External magnetic field and magnetic isotope effects upon the lifetimes of biradicals derived from benzophenone derivatives

The decay profile of several isotope-labelled biradicals containing a benzophenone ketyl and a hydrogen-donor radical has been examined by means of nanosecond laser flash photolysis in the presence of an external magnetic field. Noticeable magnetic isotope effects are observed on the decay kinetics: deuteration of the particular CH bond in the hydrogen donor results in retardation of the biradical recombination, whereas C-13 substitution of the benzylic carbon in the hydrogen donor shortens the biradical decay time. The observed results can be interpreted in terms of the relaxation mechanism and molecular motion.