The measurement of DTA and the temperature variation study of IR revealed that the reported stable phases α, α′, and β of methylammonium bromide (CH3NH3Br) exist above 387 K, between 387 and 281 K, and below 281 K, respectively. The metastable δ phase prepared by rapidly cooling the sample to 77 K seems to be transformed into α′ at 196 K, and, immediately above this temperature, it is transformed again to β. The temperature dependences of the 1H spin-lattice relaxation time T1 at 20 MHz and of the second moment of 1H NMR absorptions were determined for the bromide and also its partially deuterated analogues CD3NH3Br and CH3ND3Br in their respective phases. An analysis of the CH3 and NH3 rotational reorientations about the C-N axis occurring in the β phase was accomplished by using the T1 data of the partially deuterated salts. The onset of both reorientations in the δ phase occurs at approximately the same temperature which is lower than those found in the β phase. The motional parameters of the cation in the β and δ phases could be derived from the present results of 1H NMR.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry