Femtochemistry of Norrish Type-I Reactions: IV. Highly Excited Ketones—Experimental
Femtosecond dynamics of Norrish type-I reactions of cyclic and acyclic ketones have been investigated in real time for a series of 13 compounds using femtosecond-resolved time-of-flight mass spectrometry. A general physical description of the ultrafast processes of ketones excited into a high-lying Rydberg state is presented. It accounts not only for the results that are presented herein but also for the results of previously reported studies. For highly excited ketones, we show that the Norrish type-I reaction is nonconcerted, and that the first bond breakage occurs along the effectively repulsive S_2 surface involving the C-C bond in a manner which is similar to that of ketones in the S_1 state (E. W.-G. Diau et al. ChemPhysChem 2001, 2, 273-293). The experimental results show that the wave packet motion out of the initial Franck-Condon region and down to the S_2 state can be resolved. This femtosecond (fs) internal conversion from the highly excited Rydberg state to the S_2 state proceeds through conical intersections (Rydberg-valence) that are accessed through the C=O stretching motion. In one of these conical intersections, the internal energy is guided into an asymmetric stretching mode. This explains the previously reported pronounced nonstatistical nature of the reaction. The second bond breakage involves an excited-state acyl radical and occurs on a time scale that is up to one order of magnitude longer than the first. We discuss the details regarding the ion chemistry, which determines the appearance of the mass spectra that arise from ionization on the fs time scale. The experimental results presented here, aided by the theoretical work reported in paper III, provide a unified picture of Norrish reactions on excited states and on the ground-state potential energy surfaces.