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

Abstract

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 (14N-X-14N) and isotopically asymmetric nitroxyl biradicals, with one nitroxyl bearing 15N and the other nitroxyl bearing 14N (14N-X-15N), 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 (14N-O-15N), which exists in a condition of strong spin exchange, proved to be particularly informative. In this case, spin polarized (14N-O-15N)# (product of spin exchange with r#) and two polarized monoradicals (r14N-O-15N)# and (14N-O-15Nr)# (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 (Rex) and chemical reaction (Rrxn) were achieved through computer simulation of the TR-EPR spectra. The cross section for spin exchange, Rex, between r# and (N-X-N) biradical is estimated to be 4-6 times larger than the cross section of chemical reaction, Rrxn, between r# and (N-X-N). The para-nitroxyl biradical (14N-P-15N) exists in weak spin exchange, and behaves as an equimolar mixture of 14N and 15N mononitroxyls.