Mark D. Marshall Research Interests
From the very first courses in introductory chemistry, the concept of a barrier or activation energy that must be overcome if two chemical species are to react is presented as a ubiquitous feature of chemical kinetics. Indeed, this feature is responsible for the qualitative observation that the vast majority of chemical reactions proceed faster as the temperature is raised, as well as the quantitative relation embodied in the observation of Arrhenius that a graph of the natural logarithm of the rate constant versus the reciprocal of the absolute temperature results in a straight line whose slope is simply related to the activation energy, ln k = -(Ea/R)(1/T) + ln A. Nevertheless, there are chemical reactions that deviate from this behavior. There are many examples of reactions, particularly radical recombinations, that proceed with a vanishingly small barrier, and have reaction rates that are nearly independent of temperature. A few reactions, characterized by a negative value for the Arrhenius activation energy, have the surprising property of slowing down as the temperature is increased, or equivalently, the reaction rate increases as the temperature is lowered. Among several explanations offered for this behavior is the formation of a weakly bound complex corresponding to a minimum on the reaction coordinate leading from separated reactants to the transition state. Recently, such complexes have been shown to have profound influence on chemical reaction rates and product distributions even in the absence of a negative value for the activation energy. Consequently, it is important to determine the structure and dynamics of these weakly bound, reactant complexes and to characterize both the forces that are responsible for their formation and the subsequent influence of these forces on chemical reactivity. Accurate calculations for systems containing more than three or four heavy atoms remain unavailable from ab initio methods, thus experiments on these systems will be particularly valuable for the development of theory and subsequently, the advancement of our understanding of the effect of long range forces on chemical dynamics and reactions. My current research interests combine three lines of inquiry, tied together not only by the common theme of the changes in electron density, but also by common techniques. The first investigates electron rearrangement in reactant complexes of the OH radical. The second addresses possible electron transfer in complexes with metal atoms, and the third seeks to understand the effects of electron density on complex geometry. All systems are studied using Fourier transform microwave (FTMW) spectroscopy supplemented when possible by ab initio calculations.
An important class of reactions in which unusual kinetic behavior has been observed, such as negative activation energies, low pre-exponential A factors, and large kinetic isotope effects, and for which reactant complexes have been implicated, are those involving the hydroxyl radical, OH. Reactions of the OH radical are of fundamental importance in atmospheric and combustion processes and serve as a prototype for chemical reactivity. For example, the reaction of OH with carbon monoxide, CO, provides a principal source of CO2 in combustion and is an important OH removal pathway in the atmosphere. In earlier work with the reactant complex of OH with carbon monoxide, we have shown that reactive species can be stabilized for spectroscopic study, even if the energy of the transition state lies below the separated reactant asymptote. Later, a direct connection was demonstrated between observed intermolecular bending vibrational states for the OH-CO reactant complex and the minimum energy path to the transition state for reaction to HOCO. Additional interesting questions concerning the reactions of open-shell diatomic radicals arise regarding the influence of any electronic orbital angular momentum present in the radical reactant as the system evolves along the reaction coordinate from reactants to products in which this momentum is quenched. For the OH radical in particular, the orbital degeneracy of the 2Π ground state is resolved into separate 2A' and 2A" states, differing only in the orientation of the half-filled pπ orbital of OH with respect to the nuclear framework, as the radical approaches a closed shell partner in nonlinear geometries. Typically only one of these states leads to reaction, but as long as the energy difference between the two is smaller than the 140 cm-1 spin-orbit splitting of the free OH radical, the states remain mixed and the electronic orbital angular momentum is not quenched.
Throughout my career, my research interests have focused on the nature of intermolecular forces. An early scientific paper, published while an undergraduate more than twenty years ago, was cited in a 2002 investigation of the interactions between the ammonia molecule and rare gas atoms. More recently, two papers characterizing the complex formed between the OH radical and the acetylene molecule published in the Journal of Chemical Physics were chosen for inclusion in The Virtual Journal of Biological Physics Research as seminal contributions at the frontiers of research. The common theme in all my work has been the application of the detailed molecular information that comes from high resolution spectroscopy to address questions concerning intermolecular forces.