RADICAL-ANIONS OF CYCLIC AZOALKANES - AN ESR, ENDOR, AND TRIPLE-RESONANCE STUDY

Radical anions of ten 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-diazabicyclo[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 N-14-coupling constant, a(N), is in the range of +0.83 to +0.97 mT; this finding indicates that the spin population is essentially restricted to the pi system of the azo group. The N-14-hyperfine anisotropy largely affects the width of ESR lines, particularly at low temperatures. Substantial coupling constants of Li-7, Na-23, Cs-133-nuclei point to a close association of the radical anions with their alkalimetal counterions. With the exception of K-39, 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.