Radical Anions of Cyclic Azoalkanes: An ESR, ENDOR, and TRIPLE‐Resonance Study

Radical anions often monocyclic and bicyclic azoalkanes containing the azo group in (Z)-conformation, have been fully characterized by their hyperfine data with the use of ESR, ENDOR, and general-TRIPLE-resonance spectroscopy. These azoalkanes are represented by 3,3,5,5-tetramethyl-1-pyrazoline (1), 2,3-diaza-bicyclo[2.2.1]hept-2-ene (4), and 2,3-diazabicyclo[2.2.2]oct-2-ene (9), as well as by their derivatives 2,3, 5–8, and 10. For all radical anions 1″–10″, the 14N-coupling constant, aN, is in the range of +0. 83 to +0. 97 mT; this finding indicates that the spin population is essentially restricted to the π system of the azo group. The 14N-hyperfine anisotropy largely affects the width of ESR lines, particularly at low temperatures. Substantial coupling constants of 7Li-, 23Na-, 39K-, and 133Cs-nuclei point to a close association of the radical anions with their alkali-metal counterions. With the exception of 39K, these nuclei give rise to readily observable ENDOR signals which appear along with those stemming from protons. The prominent hyperfine features of 1″–10″ are discussed.