Knowledge Bank

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 $H3N-HSH$ and $H3N-H34SH$ near 300 GHz. The spectrum is characterized by nearly free internal rotation of the $NH3$ against the $H2S$, as initially concluded from Stark-effect measurements by Herbine {et al.}$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$, 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 $H2S$ protons. From the observed selection rules, symmetric $⟷$ 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 $H2S$ 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 $\sim 700cm-1$ barrier determined from the 57 MHz tunneling splittings associated with $H2O$ proton interchange in the related $H3N-HOH$ $complex2$.