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The kinetics of NH3D+ rotation in (NH4)2SO4 are followed from about 8 to 60 K. The rotation is between distinct and well-defined positions in the crystal and occurs by tunneling. At low temperatures the data are fit by a phonon assisted mechanism. At higher temperatures (40-60 K), the data are fit by simple quantum transition state theory.
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Burrows et al. (1990) studied this question.
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