RING SIZE AND SOLVENT EFFECTS ON SESQUIBICYCLIC HYDRAZINE SELF-ELECTRON-TRANSFER RATES

Self-electron-transfer (self-ET) rate constants k(ex) for sesquibicyclic hydrazines were determined by NMR line broadening at 25 degrees C in CD3CN: 6(0/+)(21/21), 18 500 +/- 500; 4(0/+)(21/u22), 2290 +/- 90; 2(0/+)(22/u22), 12 100 +/- 100 +/- 100; 7(0/+)(23/u22), 3370 +/- 140; 8(0/+)(22/u23), 3840 +/- 110; 3(0/+)(22/22), similar to 700 (dec); and 9(0/+)(23/22), 580 +/- 25 M(-1) s(-1). Differences in Delta G(ex)(double dagger) for the first five compounds correlate linearly with differences in inner shell vertical relaxation enthalpies calculated by AM1, and it is concluded that changes in Marcus lambda(in) dominate the changes in k(ex). The last two compounds show smaller k(ex) values than predicted by AM1, which incorrectly calculates the twist at the NN bond of the neutral forms. The effects of solvent on k(ex) are reported for 8(0/+)NO(3-)(22/u23), solvent polarity range chloroform to methanol, and 4(0/+)PF(6-)(21/u22) and 8(0/+)PF(6-)(22/u23), solvent polarity range pyridine to acetonitrile. Plots of the Eyring free-energy barrier Delta G(ex)(double dagger) vs solvent polarity parameters E(T)(30) and gamma give estimates of the ratio of the inner sphere to total Marcus reorganization barrier, lambda(in)(T)/lambda(T), of 0.78 (vs E(T)(30)) and 0.75 (vs gamma) for 2(0/+)(22/u22) and 0.82 (vs E(T)(30)) and 0.75 (vs gamma) for 4(0/+)(21/u22). The ratios obtained are unreasonably large for the 8(0/+)(22/u23) data (in the range 0.85-0.89), and it is concluded that 22/u23(+) is unstable enough under the experimental conditions to make these ratios inaccurate. It is concluded that self-ET for sesquibicyclic hydrazines is strongly diabetic.