The photochemistry of propanone and its derivatives as studied using flash-photolysis ESR; electron spin polarization (CIDEP) in radical products

By direct observation of the transient radical products of the flash photolysis of ketones related to propanone, three different primary reactions have been demonstrated. Propanone and butanone exhibit both Norrish type 1 and hydrogen-abstraction reactions whose relative contributions depend upon the solvent used, but 3,3-dimethyl-2-butanone exhibits a dominant bond scission (Norrish type 1) reaction. Ketones with hydroxyl substituents exhibit only the latter reaction, as it appears do those with chlorine substituents, but in these, in contrast to the others, it is the β bond which breaks. This might suggest a change in the character of the triplet state from which the radicals are produced from nπ * to ππ * . The CIDEP observed is consistent with this interpretation. Most spectra exhibit net polarization whose phase varies with the nature of the substituent, and which originates in the triplet mechanism. The significance of the observations to the controversial polarization observed in the propanone ketyl radical produced in the reactions of propanone with a suitable hydrogen source is discussed. Unusual observations of acyl radicals are reported, with some resolved hyperfine structure.