ESR studies on the solvation of perfluoro-n-alkyl t-butyl nitroxides — A new scale for electronegativity of perfluoro-n-alkyl groups

Pentafluoroethyl t-butyl nitroxides (3) have been generated by the use of electron-transfer reactions of O-benzoyl-N-t-butylhydroxylamine (1) and pentafluoropropanoyl peroxide (2) in F113 (CFCl2CF2Cl) solution. ESR measurement of aN and aFβ values for 3 in 11 solvents have been carried out at 17 ± 2 °C. The aN values for 3 in 10 aprotic solvents (but not in i-PrOH) show a linear correlation with the cybotactic solvent parameter ET, i.e. aN = 3.02 × 10−2 ET ± 10.46. Together with the similar correlation analysis for trifluoromethyl t-butyl nitroxide (4) and heptafluoropropyl t-butyl nitroxide (5) reported previously, the physical significance for the slope, slope × ET, the extrapolated intercept on the aN axis, is linked to the sensitivity of the nitroxide toward solvation, the magnitude of the overall solution effect on the aN values, and the intrinsic aN values of 3, 4, 5 in the ideal gaseous state, respectively. The intercepts on the aN axis, 11.34 G, 11.56 G and 11.37 G may serve a new measure of electronegativity for CF3, C2F5 and n-C3F7, respectively. The plots of aN versus noncybotactic solvent constants, such as dipolar moment (μ) and dielectric constant (ɛ), all show random cases.