NMR Study of Geminate Recombination of Radicals in Liquid Solution. Complementary Use of SCOTCH and Photo CIDNP Experiments

A 2D NMR experiment, SCOTCH (spin coherence transfer in chemical reactions), is used to study the kinetics oageminate radical pair in liquid solution. If an rf pulse is followed by a laser pulse that creates radical pairs transverse nuclear magnetization is transferred from starting material to products. The intensity of this magnetization canbe reduced by dephasing processes due to the hyperfine coupling with the free electron in the radical. A theoretical expression for this decrease is given in terms of the lifetime of the radical and the hyperfine coupling constant. Application to the photochemical reaction of pinacolone (methyl t-butylketone) in tetrachloromethane, supplemented with time resolved CIDNP measurements, led to an average lifetime of about 3 ns for the geminate radical pair and 0.4 µs for the escaping radicals.