Geometry and tunneling dynamics of CHD2 groups in aspirin: A single-crystal deuteron nuclear magnetic resonance study

${\mathrm{CHD}}_2$ groups in aspirin are studied by single-crystal deuteron NMR at temperatures between 7 and 290 K. They perform stochastic reorientations which are governed by a rotational potential possessing three wells, two of which are almost equally deep whereas the third is significantly higher. The three minima of the rotational potential are separated by angles substantially different from 120$\mathrm{°.}$ Still, at least in the two lower wells, the geometry of the ${\mathrm{CHD}}_2$ groups is close to that of a symmetric rotor. By selective magnetization transfer experiments, the incoherent tunneling rate between the two lower potential wells is determined. At temperatures below 15 K, Raman processes are the dominating rotor-phonon coupling mechanism, while at higher temperatures, thermally activated processes with activation energies close to the total potential height prevail.