The polytopicity of ionic ternary compounds

A Raman spectrometer for obtaining spectra of vapors and gases at high temperatures has been constructed. The theories of polytopicity, tautomerism, and fluxional behavior in molecules are described, and a new, more general term, "dynamorphism" (to describe a continuously changing molecular geometry), is suggested as a more general term which includes all the above terms. The Raman spectra of the gaseous tetrahedral molecules CF₄ and OsO₄ have been obtained over the temperature range 300°K to 1361⊂K. In addition, the Raman spectra of the vapors of Cs₂SO₄ and MReO₄ (M = K, Rb, Cs, T1) have been obtained in the range 1250°K to 1450°K. The ν₁ (908 cm^-1), ν₂ (435 cm^-1), and ν₄ (631 cm^-1) bands of CF₄ were observed, and the effects of temperature up to 983°K on the band shapes and the band intensities were noted. The ν₁ (970 cm^-1 ) and the combined ν₂ and ν₄ (331 cm⁴ ) bands of OsO₄ were observed, and again the effects of temperature up to 1361°K on band shapes and band intensities were noted. The spectra of the perrhenates are all very similar with a sharp band at 970 cm^-1 and a broad intense band in the 300 to 500 cm^-1 region.

The structure of the perrhenates in the vapor phase is most likely a tetrahedral anion whose symmetry is unperturbed by the cation, at least within the capabilities of the Raman technique. This conclusion is based upon a comparison of the number and locations of bands observed in the perrhenate spectra with the number of bands expected for the perrhenate ions in C_s, C_2v, C_3v and T_d symmetries. The assignment is reinforced by comparisons with the spectra of the Re0₄^- ion in solution and OsO₄ vapor. Finally, to explain the apparent absence of an effect by the cation on the anion symmetry, it is suggested that the perrhenate molecules in the vapor are polytopic or "dynamorphic" with the M⁺ (M = K, Rb, Cs, T1) residing on a spherical potential energy sur face about the anion.