Theoretical studies of the structural, electronic, and 19F NMR properties of linear and branched perfluorobutanesulfonate

The structural and electronic properties of linear and branched perfluorobutanesulfonate (PFBS) in its anionic, acidic, and potassium or sodium salt forms were studied in a polarizable continuum model (PCM) of methanol solvent with the B3LYP functional and the 6-31G(d,p) basis set. The F-19 chemical shifts and F-19-F-19 J-coupling constants were determined in a PCM of methanol solvent with GIAO B3LYP/6-31++G(d, p). The differences in energy, enthalpy, and free energy of the four PFBS isomers were compared. The data indicate that the linear PFBS species is less stable than the branched isomers and the isomer with the tert-butyl-like structure is the most stable. The J-coupling contributions via both a non-Fermi contact mechanism and a long chain of bonds (through-bond) indicate that F-19-F-19 J-coupling constants are long range in these branched PFBS isomers. The calculated and experimental F-19 chemical shifts for the linear PFBS species are in good agreement. In addition, the results obtained establish the similarities between the geometries and electronic and NMR properties of linear PFBS and linear perfluorooctanesulfonate (PFOS).