Single-crystal NMR spectra of solid<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>containing very low ortho-<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>concentrations

A motional-narrowing model adapted from the work of Anderson is used to interpret anisotropic NMR spectra from samples of solid para-${\mathrm{H}}_{2}$ containing ortho-${\mathrm{H}}_{2}$ impurity concentrations of less than 3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$. The experimental data were taken at a Larmor frequency of 30 MHz over a temperature range of 4.2-0.04 K. An analysis of the data at the lowest temperature indicates that the samples are single crystals. Values for the transition probability per unit time between the isolated molecular rotational substates are deduced as a function of temperature for this model. Based on the change of this transition probability with temperature, it is concluded that the samples undergo an order-disorder transition below 0.18 K.

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