Spin delocalization by triple-bonded functionalities in propargyl and heteropropargyl radicals, assessed from the EPR-spectral D parameter of 1,3-cyclopentanediyl triplet diradicals

The cyclopentane-1,3-diyl triplet diradicals T and V with the triplet-bonded acetylene, cyano, and isocyano functionalities at one of the radical sites are readily prepared from the corresponding azoalkanes by photodenitrogenation in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K. The EPR-spectral D values of these triplet diradicals show that the spin delocalizing ability of the triple-bonded, pi substituent follows the order -Cequivalent toCH > -NC approximate to -CN. Good correlations of the D values have been obtained with the hyperfine coupling constants (alpha(H)) and with the calculated spin densities (PM3/AUHF-CI method) of the corresponding monoradicals M. The propargyl-type mesomeric structure is favored over the allenyl-type contributor for all three triple-bonded functionalities; spin delocalization is less pronounced in the heteropropargyl derivatives due to the electronegativity effect of the nitrogen atom.