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 e lectron-transfer reactions of O-benzoyl-N-t-butylhydroxylamine (1) and pent afluoropropanoyl peroxide (2) in F113 (CFCl2CF2Cl) solution. ESR measuremen t of a(N) and a(F)(beta) values for 3 in 11 solvents have been carried out at 17 +/- 2 degreesC. The a(N) values for 3 in 10 aprotic solvents (but not in i-PrOH) show a linear correlation with the cybotactic solvent parameter E-T, i.e. a(N) = 3.02 x 10(-2) E-T +/- 10.46. Together with the similar co rrelation analysis for trifluoromethyl t-butyl nitroxide (4) and heptafluor opropyl t-butyl nitroxide (5) reported previously, the physical significanc e for the slope, slope x E-T, the extrapolated intercept on the a(N) axis, is linked to the sensitivity of the nitroxide toward solvation, the magnitu de of the overall solution effect on the a(N) values, and the intrinsic a(N ) values of 3, 4, 5 in the ideal gaseous state, respectively. The intercept s on the a(N) 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 a(N) versus noncybotactic solvent constants, such as dipolar moment (mu) and di electric constant (epsilon) all show random cases.