Proton interchange tunneling and internal rotation in HSH–NH3

An electric-resonance optothermal spectrometer and phase-locked backward-wave oscillators are used to investigate the b type, ΔK=±1, Δm=0 spectrum of the hydrogen-bonded HSH--NH3 and H34SH--NH3 complexes near 300 GHz. The spectrum is characterized by nearly free internal rotation of the NH3 subunit against the H2S, as initially concluded from Stark-effect measurements by Herbine et al. [J. Chem. Phys. 93, 5485 (1990)]. Transitions are observed for the K=1←0, m=0, A symmetry and the K=0←±1 and K=±2←±1, m=±1, Km≳0, E-symmetry subbands. The transitions are split into doublets with a 3:1 relative intensity ratio indicative of tunneling interchange of the two H2S protons. The observed selection rules, symmetric ↔ antisymmetric in the tunneling state, indicate that the tunneling motion reverses the sign of the molecular electric dipole moment component along the b inertial axis. 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 of 859–864 MHz vary little between K and m states, indicating that the interchange motion is only weakly coupled to the internal rotation. The barrier to proton interchange is determined to be 510(3) cm−1, which can be compared to the ∼700 cm−1 barrier estimated from the 57 MHz tunneling splittings associated with the H2O proton interchange in the related HOH--NH3 complex. The observation of dissociation of HSH--NH3 following excitation of the NH3 umbrella mode with a line-tunable CO2 laser places an upper bound of 992 cm−1 on the hydrogen-bond zero-point dissociation energy. The band origin for the umbrella vibration of 992.5(10) cm−1 is blueshifted by 43 cm−1 from the hypothetical inversion-free band origin of uncomplexed NH3. Previous studies have shown that the HOH--NH3 binding energy is greater than 1021 cm−1.