The temperature dependences of 35Cl NQR frequencies were observed over a fairly wide range of temperature for MSnCl6·6H2O [M(II)=Mg, Ca, Mn, Co, Ni] forming isomorphous crystals (R\bar3) at room temperature. These complexes yielded unusual 35Cl NQR temperature dependence curves showing a positive temperature coefficient. This can be attributed to the existence of Cl···H–O type weak hydrogen bonds.
Structural phase transitions and molecular motions in
(CH3NH3)2MBr4 and in [(CH3)2NH2]2MBr4 (M = Zn,
Cd, and Hg) were investigated by differential scanning calorimetry
(DSC) between about 130 K and the melting points. All of the
compounds showed at least one structural phase transition over the
temperature region investigated. The values of the transition
entropies obtained suggest that these transitions are of the
order-disorder type. N-H···Br hydrogen bonds
were found to play an important role in the stability of P21/c
room-temperature phases of the title compounds. In the
highest-temperature solid phases of all title compounds,
orientational orders of cations seem to be completely lost.
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