Vapour phase Raman spectra of the molecules MH4(M = C, Si, Ge or Sn) and MF4(M = C, Si or Ge). Raman band intensities, bond polarisability derivatives and bond anisotropies

Abstract

The vapour phase Raman spectra of the molecules MH₄(M = C, Si Ge or Sn) and MF₄(M = C, Si or Ge) have been recorded with 488.0 and/or 514.5 nm excitation at pressures of 0.5–1.0 atm and at temperatures of ca. 295 K. The intensities of all four Raman-active fundamentals of each molecule have been determined relative to that of the ν₁(a₁) band of methane as external standard, and this has permitted the calculation of Raman scattering cross sections for each band. Molecular ( ^′ _j) and bond ( ^′ _MX) polarisability derivatives have been calculated. The ^′ _MH values are similar, for a given M atom, to the ^′ _MF values and all follow trends established previously for other ligands (X), ^′ _CX≈ ^′ _SiX < ^′ _GeX < ^′ _SnX. Bond anisotropies (γ_MX) have been deduced from the Raman intensities of the ν₂(e) fundamental of each molecule. This method only yields the modulus of the γ_MX value, but, when taken in conjunction with Kerr effect results, it is concluded that γ_MX is positive in each case. Quadratic force fields for each molecule are established on the assumption of the Wolkenstein intensity theory, and mean square amplitudes of vibration are also calculated.