Four-membered rings family in the Si–O extended rocking IR band from quantum chemistry calculations

We calculate the vibrational modes of four-membered rings family in the Si–O extended rocking infrared for vitreous SiO₂ materials. In particular these species are obtained in the early stages of the sol–gel process. We have performed high quality quantum-mechanical calculations based on cluster models and gradient corrected density functional theory, (GC-DFT). Three possible configurations were obtained, a regular planar, a slightly deformed or quasi-planar, and a deformed ring. According to our result, the more probable one is the slightly deformed configuration. The calculated vibrational frequencies are compared with experimental IR and Raman results obtaining very good agreement. Also, we calculated geometries and vibrational frecuencies for three-membered rings. We found that 3- and 4-fold rings show evidence of decoupled vibrational modes, those modes are compared with a Raman spectrum obtaining very good coincidence.