DIRECT FEMTOSECOND OBSERVATION OF THE TRANSIENT INTERMEDIATE IN THE ALPHA-CLEAVAGE REACTION OF (CH3)(2)CO TO 2CH(3)+CO - RESOLVING THE ISSUE OF CONCERTEDNESS

When a reaction involving two equivalent bonds has sufficient energy to break both of them, it can proceed by either a concerted or a stepwise mechanism. For Norrish type-I and other reactions, this issue has been controversial since direct time resolution of the individual C-C cleavage events was not possible. Here, for the elementary α-cleavage of acetone, we report on the femtosecond resolution of the intermediates using mass spectrometry. The results show the nonconcertedness of the reaction, provide the times for the primary and secondary C-C breakage, and indicate the role of electronic structure (σ*, antibonding impulse) and the vibrational motions involved. © 1995 American Institute of Physics.