Abstract
AbstractMethyl deuterated caffeine powder was studied using2H-NMR relaxometry and line shape analysis. In caffeine's low-temperature phase the spin-lattice relaxation times indicate a thermally activated CD3reorientation (activation energy ≈4 kJ/mol). Below theT1minimum near 35 K the quadrupolar echo spectra develop features indicative for quantum mechanical tunneling. Dielectric measurements indicate the presence of dipole moment fluctuations with times scales that follow an Arrhenius law (energy barrier ≈107 kJ/mol). It is argued that the underlying motion involves small-angle molecular excursions and therefore remains undetectedviastimulated-echo spectroscopy. The hysteresis accompanying the order/disorder transition taking place above 400 K was monitored using solid-echo line shapes as well asviaspin relaxation times. In the high-temperature phase indications for the onset of anisotropic large-angle motions were obtained.
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