Structural Information from Hydrazine Radical Cation Optical Absorption Spectra

Transition energies (E(op)) of the nitrogen-centered pi,pi absorption of tetraalkylhydrazine radical cations are quite sensitive to twist at the NN bond, nitrogen pyramidality, and mixing of the sigma orbitals with the pi system. Thirty-one examples for which E(op) varies from 63 to 107.5 kcal/mol are discussed with the aid of calculated values (E(calc)) for the 0,0 transition energy using simple (no configuration interaction) neutral-in-cation-geometry calculations on AM1-UHF geometry-optimized radical-cation structures. Significant changes in the difference between E(op) and E(calc) are observed for bis-N,N'-bicyclic systems, which are syn pyramidalized at nitrogen (twist angles near 0 degrees; E(op) about 23 kcal/mol larger than E(calc)) and for bis-N,N-bicyclic ones, which are anti pyramidalized (twist angles of 180 degrees; difference about 7 kcal/mol when calculations of 180 degrees structures are employed). Within these classes, changes in E(op) caused by changes in pyramidality and sigma,pi interaction are predicted well by the calculations. The tetraisopropylhydrazine radical cation has lambda(max) = 282 nm, but its tetracyclohexyl analogue shows two transitions, at 276 and 386 nm. This surprising difference is attributed to tetracyclohexylhydrazine radical cation having both untwisted and significantly twisted (estimated twist angle approximately 44 degrees ) forms occupied in solution, although the isopropyl compound only has the untwisted form significantly occupied.