The Role of Nonstatistical Dynamics in the Chemistry of Reactive Intermediates

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Until quite recently, experimental chemists interested in the mechanisms of organic reactions have had to rely on indirect techniques / commonly based on reaction kinetics / to infer something about the properties of reactive intermediates, because these transient species have usually not been directly observable. With the advent of picosecond and femtosecond spectroscopies direct detection of transient intermediates is sometimes possible for photochemical reactions. However, for thermally initiated reactions intermediates can only be detected if they have lifetimes in excess of ~1 microsecond, and so the indirect methods still hold sway in most mechanistic studies. The transformation of data obtained on reactants and products into insights about undetected intermediates necessarily relies on some kind of kinetic model. Obviously, if the chosen model were wrong, the conclusions about the reaction mechanisms would probably be wrong as well. There is now persuasive evidence, not only from the PI's lab but also from many other labs around the world that, at least for some reactions, this is exactly what has happened. The problem is that most of familiar kinetic models, such as Transition State Theory, rely on the validity of the so-called statistical approximation. However, experiments and molecular dynamics (MD) simulations have revealed that many thermally generated reactive intermediates exhibit nonstatistical dynamical behaviour. Nonstatistical dynamics can be summarised as follows. Most reactive intermediates are created with selective excitation of a small subset of their available vibrational modes. In thermal reactions there can be two sources of this selectivity: for reactions in which the intermediate sits on an energetic plateau or in a very shallow minimum, the selectivity arises primarily from the need to localize the excess energy of the reactant(s) in the reaction coordinate for formation of the intermediate. For reactions in which the intermediate occupies a relatively deep local minimum on the potential energy surface (PES), the largest contributor will be the potential energy (PE) to kinetic energy (KE) conversion that accompanies the progress from the first transition state to the intermediate. This conversion deposits energy in modes that are, in large measure, determined by the geometry differences of the stationary points for an intermediate and the transition structure from which it was formed. For photochemical reactions, the selectivity commonly arises from the PE to KE conversion that accompanies passage through a conical intersection. The proposal of selective excitation is not, by itself, at odds with the standard statistical approximation to reaction kinetics. The distinction arises in what one thinks happens to these selectively excited species. Statistical models, by their very nature, assume that intramolecular vibrational energy redistribution (IVR) occurs much faster than conversion of the intermediate to any product, and hence that selective excitation has no mechanistic consequence. It is that assumption that we and others have been questioning. In the present proposal we seek to accomplish two principal goals. One is to use state-of-the-art ultrafast spectroscopies to provide, for the first time, direct experimental tests of some of the principal predictions arising from MD simulation of nonstatistical dynamics. The other is to expand the range of reaction types for which tests of nonstatistical behaviour can be conducted. The latter will move us a step closer to the long-term goal of understanding in a general way when nonstatistical dynamical effects will occur, and what their consequences will be.