An NQR study of thermally activated motions of nitro groups and of the phase transition in 2,6-dinitrochlorobenzene

Abstract

A study of the temperature dependence of the 35Cl NQR frequency of 2,6-dinitrochlorobenzene is reported. These measurements show the existence of a second-order phase transition at T approximately 255 K. The temperature dependence of the spin-lattice relaxation time and the line width between 80 and 300 K is also reported. The splitting of the resonance line in the low-temperature phase (LTP) was shown to behave as an order parameter of the phase transition. The experimental order parameter and the anomalous behaviour of the line width near the transition point were interpreted satisfactorily in terms of the librational soft-mode theory, which assumes anharmonic coupling between molecular librational modes in the crystal. The results also show that the totality of results in the LTP is consistent with the onset of reorientations of nitro groups. The parameters deduced are compared with those obtained in our studies of the n-nitrobenzene sulphonyl chloride family of compounds.