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 SiHxD4−x and SiFxCl4−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 C3v 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 SiFxCl4−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 SiClx(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)].