Anne McCoy of The Ohio State University is supported by an award from the Theoretical and Computational Chemistry program within the Division of Chemistry for the development of theoretical tools to unravel the spectroscopy and dynamics of molecular systems that undergo large amplitude motions. The work involves extensions of the diffusion Monte Carlo (DMC) technique including the development of "on-the-fly" DMC. The research focuses on systems of current interest to experimentalists as well as theorists.
By opening up a broad range of molecular and ionic systems for investigation, the work is having a broad impact on the study of systems of astrochemical and environmental importance. The majority of students engaged in this research are from under-represented groups in the physical sciences.
One way to understand the nature of bonding in molecules and chemical reactivity is by understanding their spectroscopy, or basically what colors of light molecules are able to absorb. The work done in this project has focused on energy absorbed through primarily molecular vibrations. We focus on developing tools needed to make connections between the concept of chemical bonding and reactions and the colors of light that are absorbed in making molecules vibrate. The research that was supported by this grant also involved studying molecules or molecular systems that either are important in atmospheric or astrochemistry, or which serve as models for more general chemical phenomina. Finally, research focused on gaining insights into the what makes a molecule absorb light of some wave lengths and not others. Much of the underlying theory has been developed and the spectroscopy is well understood for well-behaved molecules that upon vibrational excitation undergo small amplitude oscullations around their minimum energy structure. We, on the other hand have studied molecules for which this is not the case. Specific targets and studies include: Using spectroscopy to determine the relative amounts of products the reactions of NO2 with OH as part of a larger collaboration to understand this reaction which removes two radicals that are involved in ozone formation in the troposphere. Studying the spectroscopy of CH5+ and H5+, two molecules that are expected to be found in space, but the only way to know if they exist and how much there is is by understanding their spectroscopy. Understanding the nature of CH bond formation by investigating the spectroscopy of formate (HCO2-) in isolation and in complexes with water or chemically inert species. Understanding the spectrum of water through studies of H3O+.X3 and complexes of halide ions with water molecules. These studies provide spectral signatures for various bonding motifs and strength of H-bonding in water. On the theory side, we expanded significantly the types of molecules and motions that can be studied using a method called Diffusion Monte Carlo to include the ability to evaluate energies of states with significant rotational excitation. We also developed an approach that allows us to study how a molecule behaves (according to quantum mechanics) as it dissociates and applied the approach to studies of the dissociation of CH5+. Through a combined experimental/theoretical study, we investigated mechanisms for solvent-induced long-range charge transfer by looking at the photodissociation dynamics of IBr-.CO2 Finally, we extended those studies to look at the electronic structure and photo-dissociation dynamics of ICN-, which is a prototype molecule with very complicated dynamics.
