Quantum tunneling and motional narrowing of HD NMR line shapes in solid hcp<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

The observed narrowing of the NMR line shapes of HD impurities in solid parahydrogen is interpreted in terms of a motional narrowing effect due to quantum tunneling. This model is successful in describing the dependence of the HD linewidths on the orthohydrogen concentration (0.1-4 at.%) and predicts a tunneling frequency $J\ensuremath{\sim}1$ kHz. This value can be understood in terms of current views of tunneling in quantum solids which predict that three-body cyclic permutations dominate the tunneling processes in hcp $^{3}\mathrm{He}$ and ${\mathrm{H}}_{2}$. Estimates of the tunneling frequencies to be expected for ${\mathrm{H}}_{2}$ based on the experimental values for $^{3}\mathrm{He}$ are in close agreement with those needed to explain the NMR results.

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