The Crystal Structure and Vibrational Spectra of Mono-L-valinium Nitrate: DSC, FTIR, and X-ray Diffractional Study of Low-Temperature Phase Transition

摘要

The X-ray structural analysis of two mono-L-valinium nitrate phases (293 and 130 K) has been carried out. In the both phases substance crystallizes in the monoclinic space group 1'21: phase I a = 10.8369(3), b = 18.4590(10), c = 5.62190(2) A, p = 129.07 (3)0, V=873.11(5)A3, Z=4, R=0.0509 for 1514 observed reflections, and phase II a = 10.765(2), b = 18.2290(10), c = 11.109(2) A, 11 = 128.64(1)0, V= 1702.7(5) A, Z = 8, R = 0.0257 for 4316 observed reflections. The crystal structures are formed by L-valinium cations and nitrate anions connected by an exten- sive system of hydrogen bonds. Phase I is characterized by the presence of isopropyl groups structural disorder between two positions. Low-temperature phase II exhibits an increase of the unit cell volume, and absence of isopropyl groups disorder. The positions of nitrate groups remain practically unchanged in both the phases. The FTIR and FT Raman spectra of natural and deuterated compounds were recorded and interpreted. The FTIR spectra were studied down to a temperature of 90 K. DSC measurements were carried out in the temperature range 95-400 K. The order-disorder phase transition between the phases I and II was observed at ca. 250 K.