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

Joanna Hetmańczyk; Łukasz Hetmańczyk; Anna Migdał-Mikuli; Edward Mikuli

doi:10.1016/j.saa.2014.10.044

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

Spectrochim Acta A Mol Biomol Spectrosc. 2015 Feb 5:136 Pt C:1515-22. doi: 10.1016/j.saa.2014.10.044. Epub 2014 Oct 24.

Authors

Joanna Hetmańczyk¹, Łukasz Hetmańczyk², Anna Migdał-Mikuli³, Edward Mikuli³

Affiliations

¹ Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Kraków, Poland; Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Russia. Electronic address: serwonsk@chemia.uj.edu.pl.
² Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Kraków, Poland; Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Russia.
³ Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Kraków, Poland.

PMID: 25459713
DOI: 10.1016/j.saa.2014.10.044

Abstract

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 3385cm(-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 4.5kJmol(-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 TC(c)=137.6K (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.

Keywords: Density functional theory (DFT); Fourier transform middle and far infrared spectroscopy (FT-MIR and FT-FIR); Hexaamminemanganese(II) perchlorate; Molecular reorientations and vibrations; Phase transition.