Tuning aryl, hydrazine radical cation electronic interactions using substitutent effects

Absorption spectra for 2,3-diaryl-2,3-diazabicyclo [2.2.2] octane radical cations (2(X)(center dot+)) and for their monoaryl analogues 2-tert-butyl-3-aryl-2,3-diazabicyclo [2.2.2] octane radical cations (I(X)(center dot+)) having para chloro, bromo, iodo, cyano, phenyl, and nitro substituents are reported and compared with those for the previously reported 1- and 2(H)(center dot+) and 1- and 2(OMe)(center dot+). The calculated geometries and optical absorption spectra for 2(Cl)(center dot+) demonstrate that p-C6H4Cl lies between p-C6H4OMe and C6H5 in its ability to stabilize the lowest energy optical transition of the radical cation, which involves electron donation from the aryl groups toward the pi*(NN)(+)-centered singly occupied molecular orbital of 2(X)(center dot+). Resonance Raman spectral determination of the reorganization energy for their lowest energy transitions (lambda(sym)(v)) increase in the same order, having values of 1420, 5300, and 6000 cm(-1) for X = H, Cl, and OMe, respectively. A neighboring orbital analysis using Koopmans-based calculations of relative orbital energies indicates that the diabatic aryl pi-centered molecular orbital that interacts with the dinitrogen pi system lies closest in energy to the bonding pi(NN)-centered orbital and has an electronic coupling with it of about 9200 +/- 600 cm(-1), which does not vary regularly with electron donating power of the X substituent.