The D parameter of vinyl-substituted 1,3-cyclopentanediyl triplet diradicals as a sensitive tool To determine electronic substituent effects in allylic radicals

Abstract

The zero-field D parameter of the localized vinyl-substituted 1, 3-cyclopentanediyl triplet diradicals V was determined at 77 K in a 2-methyltetrahydrofuran (2-MTHF) glass matrix. Good linear correlations were obtained with the reported alpha-hyperfine coupling constants (r(2) = 0.991, n = 7) and with the semiempirically calculated (PM3) spin densities (r(2) = 0.989, n = 16) of the corresponding allylic monoradicals A. The observed substituent effects are generally larger and, thus, more accurately measured compared to the previously examined aryl-substituted 1, 3-cyclopentanediyl triplet diradicals P. The vinyl-substituted triplet diradicals reflect accurately the delocalizing propensity of substituents, either through hyperconjugative, mesomeric, or inductive effects. For the methyl group, the small but significant stabilization of adjacent radical sites has been clearly demonstrated. In the case of the halogen set, the small but definitive heavy-atom effect has been determined for the bromo (0. 0010 cm(-)(1)) and the iodo (0.0024 cm(-)(1)) substituents. The organometallic substituents SiMe(3) and SnMe(3) are shown to be weak spin acceptors, while spin delocalization for the sulfur series follows the increasing order MeS >> MeSO(2) > MeSO.