The configuration and the dynamics of the -CD3 groups in methyl deuterated dimethylmalonic acid. A single crystal deuteron NMR study

The angular variation of the deuteron NMR of single crystals of methyl deuterated dimethylmalonic acid, C(CD3)2(COOH)2 (DMMA), was measured at room temperature and at 56 K. From the room temperature data the motionally averaged deuteron quadrupole coupling tensors for rapidly reorienting methyl groups are determined. The C-Cmethyl bond directions are inferred from the unique principal axes of these tensors. A semi-quantitative analysis of the 56 K data leads to the conclusion that at this temperature the methyl groups are immobile and that the configuration of the two methyl groups within the DMMA molecule is an eclipsed one. By line shape analysis of deuteron spectra recorded for 290 K > T > 60 K the Arrhenius parameters for reorienting -CD3 groups E a = 9·5 kJ mol-1 and k (D) 0 = 0·8 × 1011 s-1 were determined. No features giving evidence for tunnelling could be observed in the low temperature deuteron spectra. An upper limit of 3 kHz is derived for the -CD3 tunnel frequency v (D) t for T → 0