PYRIDINIUM PICRATE - THE STRUCTURES OF PHASE-I AND PHASE-II - CORRECTION OF PREVIOUS REPORT FOR PHASE-I - STUDY OF THE PHASE-TRANSFORMATION

Pyridinium picrate, C5H6N+.C6H2N3O7, was reported [Kofler (1944). Z. E lektrochem. 50, 200-207] to exist in two crystalline phases, one (I) b eing stable below 343 K and the other (II) between 343 K and the melti ng point (approximately 438 K). The room-temperature structure of phas e I, studied by two-dimensional methods, has been reported [Talukdar & Chaudhuri (1976). Acta Cryst. B32, 803-808]. We were led to reinvesti gate the system by a number of unusual features in Kofler's descriptio n of the phase behaviour. Single crystals of phase I were grown from s olution and those of phase II from the melt. We have determined the st ructure of both phases, including analysis of the thermal motion of th e picrate ions, which was found to be appreciably larger in phase II t han in phase I. The reported structure of phase I was found to be inco rrect, although there were no warning signs; the error was caused by c onfusion between a centre and twofold screw axis in projection down [0 10]. The packing units in the two phases are nearly identical and cons ist of hydrogen-bonded cation-anion pairs. These are packed in stacks, with the ion-pairs superimposed in parallel array in phase I whereas those in phase II are antiparallel; the transition between the two pha ses, therefore, cannot be expected to be single crystal to single crys tal, as indeed it is not. Differential scanning calorimetry (DSC) and variable-temperature powder X-ray diffraction photography show that th e transition occurs at 383 K. Kofler appears to have been misled by a colour change in the phase I crystals at 343 K, which we have also obs erved but cannot explain. The DSC measurements give DELTAH(trans) = 6. 8 kJ mol-1 and DELTAH(fus) = 31.2 kJ mol-1. The transition has proved not to be reversible under our experimental conditions; for example, p hase II crystals remain unchanged after 24 h at 353 K. This suggests t hat the temperature at which the crystalline phases are in thermodynam ic equilibrium is appreciably below 383 K; we have not been able to de termine the transition temperature. The details of the structure deter minations (both at 298 K) are as follows: phase I, M(r) = 308.22, lamb da(Mo Kalpha) = 0.71069 angstrom, F(000) = 632, yellow laths, monoclin ic, mu(Mo Kalpha) = 0.95 cm-1, P2(1/c), a = 12.122 (2), b = 3.783 (1), c = 26.621 (3) angstrom, beta = 92.56 (5)-degrees, V = 1219.6 angstro m3, Z = 4, D(m) = 1.62 (flotation at 298 K), D(x) = 1.67 g cm-3, R(int ) = 0.0167 (based on 25 pairs of equivalent reflections), R(F) = 0.043 6, wR = 0.0492 [based on 1645 independent reflections with F > 3sigma( F)], refinement on F; phase II, M(r) = 308.22, lambda(Mo Kalpha) = 0.7 1069 angstrom, F(000) = 316, yellow prisms, triclinic, mu(Mo Kalpha) = 0.90 cm-1, P1BAR, a = 10.156 (2), b = 8.984 (2), c = 7.230 (1) angstr om, alpha = 86.38 (5), beta = 80.10 (5), gamma = 89.97 (5)-degrees, V = 648.6 angstrom3, Z = 2, D(m) = 1.60 (flotation at 298 K), D(x) = 1.5 8 g cm-3, R(F) = 0.0716, wR = 0.0694 [based on 1478 independent reflec tions with F > 3sigma(F)], refinement on F. Cell dimensions have been measured as a function of temperature for both phases.