EPR of transient free radicals during photochemical reactions in high temperature and pressure gases

Well-resolved EPR spectra are reported for transient organic radicals produced by photolysis of ketones in gas phase solutions at pressures and temperatures of up to 150 bar and 720 K. Under these conditions a-cleavage is more important than at ambient temperatures, and for excited acetone the rate constant is estimated as greater than or equal to 7 x 10(7) s(-1) at 600 K and 140 bar. The EPR line widths are found to be controlled by spin-rotational interaction and are well predicted from theory for radicals with radii >0.25 nm. Analysis of the hydroxy-proton coupling of the 2-hydroxy-2-propyl radical gives a rotation barrier of 18.1 kJ mol(-1) about the C-alpha-OH bond. This increases to 22.7 kJ mol(-1) on addition of acetone which is attributed to hydrogen bonding.