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Phase Transitions, Spin−Lattice Relaxation, Echo Spectroscopy
Abstract:
Methyl deuterated caffeine powder was studied using 2H−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 ≈4 kJ/mol). Below the T1 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 ≈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
