Photoreduction of benzophenone by acetonitrile: correlation of rates of hydrogen abstraction from RH with the ionization potentials of the radicals R.

Triplet benzophenone (/sup 4/K) abstracts a hydrogen from acetonitrile with a rate constant of 130 +/- 30 M/sup -1/ s/sup -1/. Despite this low rate constant acetonitrile is not really an inert solvent and at low light fluxes, where T-T annihilation is not a major fate of the triplet, a major decay path for /sup 4/K is hydrogen abstraction with resulting pinacol (K/sub 2/H/sub 2/) formation (phi/sub K/sub 2/H/sub 2/ approx. = 0.1 at t = 0). Both K/sub 2/H/sub 2/ formation and /sup 3/K lifetime rapidly decrease with irradiation due to the light absorbing transients (LAT's) which are formed along with K/sub 7/H/sub 2/ from ketyl radicals (KH). The rate constants per hydrogen (k/sub H/) for abstraction from R-H by the electrophilic /sup 3/K correlate well with the ionization potentials (IP) of the corresponding radicals (R).