Ab initio quantum chemical and experimental study of structure, harmonic vibrational frequencies and internal Ph–SO3 torsion of benzenesulfonate anion

Abstract

A combined quantum chemical and experimental study of the structure, harmonic vibrational frequencies, charge distribution and internal torsional motion of benzenesulfonate anion was performed. The geometry of the anion was optimized at the HF/3-21+G(d) and HF/6-31+G(d) levels of theory, followed by numerical harmonic vibrational analysis. Furthermore, the FT-IR spectra of several metal benzenesulfonate salts were recorded. The computed structural parameters of the anion at both levels of theory are in very good agreement with the X-ray data. Regarding the vibrational analysis, the HF/6-31+G(d) force field is significantly superior over the HF/3-21+G(d). On the basis of the HF/6-31+G(d) vibrational analysis, several important reassignments of the IR bands owing to the benzenesulfonate anion are suggested. The larger basis set methodology gives the correct order of conformational stabilities (staggered vs. eclipsed anion conformation), while the lower basis reproduces the experimental data only upon inclusion of the zero-point energy corrections. On the basis of ab initio HF/6-31+G(d) energetics, the torsional energy levels of the Ph–SO3 rotor were computed within a one-dimensional approach, diagonalizing the torsion Hamiltonian in the free-rotor basis. Both the Mulliken and the NPA charge-partitioning schemes predict a strong delocalization of the anionic charge over the phenyl ring, while the electrostatic potential based schemes (CHelp, CHelpG and MK) predict only a slight delocalization.