Theoretical studies of vibrational spectra of [N(CH3)4]2ZnCl4-yBry compounds with y=0, 2 and 4

The infrared and Raman spectra of [N(CH3)(4)](2)ZnCl4-yBry, where y = 0, 2 and 4, have been analyzed with ab initio calculations of the vibrational characteristics of constitutive polyhedra, tetramethylammonium [N(CH3)(4)](+) and [ZnCl4-xBrx](2-) (x = 0, 1, 2, 3 and 4) tetrahedra. The optimized geometries, calculated vibrational frequencies, infrared intensities and Raman activities are calculated using Hartree-Fock and density functional theory B3LYP methods with 3-21G, 6-31G(d) and 6-311G(+)(d,p) basis sets. Calculation of the root mean square difference (rms) between the observed and calculated frequencies allows to give scaling factors and to deduce that the best agreements are obtained by B3LYP/6-311G(+)(d,p) for [N(CH3)(4)](+) and B3LYP/3-21G for [ZnCl4-xBrx](2-). The present study establishes a strongly reliable assignment of the vibrational modes of [ZnCl4-xBrx](2-) tetrahedra based on comparison between experimental and ab initio calculations, both of the frequencies and the intensities of the Raman signals.