DETERMINATION OF HYPERFINE SPLITTINGS OF BIRADICAL TERMINI BY COMBINING BIRADICAL TRAPPING AND TIME-RESOLVED ESR TECHNIQUES

ESR detection of spin-polarized short-lived biradicals has been shown to be a valuable tool in studying the mechanisms of photochemical reac tions and in the investigation of the properties of biradicals. Howeve r, some important spectroscopic features of these species are not read ily available because of certain peculiarities of biradical ESR spectr a. This difficulty can be overcome through the conservation of spin po larization by the controlled scavenging of a biradical into a subseque nt radical species whose spectroscopic features can be accurately eval uated. In particular, it is shown that the hyperfine splittings of a r adical site in a biradical can be measured directly by time resolved E SR if the other site is scavenged by a selective spin trap. BrCCl3 was used as a selective scavenger of the acyl moiety of polymethylene lin ked acyl-alkyl and acyl-benzyl biradicals formed in the photolysis of substituted cycloalkanones in solution. Transformation of these polari zed biradicals into correpsonding polarized monoradicals upon bromine abstraction by acyl site leads to much less complicated and much bette r resolved ESR spectra. The hyperfine splittings of the benzylic termi ni of several acyl-benzyl biradicals generated in this manner were eva luated and quantitatively compared with the splittings of benzyl and 1 -phenylpropyl radicals formed in the photolysis of linear ketones.