Infrared, polarized Raman and ab initio calculations of the vibrational spectra of [N(C₃H₇)₄]₂Cu ₂Cl₆ crystals

The room temperature structure of crystallized [N(C₃H ₇)₄]₂Cu₂Cl₆ is triclinic with P1 space group. It exhibits two sublattices made up of [Cu ₂Cl₆]^2- bitetrahedra and tetrapropylammonium N(C₃H₇)₄⁺ layers. The vibrational characteristics have been measured at room temperature by infrared spectroscopy (400-4000 cm^-1) on polycrystalline samples, and by polarized Raman spectroscopy (30-3500 cm^-1) on microcrystals orientated with respect to the organic and inorganic sublattices. The strongly polarized Raman spectra at low wavenumbers are consistent with the existence of orientated [Cu ₂Cl₆]^2- bitetrahedra that exhibit a quasi C_2h symmetry. Ab initio calculations using Gaussian 98 set of quantum chemistry code are performed by DFT approach with B3LYP to predict the set of normal modes characteristics of free [Cu₂Cl₆] ^2-. The reliability of 3-21G, 6-31G(d) and LanL2DZ basis functions has been first checked in the case of [CuCl₄]^2- for which the results can be compared with experimental data previously reported. The low wavenumbers Raman spectra of [N(C₃H₇)₄] ₂Cu₂Cl₆ are analysed and the lines are assigned on the basis of group theory and the ab initio calculation results. The whole vibrational characteristics of the [Cu₂Cl₆]^2- bitetrahedra are given.