New assignments of Raman spectra of tetrahedra and the effects of electronegativity. I. P4O10 and Si-containing molecules.

Our new density functional theory calculations by Gaussian reproduce the bond lengths and Raman spectra of a number of model tetrahedral Si and P compounds [the ten compounds in the SiH_xD_4−x and SiF_xCl_4−x (x = 0–4) series, and the tetramer P₄O₁₀]. The number of symmetric A₁ peaks is determined by group theory, and the center of mass is especially important for the assignment of the three A₁ peaks for P₄O₁₀ to the two P–O stretches and the O–P–O bend. Additional Gaussian calculations on isolated C_3v PO₄ tetrahedra, using the P–O bond lengths and O–P–O bond angles from the P₄O₁₀ optimization, also reproduce the three peak P₄O₁₀ Raman spectra and provide an important guide to the assignment of the two low energy peaks at ∼500 and ∼700 cm⁻¹ to the P–BO (BO = P–O–P) stretch and BO–P–BO bend, respectively, in contrast to all earlier assignments. In the SiF_xCl_4−x series, there is a regular increase in the Si–F A₁ frequency and a decrease in the Si–Cl A₁ frequency across the series. Similar trends are seen in the SiCl_x(CH₃)_4−x series, and these trends are due to changes in the electron densities in the tetrahedron when an electronegative F or Cl is replaced by a less electronegative Cl or CH₃, as measured by previous Si 2p, F 1s, Cl 2p, and C 1s x-ray photoelectron spectroscopy. These considerations serve as a guide for the interpretation of the Raman spectra of silicate glasses in Paper II [Bancroft et al., AIP Adv. 13, 125216 (2023)]. [ABSTRACT FROM AUTHOR]