A Time-Resolved Electron Paramagnetic Resonance Investigation of the Spin Exchange and Chemical Interactions of Reactive Free Radicals with Isotopically Symmetric (14N−X−14N) and Isotopically Asymmetric (14N−X−15N) Nitroxyl Biradicals

Interactions between reactive free radicals (r) with stable mononitroxyl radicals (N) and bisnitroxyl radicals (N−X−N) were studied by time-resolved electron paramagnetic resonance (TR-EPR). Reactive spin-polarized free radicals (r # ), with non-Boltzmann population of spin states were produced by laser flash photolysis of benzil dimethyl monoketal or of (2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (the superscript # symbol indicates electron spin polarization). Both isotopically symmetric nitroxyl biradicals ( 14 N−X− 14 N) and isotopically asymmetric nitroxyl biradicals, with one nitroxyl bearing 15N and the other nitroxyl bearing 14N ( 14 N−X− 15 N), were employed as probes of the spin exchange and chemical interactions between r and the nitroxyl biradicals. The interaction of r # with the asymmetric ortho-nitroxyl biradical ( 14 N−O− 15 N), which exists in a condition of strong spin exchange, proved to be particularly informative. In this case, spin polarized ( 14 N−O− 15 N) # (product of spin exchange with r # ) and two polarized monoradicals (r 14 N−O− 15 N) # and ( 14 N−O− 15 Nr) # (products of chemical reaction with r # ) were observed. The latter three species possess three distinct TR-EPR spectra with different line splittings. The relative cross sections for spin exchange (R ex) and chemical reaction (R rxn) were achieved through computer simulation of the TR-EPR spectra. The cross section for spin exchange, R ex, between r # and (N−X−N) biradical is estimated to be 4−6 times larger than the cross section of chemical reaction, R rxn, between r # and (N−X−N). The para-nitroxyl biradical ( 14 N−P− 15 N) exists in weak spin exchange, and behaves as an equimolar mixture of 14N and 15N mononitroxyls.