Refined, scaled and canonical force fields for the cis- and trans-3-fluorostyrene conformers. An interplay between theoretical calculations, IR/Raman and INS data

Abstract

We have performed a force field calculation for 3-fluorostyrene following the refinement and the scaling methodologies, starting in both cases from the force constants matrix calculated at the B3LYP/6-311++G** level. To do this, the structures of the cis- and trans- conformers were first obtained at different levels of theory. While HF and MP2 predict non-planar structures, only DFT calculations yield a planar one for both conformers, in agreement with experimental data. The final rms deviations for the calculated wavenumbers are 0.46 cm⁻¹ and 0.23 cm⁻¹, in the case of the refinement method, and 5.01 cm⁻¹ in the case of the scaling method, for the cis- and trans- conformers, respectively. The refined force constants obtained in terms of independent symmetry coordinates have been expressed in terms of simple valence internal coordinates in the unambiguous canonical form for comparison. The refined values for the C–C stretching force constants suggest that the vinyl C–C bond is similar in strength to ethene while the bond connecting the vinyl moiety to the benzene ring is similar in strength to those of the benzene C–C bonds. As to the scaling procedure, the values obtained for each force constant match the characteristic value for a C–C single bond. All these results agree well with those from an analysis of the barriers to internal rotation and also geometric data.