Order‐Disorder in Solid 1,2,3‐Trichlorobenzene. A Single Crystal 35CI NQR Study

By single crystal Zeeman split NQR spectroscopy the nuclear quadrupole coupling tensors, NQCT, of 35CI in magnitude, orientation, and symmetry were studied in the range 140 ≦ T/K ≦ 250. Neither the asymmetry parameters nor the orientations of the NQCTs depend significantly on temperature. The disappearance of the 35CI NQR signal well below the melting point was investigated by studying the line width Δv and the spin lattice relaxation time T1 as a function of temperature. From Δv = f(T) an activation energy of 31 kj mol−1 and 36 kJ mol−1, respectively, was found for the two crystallographically inequivalent molecules in the unit cell. The onset of static order‐disorder or the creation of lattice vacancies may be responsible for the line broadening and the change in T1.