A H-1 NMR study of molecular motions in chloropentamethylbenzene

The temperature dependence of the H-1 NMR spin-lattice relaxation time T-1 has been measured for solid chloropentamethylbenzene in the range 90-380 K. A single T-1 minimum of 36.5 ms was observed around 278 K at the Larmor frequency of 18 MHz and has been assigned to in-plane molecular reorientation about the pseudohexad C-6' axis over the nonequivalent potential wells. With the aid of the reported X-ray analyses, the results have been analysed by employing the method developed by Anderson. The activation energy has been evaluated as 27 kJ mol(-1) which amounts to nearly the same value as hexamethylbenzene. A gradual decrease of T-1 below 150 K was attributed to the methyl groups rotation about their C-3 axes over the barriers of 3.0 kJ mol(-1). (C) 1997 Elsevier Science B.V.