TIME-RESOLVED ELECTRON-PARAMAGNETIC RESONANCE STUDIES ON THE LOWEST EXCITED TRIPLET-STATES OF ALIPHATIC CARBONYL-COMPOUNDS

The time-resolved electron paramagnetic resonance (TREPR) technique is applied to study the properties of the lowest excited triplet states of aliphatic carbonyls such as acyclic and cyclic ketones, aldehydes, and the methyl derivatives of cyclohexanone. The zero-field splittings (zfs) and the populating rates obtained from the analysis of the TREPR spectra are found to depend considerably on the individual molecule, particularly alkyl groups attached to the carbonyl group. From the analysis of the zfs, the distortion angles in the pyramidally distorted triplet states are estimated, for example, 40° and 32° for acetone and isopropionaldehyde, respectively. The populating rates are also analyzed to examine the triplet geometry. A good correlation is found between the angles obtained from the analyses of the zfs and those from the populating rates. A brief discussion is given on the reactivities of the triplet states such as hydrogen abstraction and α-cleavage reactions in relation to the observed triplet geometries. © 1990 American Chemical Society.