pH Dependence of the Lifetime of a Norrish II Biradical

Article abstract of DOI:10.1021/ja00304a022

The transient spectroscopy of γ-phenylbutyrophenone in a 2:1 (v/v) methanol:water mixture has been studied as a function of pH.The lifetime of the Norrish II 1,4-biradical shows an excellent titration curve.The moderately strongly absorbing acidic form of the biradical has a lifetime of 125 ns, with λ_max of the difference spectrum with starting material at 320 nm.The strongly absorbing basic form has a lifetime of 62 ns and λ_max 325 nm.The pK_a in the mixed solvent is 11.8.The pK_a in pure water, based on solvent dependence of pK_a values for phenols of similar acidity, is estimated as 10.2+/-0.2, essentially identical with a value reported for the acetophenone ketyl monoradical.