The Single Photon Initiated Dissociative Rearrangement Reactions (SPIDRR) technique was developed by the Bellert group at Baylor University to study the kinetics and dynamics of metal mediated chemical reactions. The instrument capable of these studies is depicted above. The types of reactions studied is indicated below where M is a transition metal and R indicates an organic fragment.
The Bellert Group is interested in applying SPIDRR to the study of photocatalysis to better understand how the absorption of visible light leads to chemical transformations. In photocatalysis the energy gained from the absorption of light by a chromophore is converted into kinetic energy that drives atomic motions. A rearrangement reaction, often consisting of an oxidative addition/reductive elimination sequence, ensues if the activating photon energy is greater than reaction requirements. Here, the transition metal within the M(organic)+ complex serves as the both the chromophore (allowing visible photon absorption) and activating agent within the complex. Interaction between the organic molecule and open-shell of the transition metal cation opens lower energy pathways to the rearrangement dissociative reaction.
The SPIDRR technique temporally and selectively monitors the production of products in such metal mediated reactions. The measurement of rate constants, product branching distributions, the change in rate constant due to the kinetic isotope effect, and the energy dependence of each of these measurements offers a thorough experimental data set from which an atomistic interpretation of these chemical transformations can be obtained.
It is the electronically unsaturated structure of the metal center that opens lowered energy pathways to the dissociative reaction. Moreover, the electronic density of the transition metal suggests that reaction profiles characterized by different spin states (often called spin surfaces) may interact during the reaction progression. This, and other non-adiabatic interactions, may be the dominating characteristic influencing reaction dynamics. This means that the often used quantum mechanic adiabatic approximation may be invalid when computing the reaction profile for such photocatalytic active sites. Thus, the results of SPIDRR can serve as a training ground for computational chemists that wish to go beyond traditional quantum chemical approaches to describe these important model catalytic chemical reactions.