Vibrations and reorientations of NH3 molecules in [Mn(NH3)6](ClO4)2 studied by infrared spectroscopy and theoretical (DFT) calculations

• FT-IR spectra calculated by DFT method (for two models) compared with experimental results.
• Molecular reorientations in [Mn(NH3)6](ClO4)2 at phase transition studied by infrared spectroscopy.
• Dynamics of NH3 ligands is slightly disturbed in the phase transition.
• The activation energy determined for fast NH3 stochastic reorientations in the low and in the high-temperature phase.

The vibrational and reorientational motions of NH3 ligands and ClO4− anions were investigated by Fourier transform middle-infrared spectroscopy (FT-IR) in the high- and low-temperature phases of [Mn(NH3)6](ClO4)2. The temperature dependencies of full width at half maximum (FWHM) of the infrared bands at: 591 and 3385 cm−1, associated with: ρr(NH3) and νas(N–H) modes, respectively, indicate that there exist fast (correlation times τR ≈ 10−12–10−13 s) reorientational motions of NH3 ligands, with a mean values of activation energies: 7.8 and 4.5 kJ mol−1, in the phase I and II, respectively. These reorientational motions of NH3 ligands are only slightly disturbed in the phase transition region and do not significantly contribute to the phase transition mechanism. Fourier transform far-infrared and middle-infrared spectra with decreasing of temperature indicated characteristic changes at the vicinity of PT at TCc = 137.6 K (on cooling), which suggested lowering of the crystal structure symmetry. Infrared spectra of [Mn(NH3)6](ClO4)2 were recorded and interpreted by comparison with respective theoretical spectra calculated using DFT method (B3LYP functional, LANL2DZ ECP basis set (on Mn atom) and 6-311 + G(d,p) basis set (on H, N, Cl, O atoms) for the isolated equilibrium two models (Model 1 – separate isolated [Mn(NH3)6]2+ cation and ClO4− anion and Model 2 – [Mn(NH3)6(ClO4)2] complex system). Calculated optical spectra show a good agreement with the experimental infrared spectra (FT-FIR and FT-MIR) for the both models.

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