Research

Molecular Reaction Dynamics

 

Figure 1, Reaction coordinate and transition state.  Product state distributions (vibration, rotation, etc.), hint at the transition state geometry

Chemical reaction mechanisms are of central importance to organic, inorganic, and biochemistry.  Armed with a solid understanding of the mechanisms behind chemical reactions, chemists are able to predict and design syntheses of new and important compounds.  However, despite of the importance of the reaction mechanism, the transition state is often poorly characterized experimentally. 

Something of a misnomer, the transition state is not so much a “state” as it is a point along its path to products.  In most cases, it represents some restricted geometry, through which a molecule must transform on its way to products.  For example, in Figure 1 to the right, the transition state lies at the top of a barrier between reactants and products. 

While transition states play such an important role in chemistry, very little direct evidence of their structure exists due to their inherently transient nature. Historically, researchers were limited to performing kinetics experiments that could, at best, provide supporting evidence for a proposed mechanism.  Over the last 40 years, however, physical chemists / chemical physicists  have become increasingly sophisticated in their ability to probe chemical transition states at an ever increasing level of detail.

The field of research which has grown out of these studies is known as “molecular reaction dynamics.”  It’s goal is to understand chemical reactions at their most fundamental, quantum mechanical level of detail.  Great advances have been made within the field both experimentally and theoretically and it is now possible, for some small molecular systems, to completely characterize a reaction from the initial quantum state of the reactants to the final product state distributions. With the advent of femtosecond spectroscopy, even the transition state may be probed as the reaction unfolds. 

In my research, rather than trying to probe the transitions state directly, we attempt to learn as much as possible about the transition state structure through a detailed analysis of the quantum states of the product molecules; much like a police officer analyzes the debris of an accident to determine what happened.  Unlike more traditional methods for doing this, such as Laser Induce Fluorescence (LIF), we do this with pure rotational spectroscopy, as outlined in the “research overview” section.