Secondary bicyclic peroxide cation radicals

Abstract

systems (4-9, respectively) and the syn-1-bridge bromo peroxides in the [2.2.1], [3.2.1], and [4.2.1] systems (10)(11)(12). The lifetimes of the radical cations decreased in the order 7+ (no decomposition seen by CV) > 8+ > 4+ » 5+ > 6+, 9+ (no reduction wave seen by CV). Accurate E0' values could be measured only for 7 (2.27 V), 8 (2.35 V), and 4 (2.40 V) because of large AEp values at fast scan rates or low temperatures. ESR spectra with resolved proton hyperfine splittings are reported for 4+, 7+, and 8+.We recently showed that bicyclic tertiary dialkyl peroxides 1 and 2 give long-lived enough radical cations at room temperature to allow observation of reduction waves in their cyclic voltammograms, so that their thermodynamically significant E°' values could be measured with reasonable accuracy.1 2345Di-fert-butyl peroxide (3) cation radical, in contrast, decomposed so rapidly at room temperature that only an irreversible oxidation wave was observed. Although 3+ was longer lived at -78 °C, and both oxidation and reduction waves were observed, the difference in oxidation and reduction peak potentials, , was so large at low temperature that E0' could not be very accurately determined.In this work we report the cyclic voltammetric behavior of nine secondary bicyclic peroxides 4-12, compare their solution oxidation potentials with gas-phase ionization potential data from the literature,2-5 and give ESR data for three secondary peroxide radical cations. Results and DiscussionElectrochemistry. Peroxides oxidize above +2 V vs. SCE, and we observe large background currents in this range in methylene chloride. Following Parker,6 we em-(1)

Ionized Bicyclo[2.2.2]oct-2-ene: A Twisted Olefinic Radical Cation Showing Vibronic Coupling

“…As expected from the basic theory of PJTE,7 this mixing of the 2 B 1 π‐ionized ground state and the 2 B 2 σ‐ionized excited state is facilitated by the small energy gap of ∼1.0 eV between these states, as estimated from photoelectron spectroscopy 16a. A satisfying corollary is that no indication of twisting is observed13a for the isostructural peroxide 3 .+ , which has a larger energy gap of ∼2.0 eV between the relevant 2 A 2 π*‐ and 2 A 1 σ‐states.…”

“…The vinyl hydrogen splitting for norbornene radical cation is (−)10.6 G11 and those for cyclopentene, cyclohexene, and cis ‐but‐2‐ene radical cations lie between (−)8.6 and (−)9.5 G 12. The average a (H x ) for 1 .+ is twice that of peroxide 3 .+ , a (4H x )=4.7 G,13a 2.4 times that of the tetracyclic alkene analogue 4 .+ , a (8H x )=3.9 G,13b and 3.8 times that for the analogous dimethylhydrazine radical cation, 5 .+ , a (4H x )=2.6 G 13c. In contrast to these results for π radicals, a larger a (4H x )=15.5 G applies for the isoelectronic 2,3‐diazabicyclo[2.2.2]oct‐2‐ene radical cation, 6 .+ , which is known to be a σ radical at the nitrogens 14.…”

Correlation of Oxidation and Ionization Potentials for Azoalkanes

“…Results, are also contained in Figure to illustrate the significance of the correlation. Both the intercept (6.4) and the slope (0.95) of the regression line (eq 1) fall slightly outside the range observed for the intercept (3.4−6.2) and the slope (0.66−0.92) in previous correlations. − Interestingly, even the only reversible oxidation potential ( E 1/2 = 1.40 V versus SCE; IP v = 8.24) 30,35 for azonorbornane ( 11 ) falls very close to the regression line (triangle in Figure )

1 Peak oxidation potentials ( E p ) of azoalkanes 1 − 10 (·) in acetonitrile from Table (0.1−0.2 V/s) versus the corresponding vertical ionization potentials (IP v ) and the resulting (solid) linear regression line (eq 1).

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“…A number of correlations between oxidation and ionization potentials have been established for several classes of compounds. − Although the adiabatic ionization potentials (IP a ) are preferred for correlation 6 since they include (as the oxidation potentials) contributions due to the geometric relaxation or reorganization energy of the cations ( E relax ), vertical ionization potentials (IP v ) are more straightforward to determine and, hence, have been extensively used. Similarly, although it is more common to employ half-wave potentials ( E 1/2 ), linear correlations are also obtained for the peak oxidation potentials ( E p ) in those cases for which E 1/2 cannot be accurately determined. ,− , …”

“…There is a substantial interest in correlating redox potentials in solution with experimental electron affinities , or ionization potentials − in the gas phase, or with theoretical heats of ionization and frontier orbital (HOMO, LUMO) energies. − Although the solution and gas-phase measurements represent complementary techniques to assess redox properties, they are subject to various experimental limitations. This may render difficult to determine either the gas phase values, in particular the electron affinities, , or more commonly the solution values.…”