Determination of Hyperfine Splittings of Biradical Termini by Combining BiradicalTrapping and Time-Resolved ESR Techniques

摘要

ESR detection of spin-polarized short-lived biradicals has been shown to be avaluable tool in studying the mechanisms of photochemical reactions and in the investigation of the properties of biradicals. However, some important spectroscopic features of these species are not readily available because of certain peculiarities of biradical ESR spectra. This difficulty can be overcome through the conservation of spin polarization by the controlled scavenging of a biradical into a subsequent radical species whose spectroscopic features can be accurately evaluated. In particular, it is shown that the hyperfine splittings of a radical site in a biradical can be measured directly by time resolved ESR if the other site is scavenged by a selective spin trap. BrCCl3 was used as a selective scavenger of the acyl moiety of polymethylene linked acyl-alkyl and acyl-benzyl biradicals formed in the photolysis of substituted cycloalkanones in solution. Transformation of these polarized biradicals into corresponding polarized monoradicals upon bromine abstraction by acyl site leads to much less complicated and, much better resolved ESR spectra.... Biradicals, Time-resolved ESR, Photolysis, Cycloalkanones.