✍ Scribed by A Kuhnen; P Vermathen; W Müller-Warmuth
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Rotational excitations of methyl groups in six solid acetates have been investigated by 1 H nuclear magnetic resonance Ž . NMR relaxation time measurements at 15 MHz and 30 MHz and at temperatures between 10 K and the melting point. Hindering barriers between 1.6 kJrmol and 3.7 kJrmol have been found that could be correlated to the tunnelling frequencies observed by inelastic neutron scattering. A consistent description of the relaxation rate dependences from the classical regime at high temperatures to the quantum-mechanical regime at low temperatures is possible by Haupt's equation. The rotational potentials are mainly determined by inter-molecular interaction with an important influence of water of crystallization, if present. Published by Elsevier Science B.V.
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We have measured the proton spin-lattice relaxation rate as a function of temperature in polycrystalline 3-ethylchrysene at nuclear magnetic resonance Larmor frequencies of 53.0 and 22.5 MHz and in polycrystalline 3-isopropylchrysene at 53.0, 22.5 and 8.50 MHz. The syntheses of these new compounds a
NMRRR elperunents, using field cyclin g, on pentamethylbenzene below 15 K yield one Zeeman-tunnel resonance anomaly in the spin-lattice relaxation, at 1.88 T, corresponding to a Larmor frequency equal to half the tunnel frequency, probably of methyl groups, whose reorientation causes the fi\* versus