u}r Angewandte Physik; Applied Physics Institute, Molecular Spectroscopy Laboratory; Molecular Physics Division, National Institute of Standards and Technology
Author Institution: Institut f\""{u}r Angewandte Physik; Applied Physics Institute, Molecular Spectroscopy Laboratory; Molecular Physics Division, National Institute of Standards and Technology