Phase transitions and molecular reorientations in [Mn(OS(CH3)2)6](ClO4)2 studied by proton magnetic resonance and Raman spectroscopy

Abstract

The temperature dependence of full width at half maximum of bands associated with δ d(OClO)F2, δ(CSC), and ρ(CH3) vibrational modes in the FT-RS spectra of [Mn(OS(CH3)2)6](ClO4)2 have shown that the dynamic of reorientational motions of and CH3 groups (from (CH3)2SO) undergoes distinct changes at the phase transitions at T ≈ 365 K and T ≈ 322 K. These are the phase transitions from stable crystal to stable rotational phase and from metastable crystal to metastable rotational phase, respectively. Moreover, characteristic changes of the Raman spectra at these phase transitions, connected with both the shift of the band positions and the scattered light intensity of the bands associated with ν s(SO), ν s(MnO), δ(CSC), and δ(OMnO) modes, suggest that these phase transitions are associated with crystal structure changes, too. Analysis of temperature dependence of the second moment (M2) of 1H NMR showed that on first heating (from room temperature) of the compound the reorientations of the CH3 groups were set in motion in the phase transition at T ≈ 365 K. On subsequent heating (after cooling the compound to 100 K) molecular reorientation starts just above 150 K, and above the temperature of 223 K all molecular groups containing protons perform nearly free rotation with frequencies of a few kHz, including an isotropic reorientation of the whole [Mn(OS(CH3)2)6]2+.