Rotational Tunnelling of CD₃Groups in Molecular Crystals as Studied by NMR Spectra

Tunnelling frequencies of rotating CD 3 groups in solids between about 20 kHz and 2 MHz may be obtained from the 2 H NMR spectra. The theory of the spectral response is developed where quadrupole and dipole-dipole interactions as well as rotational tunnelling are taken into account. Features characteristic of tunnelling, which distinguish the spectra from those of rapidly reorienting deuterated methyl groups, are found from analytically calculated spectra even for the case of very large tunnel splittings. Numerical calculations have been performed for various conditions to deter­ mine the tunnel frequency. Experimental spectra measured at 45 MHz and low temperatures have revealed the appearance of rotational tunnelling in CD 3 I, CD 3 COONa, and (CD 3 COO) 2 Cu • H 2 O. In the latter case, a tunnelling frequency of 608 kHz has been extracted from the spectrum at 27 K.