Deuteron Magnetic Resonance Studies of Anhydrous Caffeine

Methyl deuterated caffeine powder was studied using H-2-NMR relaxometry and line shape analysis. In caffeine's low-temperature phase the spin-lattice relaxation times indicate a thermally activated CD3 reorientation (activation energy approximate to 4 kJ/mol). Below the T-1 minimum 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 approximate to 107 kJ/mol). It is argued that the underlying motion involves small-angle molecular excursions and therefore remains undetected via stimulated-echo spectroscopy. The hysteresis accompanying the order/disorder transition taking place above 400 K was monitored using solid-echo line shapes as well as via spin relaxation times. In the high-temperature phase indications for the onset of anisotropic large-angle motions were obtained.