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)^̇+) and for their monoaryl analogues 2-tert-butyl-3-aryl-2,3-diazabicyclo[2.2.2]octane radical cations (1(X) ^̇+) having para chloro, bromo, iodo, cyano, phenyl, and nitro substituents are reported and compared with those for the previously reported 1- and 2(H)^̇+ and 1- and 2(OMe)^̇+. The calculated geometries and optical absorption spectra for 2(Cl)^̇+ demonstrate that P-C₆H₄Cl lies between p-C₆H ₄OMe 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 π*(NN)⁺-centered singly occupied molecular orbital of 2(X)^̇+. Resonance Raman spectral determination of the reorganization energy for their lowest energy transitions (λ_v^sym) 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 π-centered molecular orbital that interacts with the dinitrogen π system lies closest in energy to the bonding π(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.