Infrared, polarized Raman and ab initio calculations of the vibrational spectra of [N(C3H7)4]2Cu2Cl6 crystals

The room temperature structure of crystallized [N(C3H7)4]2Cu2Cl6 is triclinic with P1¯ space group. It exhibits two sublattices made up of [Cu2Cl6]2− bitetrahedra and tetrapropylammonium N(C3H7)4+ layers. The vibrational characteristics have been measured at room temperature by infrared spectroscopy (400–4000cm−1) on polycrystalline samples, and by polarized Raman spectroscopy (30–3500cm−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 [Cu2Cl6]2− bitetrahedra that exhibit a quasi C2h 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.