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

A combined quantum chemical and experimental study of the structure, harmon ic vibrational frequencies, charge distribution and internal torsional moti on of benzenesulfonate anion was performed. The geometry of the anion was o ptimized 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 structura l parameters of the anion at both levels of theory are in very good agreeme nt 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 larg er basis set methodology gives the correct order of conformational stabilit ies (staggered vs. eclipsed anion conformation), while the lower basis repr oduces the experimental data only upon inclusion of the zero-point energy c orrections. On the basis of ab initio HF/6-31+G(d) energetics, the torsiona l energy levels of the Ph-SO3 rotor were computed within a one-dimensional approach, diagonalizing the torsion Hamiltonian in the free-rotor basis. Bo th the Mulliken and the NPA charge-partitioning schemes predict a strong de localization of the anionic charge over the phenyl ring, while the electros tatic potential based schemes (CHelp, CHelpG and MK) predict only a slight delocalization. (C) 2000 Elsevier Science B.V. All rights reserved.