Phase transition in para-cyanoanilinium iodide

^79,81Br and ¹²⁷I NQR spectra of para-cyanoanilinium bromide, 4-CNC₆H₄NH^⊕₃ Br^?, and para-cyanoanilinium iodide, 4-CNC₆H₄NH^⊕₃I^?, respectively, have been investigated in the temperature range 77 K ? 390 K. In para-cyano-anilinium iodide a phase transition has been found to occur at 99.5 K ± 1.0 K. The transition is characterized by a sudden change in the slope ν(¹²⁷I)=f(T), and by a corresponding change in the nuclear quadrupole coupling constant. The asymmetry parameter η, which has a value of zero in the high temperature phase I, shows a small but finite increase below the transition. The nature of the phase transition in para-cyanoanilinium iodide is found to be similar to that taking place in anilinium iodide as well as in para-fluoro- and para-chloroanilinium iodide. In addition, the nuclear quadrupole coupling constant, η, and the transition temperature for para-cyanoanilinium iodide and para-chloroanilinium iodide are remarkably similar. Results for the temperature dependent studies on para-cyanoanilinium bromide are also presented.