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

摘要

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 ~(15)N and the other nitroxyl bearing ~(14)N (~(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 ~(14)N and ~(15)N mononitroxyls.