A 1H NMR study of molecular motions in bromo- and iodopentamethylbenzene

Abstract

The molecular motions and the phase transitions in crystalline bromopentamethylbenzene (BPMP) and iodopentamethylbenzene (IPMB) have been studied 1H NMR spin–lattice relaxation time T 1 measurements and differential scanning calorimetry. A solid–solid phase transition was observed at 399 K for BPMB and three solid–solid phase transitions were 237, 277, and 316 K for IPMB. T 1 was calculated theoretically with the stochastic process for the reorientations in the six potential wells of five different depths depending on the halogen position. The C 3 reorientation of CH 3 groups was observed as a gradual decrease of T 1 in both BPMB and IPMB at temperatures lower than 145 and 130 K, respectively. It has been clarified that all the in-plane reorientations activated simultaneously result in a deep T 1 minimum of 43 ms for BPMB. For IPMB, the calculation implies that the reorientations among the six potential wells are activated separately in a wide range of temperature; those from moderately deep two potential wells contribute mainly to the deepest T 1 minimum of 135 ms and those from the deepest two potential wells are not yet activated in the measured temperature region. IPMB melts at 412 K, where the latter start to contribute to the magnetic relaxation, while BPMB melts at 436 K, where all the reorientations have been already activated much.