Molecular tunneling dynamics of NH3D+ in ammonium sulfate and ammonium thiocyanate

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Abstract
The kinetics of NH3D+ rotation in ammonium sulfate and ammonium thiocyanate are measured at low temperature (8–60 K). Infrared hole burning of the N–D stretching bands causes reorientation of the ions among distinct positions relative to the crystal lattice, producing nonequilibrium population distributions. The slow relaxation (time scale of minutes to hours) is monitored with a Fourier‐transform infrared spectrometer as the ions tunnel back to their equilibrium distributions. Kinetic models are used to extract rate constants for rotation between orientations. The rate constants range from 0.01 h−1 at 8 K, to about 10 h−1 above 50 K. The higher temperature data is described by barrier transmission calculations with barrier heights of 1000 to 1600 cm−1. The low temperature data agrees with rates calculated from a phonon‐assisted tunneling model for transitions between the ground states of the orientations, with energy differences between ground states on the order of 10 cm−1. Calculations of the tunneling rates between quantum states in a potential for the rotation of the NH3D+ ion in ammonium sulfate generated by a point‐charge model are also performed and compared to the experimental results.