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 T' 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 -Ctbd1;CH > -NC approximately -CN. Good correlations of the D values have been obtained with the hyperfine coupling constants (a(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.
Adam
Waldemar
W
Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany. adam@chemie.uni-wuerzburg.de
Ortega-Schulte
Claudio M
CM