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 a t 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 int eraction) neutral-in-cation-geometry calculations on AM1-UHF geometry- optimized radical-cation structures. Significant changes in the differ ence between E-op and E-calc are observed for bis-N,N'-bicyclic system s, which are syn pyramidalized at nitrogen (twist angles near 0 degree s; 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; diff erence about 7 kcal/mol when calculations of 180 degrees structures ar e employed). Within these classes, changes in E-op caused by changes i n pyramidality and sigma, pi interaction are predicted well by the cal culations. The tetraisopropylhydrazine radical cation has lambda(max) = 282 nm, but its tetracyclohexyl analogue shows two transitions, at 2 76 and 386 nm. This surprising difference is attributed to tetracycloh exylhydrazine radical cation having both untwisted and significantly t wisted (estimated twist angle approximate to 44 degrees) forms occupie d in solution, although the isopropyl compound only has the untwisted form significantly occupied.