Structural information from hydrazine radical cation optical absorption spectra

Abstract

Transition energies (Eop) of the nitrogen-centered π,π* absorption of tetraalkylhydrazine radical cations are quite sensitive to twist at the NN bond, nitrogen pyramidality, and mixing of the σ orbitals with the π system. Thirty-one examples for which Eop varies from 63 to 107.5 kcal/mol are discussed with the aid of calculated values (Ecalc) 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 Eop and Ecalc are observed for bis-N,N'-bicyclic systems, which are syn pyramidalized at nitrogen (twist angles near 0°; Eop about 23 kcal/mol larger than Ecalc) and for bis-N,N-bicyclic ones, which are anti pyramidalized (twist angles of 180°; difference about 7 kcal/mol when calculations of 180° structures are employed). Within these classes, changes in Eop caused by changes in pyramidality and σ,π interaction are predicted well by the calculations. The tetraisopropylhydrazine radical cation has λmax = 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 ≈ 44°) forms occupied in solution, although the isopropyl compound only has the untwisted form significantly occupied.

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