35Cl NQR and hindered motions in the addition compound Cl2·benzene
DOI:
https://doi.org/10.17072/1994-3598-2016-2-5-13Abstract
Temperature dependences of the resonance frequency and the spin-lattice relaxation time of the chlorine nuclei are investigated in the equimolecular addition compound of chlorine with benzene in the temperature range between 77 K and the melting point. The experimental data reveal a complex picture of thermally activated molecular motion involving both components of this compound. At temperatures 77–130 K the quadrupole relaxation is determined by the action of three types of molecular motions: 1– background molecular librations, 2– reorientational movement of the benzene rings, modulating the electric field gradient at the chlorine nuclei, created by donor-acceptor interaction of the partner molecules, and 3– reorientational movement of the chlorine molecules at a rate less than the NQR frequency, causing the broadening of the 35Cl spectral line and its fading out at 130 K. In this temperature region consistent separation of the respective contributions to the chlorine relaxation rate is made and the quantitative parameters of the movements defined. At higher temperatures, the development of hindered rotation of the chlorine molecules leads to the appearance of the resonance signal again at 150 K to ~4 MHz lower in frequency, and the relaxation time of the chlorine nuclei increases with increasing temperature. Both these facts indicate that the reorientation rate of the chlorine molecules becomes much higher than the nuclear quadrupole resonance frequency. By analogy with the known NQR data for 1,2-dichloroethane these results show that the “fast” reorientation of the chlorine molecule occurs about the axis that forms the angle ~14° with the Cl–Cl direction. The spin-lattice relaxation time of the chlorine nuclei, as measured in the field of both “slow” and “fast” reorientations, revealed that the activation energy of motion in these areas varies considerably (~22.5 kJ/mol and ~6.2 kJ/mol, respectively). The set of obtained data allows to conclude that at the temperatures 130–150 K in the sample, apparently, there are structural changes, leading to a sharp change in the mobility of the chlorine molecules.Received 07.06.2016; accepted 23.06.2016References
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