The Raman spectrum of (TMA)2UCl6 and (TMA)2SnCl6 crystals and of TMACl in solution

Abstract

The Raman spectrum of (TMA)₂UCl₆, not reported previously, is presented and the assignments of the observed bands to normal modes of vibration are discussed based on polarized room temperature scans of single crystals. Seventeen of the eighteen fundamental vibrational bands and seven nonfundamental ones are identified. The missing fundamental vibrational band, containing mainly methyl internal symmetric stretching motions, is at least three orders of magnitude weaker than another band composed of the same methyl internal motions. Five totally symmetric nonfundamental bands are resolved near 3000 cm⁻¹. It is suggested that they interact by Fermi resonance with the highly intense totally symmetric methyl stretching vibration. Their proposed assignments imply that one of the nonfundamental bands is an overtone of a combination level, thus involving four vibrational quanta. Another one of these bands is assigned to a combination level involving three different vibrational quanta. Light scattering by three and four phonon excitations has not been reported before, except in resonance Raman scattering. An unpolarized Raman spectrum of (TMA)₂SnCl₆ is shown and the bands are assigned based on the close correlation with the (TMA)₂UCl₆ spectrum. The Raman spectrum of TMACl in solution is reexamined. Four of the frequencies reported previously could not be verified, and five bands of this spectrum, including a fundamental one, are reported here for the first time. The polarization information of the latter spectrum is completed. The crystal structures of (TMA)₂UCl₆ and (TMA)₂SnCl₆ are discussed. Four variations of the structure compatible with O⁵_h, the space group suggested for both crystals by x‐ray investigations, are described.