Nonclassical dynamics of the methyl group in 1,1,1-triphenylethane. Evidencefrom powder 1H NMR spectra

According to the damped quantum rotation (DQR) theory, hindered rotation of methylgroups, evidenced in nuclearmagnetic resonance(NMR)line shapes, is anonclassical process. It comprises a number of quantum-rate processes measured by twodifferent quantum-rate constants. The classical jump model employing only one rateconstant is reproduced if these quantum constants happen to be equal. The values of theirratio, or the nonclassicallity coefficient, determined hitherto from NMR spectra of single crystals and solutionsrange from about 1.20 to 1.30 in the latter case to above 5.0 in the former, with thevalue of 1 corresponding to the jump model. Presently, first systematic investigations of theDQR effects in wide-line NMR spectra of a powder sample are reported. For 1,1,1-triphenylethanedeuterated in the aromatic positions, the relevant line-shape effects were monitored inthe range 99–121 K. The values of the nonclassicality coefficient dropping from 2.7 to 1.7were evaluated in lineshape fits to the experimental powder spectra from the range99–108 K. At these temperatures, the fits with the conventional line-shape model are visibly inferior tothe DQR fits. Using a theoretical model reported earlier, a semiquantitative interpretation of the DQRparameters evaluated from the spectra is given. It is shown that the DQR effects as suchcan be detected in wide-line NMR spectra of powdered samples, which are relatively facile tomeasure. However, a fully quantitative picture of these effects can only be obtained fromthe much more demanding experiments on single crystals.