Methyl Rotation in 1,3,5-Trichloro-2,4,6-trimethylbenzene as Studied by Nuclear Magnetic Resonance: A Comparison with Merthyl Rotation in Hexamethylbenzene

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The spin-lattice relaxation rate and the line shape of proton NMR of 1,3,5-trichloro-2,4,6-trimethylbenzene were measured between 1.8 and 430 K. Two maxima of the spin-lattice relaxation rate were clearly distinguished at 36.4 and 19.0 K. The maxima are ascribed to fluctuation of intramethyl and intermethyl dipole interactions due to methyl rotation. The activation energy of the rotation of the methyl group over the potential barrier is 2.4 kJ mol⁻¹ and the tunneling frequency of the methyl group was estimated to be 830 MHz at 36 K. In comparison with the methyl rotation in hexamethylbenzene, the alternate substitution of three chlorine atoms for the three methyl groups substantially reduces the potential barrier hindering the rotation of the methyl groups.