Solvent Induced Intramolecular Electron Exchange Kinetics in 1,3-Isodisubstituted Aromatic Radical Anions II. Activation Parameters

Electron Transfer within 2,7-Dinitronaphthalene Radical Anion

The optical spectrum of 2,7-dinitronaphthalene radical anion generated by Na(Hg) reduction in acetonitrile containing a large excess of cryptand[2.2.2] exhibits a Hush-type intervalence charge-transfer band at 1070 nm, estimated to correspond to an off-diagonal matrix coupling element of 310 cm(-)(1). The interpolated rate constant for intramolecular electron transfer at 293 K measured by ESR between 225 and 320 K for this solution is 3.1(+/-0.2) x 10(9) s(-)(1). Rate constants estimated in two ways from the optical parameters using the Marcus-Hush assumption that the diabatic surfaces should be parabolae are 1.0 and 0.11 x 10(9) s(-1), and those using diabatic surfaces that fit the observed charge-transfer band are 9.6 and 3.4 x 10(9) s(-)(1), when used with an electron-transfer distance on the adiabatic surfaces of 6.42 A. Similar measurements and comparisons were also carried out using dimethylformamide and butyronitrile as solvents. The success of simple, classical two-state Marcus-Hush theory precludes an electron-hopping mechanism. UHF calculations predict a planar unsymmetrical gas-phase structure for 1,3-dinitrobenzene radical anion but give serious spin contamination. Semiempirical AM1 calculations using singles excitation configuration interaction with an active space of 70 orbitals and the COSMO solvent model also give a planar unsymmetrical structure. These calculations make the internal vibrational component of the reorganization energy nearly constant, and much smaller than the solvent reorganizational component, and predict the transition energy to lie between that observed in acetonitrile (9360 cm(-1)) and those observed in dimethylformamide (8100 cm(-1)) and butyronitrile (8040 cm(-1)).

Intervalence Near-IR Spectra of Delocalized Dinitroaromatic Radical Anions

The Class III (delocalized) intervalence radical anions of 1,4-dinitrobenzene, 2,6-dinitronaphthalene, 2,6-dinitroanthracene, 9,9-dimethyl-2,7-dinitrofluorene, 4,4'-dinitrobiphenyl, and 1,5-dinitronaphthalene show charge-transfer bands in their near-IR spectra. The dinitroaromatic radical anions have comparable but slightly larger electronic interactions (H(ab) values) through the same aromatic bridges as do the corresponding dianisylamino-substituted radical cations. H(ab) values range from 5410 cm(-)(1) (1,4- dinitrobenzene) to 3400 cm(-)(1) (9,9-dimethyl-2,7-dinitrofluorene), decreasing as the number of bonds between the nitro groups increases, except for the 1,5-dinitronaphthalene radical-anion, which has a coupling similar to that of 9,9-dimethyl-2,7-dinitrofluorene. All charge-transfer bands show vibrational fine structure. The vertical excitation energies (lambda(v)) were estimated from the vibrational components, obtained by simulation of the entire band. The large 2H(ab)/lambda(v) values confirm these radicals to be Class III delocalized mixed-valence species. Analysis using Cave and Newton's generalized Mulliken-Hush theory relating the transition dipole moment to the distance on the diabatic surfaces suggests that the electron-transfer distance on the diabatic surfaces, d(ab), is only 26-40% of the nitrogen-to-nitrogen distance, which implies that something may be wrong with our analysis.