Vibrational studies of non-centrosymmetric [N2H5][M(HCOO)4] (M = Dy, Er and Y) supported by DFT calculations and analysis of IR reflectance spectra using three different methods

Abstract

Experimental vibrational as well as theoretical studies of three non-centrosymmetric (Pca21) and polar MOFs, [N2H5][M(HCOO)4], where M=Dy, Er and Y, have been performed. IR reflectance spectra have been recorded at room temperature and fitted using three- (3PM) and four-parameter model (4PM) to obtain information on LO-TO splitting and symmetry of the observed bands. The Kramers-Kronig analysis (KK) of raw reflectance spectra is presented for comparison as well. We have also recorded polarized Raman spectra and performed DFT (the three-parameter hybrid B3LYP, 6–311G(2d,2p) basis set) calculations in harmonic and anharmonic approximations for hydrazine molecule and its singly protonated cation. The obtained theoretical spectra have revealed strongly anharmonic nature of the ν(NH2) modes and showed that protonation of only one amine group in hydrazine molecule should affect most significantly the ρ(NH2), ω(NH2) coupled with ν(NN), and twisting τ(NH2) modes, which are expected to shift towards higher wavenumbers after protonation. Based on the obtained results, all observed Raman and IR bands have been assigned to respective motions of structural units in the crystal lattice. The differences in the hydrogen bond arrangement have been discussed. The comparison of three types of IR reflectance spectra analyses showed that the Lorentz 3PM is the most suitable method and gives the best fitting results in comparison to 4PM and KK.