Vibrational Spectra, Assignments and Normal Coordinate Analyses for Crystalline Zirconium Tetrachloride and Tetrabromide

The structure of solid zirconium tetrachloride is known from X-ray studies[l] to consist of extended chains of distorted octahedra with two bridging chlorine atoms between adjacent zirconium atoms. Although the unit cell possesses a center of symmetry, the two pairs of bridging chlorine atoms attached to a given zirconium atom are not trans to each other. The factor group symmetry thus is C2h with two zirconium atoms per unit cell. The X-ray structure of zirconium tetrabromide has not been reported but Krebs in his report on the chloride[l] states that the bromide is isostructural with the chloride. The zirconium halides have been subjected to a limited amount of spectroscopic study. In the vapor phase the work of Clark et al.[2] indicates that the tetrachloride, tetrabromide and tetraiodide all have the expected tetrahedral form. No complete studies of the liquid phase have been reported although a number of authors[3-6] have examined spectra of the octahedral hexahalide anion ZrX26 in different solvents. Prior to the present work only the tetrachloride had been studied in the solid state[7,8]. In their IR and Raman study of ZrCI4, Weidlein, Mueller and Dehnicke[7] suggested that a polymeric model with bridging chlorines and an octahedral structure around the zirconium was the most reasonable. However, since their work was prior to the X-ray work of Krebs[1] they assumed a D2h symmetry model and made their assignments accordingly. Quite recently, Brockner and Demiray[8] analyzed and compared the spectra of solid and liquid (ZrCI4),. The authors based their frequency assignments on a dimeric model with C~ symmetry. In the present work the Raman and IR spectra of solid zirconium tetrachloride and tetrabromide are reported.~ Examination of the structure reported by