NQR, DSC, and X-ray structure studies of pyridinium tetrabromozincate and pyridinium tetrabromocadmate (C _5H _5NH) _2MBr _4 · nH _2O (M = Zn and Cd; N = 0, 1); Phase transitions and weak hydrogen bond interactions

摘要

Crystals of pyridinium tetrabromozincate and pyridinium tetrabromocadmate were obtained as monohydrates and anhydrous compounds. The crystal structure of metastable (C _5H _5NH) _2-CdBr _4· H _2O was determined at 300(2) K; triclinic space group P-1 with a = 7.875(2), b = 8.151(1), and c = 16.356(2) ?, α = 79.260(10), β = 86.030(10), and γ = 61.440(10)?, Z = 2. All compounds except for stable (C _5H _5NH) _2CdBr _4·H _2O gave four ~(81)Br NQR lines at temperatures between 77 and around 325 K. The stable (C _5H _5NH) _2CdBr _4·H _2O undergoes a first-order phase transition at T c = 116 K. Four 81Br NQR lines below Tc merged into two with equal intensities above T _c, indicating a 180? flip motion of water molecules in the r. t. phase. The ~(81)Br NQR lines of the two anhydrous compounds faded out around 325 K probably due to the reorientational motion of ZnBr 4 ~(2-) or CdBr 4 ~(2-) ions. The respective two ~(81)Br NQR lines of the hydrates exhibited anomalous positive temperature dependence. This is considered to be induced by a weakening in the interionic C-H Br hydrogen bonds with increasing temperature. The DSC measurements of the anhydrous compounds have revealed phase transitions above r. t. The thermal behavior of (C _5H _5NH) _2CdBr _4 is complicated by the formation of metastable forms.