Application of the multi-parameter SQM harmonic force field, and ESFF harmonic frequencies scaling procedures to the determination of the vibrational spectra of silicon- and sulfur(II)-containing compounds.

Multi-parameter scaling techniques, such as Scaled Quantum Mechanical (SQM) force field [J. Am. Chem. Soc. 105 (1983) 7037-7047; J. Phys. Chem. A 102 (1998) 1412-1424] or Effective Scaling Frequency Factor (ESFF) [Chem. Phys. Lett. 446 (2007) 191-198; J. Mol. Spectrosc. 264 (2010) 66-74] techniques, are very powerful in the theoretical prediction of the vibrational spectra of complex molecules. In the present work sets of transferable SQM and ESFF scaling factors (within the valence coordinates based schemes) that can be applied to silicon- and sulfur(II)-containing compounds have been determined. A number of VDZ- and VTZ-quality basis sets were used in conjunction with the B3LYP density functional. Eight molecules typically used in the synthesis of silica-based materials were chosen, and theoretical modes were assigned to bands detected on their IR or Raman spectra. This set was augmented with a set of 10 auxiliary, sulfur(II)-containing molecules, for which only "pure" vibrations involving S-containing motifs were assigned. This led to the set of more than 600 individual vibrations. Five factors attributed to these motifs were optimized. Scaling factors attributed to the characteristic types of internal coordinates including the second-row atoms and chlorine, which are applicable to the present molecules were preset. Their values, optimized for Baker's training set of molecules [J. Phys. Chem. A 102 (1998) 1412-1424] for all basis sets considered in this work, were also found, extending thus the applicability of the multi-parameter scaling methods. New scaling factors exhibit low statistical uncertainties. Reasonable agreement between experimental and SQM- or ESFF-scaled frequencies was obtained even for the 6-31G* basis set (RMS<12cm(-1)); extension of the basis set by adding polarization function on hydrogen atoms and/or diffuse functions provides significant improvement of the results, for which the RMS values are often (well) below 10 cm(-1). In addition, SQM scaling factors were found to occasionally exhibit large deviations from unity, which is to be contrasted with ESFF scaling factors.