Phase Transitions and Molecular Dynamics in Tris (Methylammonium) Nonabromodiantimonate (III) and its Partially Deuterated Analog as Studied by NQR and NMR

Molecular motions and phase transitions in tris (methylammonium) nonabromodiantimonate(III), (CH3NH3)3 Sb2Br9, and its partially deuterated analog, (CH3ND3)3 Sb2Br9, are investigated by measuring the temperature dependences of static and rotating frame proton spin–lattice relaxation times (T1 and T1Q), the deuteron lineshapes, 81Br NQR frequencies, and nuclear quadrupole spin–lattice relaxation times (T1Br). Discontinuities in the spin–lattice relaxation plots confirm the existence of three different phase modifications related to a freezing of reorientation of methylammonium cations. Molecular motions occurring in the respective phases are identified and their activation energies are determined. 2D NMR spectra show that the methylammonium cations are in phase I isotropically disordered possessing dynamically almost spherical symmetry, whereas at low temperature phase III the cations reorient only along the C3-axis. The role of N–H … Br hydrogen bonds and the effect of 1H ⇒ 2D isotope substitution on the phase transition temperatures, activation energies of methylammonium groups, and 81Br NQR frequency isotope shifts are discussed. The NQR and NMR results pertaining to the three phases are discussed and related to other studies.