Phase Transitions and Molecular Motions in Guanidinium-d6 Trihalogenomercurates(II) [C(ND2)3]HgBr3 and [C(ND2)3]HgI3 as Investigated by Means of 81Br and 127I NQR and 2H NMR

The temperature dependence of 2 HNMR spectra and 81 Br and 127 INQR frequencies was investigated for the deuterated compounds of [C(NH 2 ) 3 ]HgBr 3 and [C(NH 2 ) 3 ]HgI 3 between 77 and ca. 400K. Due to deuteration, the phase-transition temperatures of the iodine compound were increased by 2 and 6K for the second-order (251K) and the first-order (210K) phase transitions, respectively. Distinctive 127 INQR frequency-shifts of more than 200kHz were observed in the intermediate phase of the iodine compound on deuteration. The temperature dependences of the 2 HNMR spectra for [C(ND 2 ) 3 ]HgBr 3 and [C(ND 2 ) 3 ]HgI 3 could be explained by invoking C 3 reorientation of the guanidinium cation. The activation energies for the reorientation evaluated from 2 HNMR were consistent with the former 1 HNMR T 1 results with the exception of the low-temperature phase of the iodine compound. The quite low activation energy from 2 HNMR spectrum for this phase may indicate that the quenching of the orientation disordering of the cations is responsible for the fade-out phenomenon in the 127 INQR resonance lines below ca. 100K.