Femtosecond Dynamics of Transition States and the Concept of Concertedness: Nitrogen Extrusion of Azomethane Reactions

In this communication, we report femtosecond, real-time studies of the nature of the transition states and the concept of concertedness in the nitrogen extrusion reaction of azomethane. Many thermal and photochemical reactions involve the breaking or making of several chemical bonds. Important to the mechanism are the following questions:  How do the two (or more) bonds break? What is the time scale for the elementary steps? What is the true nature of the transition states (TS)? The literature is very rich with discussions on the relationship between the nature of the transition state or structural intermediate and the two extreme descriptions of the reaction, concerted or consecutive, i.e., stepwise (see, e.g., refs 1 and 2). These descriptions are critical to the understanding of the reaction path, the stereochemistry, and the structure of the TS, but in many studies their definition is operational depending on the experimental approach and time scales. With femtosecond resolution, nuclear motions can be studied, allowing for a true definition of concertedness and a clear distinction between TS(s) and intermediate(s).