Crystal structure and phase transition of 4-aminopyridinium Tetrabromoantimonate(III) as studied by bromine and antimony NQR, proton NMR, and single crystal X-ray diffraction

The crystal structure of the room temperature phase (RTP) of the title comp ound was determined at 297 K (monoclinic, space group C2/c, a = 1384.2(2), b = 1377.8(3), c = 755.5(2) pm, beta = 121.58(1)degrees). A complicated dis order was found for the cation. A phase transition from the low-temperature phase (LTP) to the RTP was found at (224 +/- 1) K (T-c). The H-1 NMR spect ra showed a sharp motional narrowing at ca. T = T-c, indicating the occurre nce of a reorientational motion of the cation in the RTP in accord with the disorder. It was found that another reorientational motion is excited in t he LTP. Four Br-81 NQR lines (132.71, 115.38, 61.54 and 59.31 MHz at 77 K) and two Sb NQR lines (53.78 and 33.76 MHz at 77 K) were found in the LTP, w hile a single 81Br NQR line was observed at T > 276 K (ca. 121.80 MHz at 30 0 K). Crystal dynamics are discussed on the basis of the temperature depend ence of the NQR, H-1 NMR line width, and H-1 NMR T-1.