Understanding the Chemical Dynamics of Proton-Transfer Reactions

University Of Iowa, Iowa City IA

Investigators

Abstract

CHE-1361765: Christopher Cheatum, University of Iowa, "Understanding the chemical dynamics of proton-transfer reactions" With this award, the Chemical Structure, Dynamics and Mechanisms A program is funding the research of Christopher Cheatum, University of Iowa, to investigate a key chemical process known as a proton transfer reaction. This simple reaction in which a hydrogen atom, stripped of its single electron, is moved from one molecule to another forms the basis for a large number of more complex reactions in biology and other settings. A familiar example is the reaction between an acid, such as vinegar, and a base, such as baking soda. The vigorous chemical reaction that results when these two are brought into contact generates a great deal of heat and energy but involves, at the molecular level, just a simple movement of a single hydrogen from the vinegar to the baking soda. In this project, the investigators are using a sophisticated technique known as two-dimensional infrared spectroscopy (2D IR) to follow the movement of the hydrogen atom as it occurs during the chemical reaction. The specific project funded here is to study a model system using a small molecule, formic acid, reacting with a simple base similar to ammonia. A model system is being used in order to fully develop the observation technique and, eventually, apply it to more complex systems. In contrast to earlier studies which involved the application of laser light to force the movement of the hydrogen atom from one molecule to another, this research is investigating this important reaction under ordinary conditions, where the hydrogen moves by itself. The results are, thus, expected to provide a more realistic glimpse into what actually happens during this important, fundamental chemical process. The work is having a broad impact on many fields of science through the development of new scientific instruments that can be used to study a variety of phenomena from biology to materials science and beyond. The work is having a further broad impact on the training of the next generation of scientists through the involvement of students at all levels, even elementary school, in this research. Proton-transfer reactions are fundamental in chemistry and have important applications in biology, energy, and the environment. Proton transfers are challenging to understand and control because they can involve quantum tunneling of the proton, solvent control of the reaction coordinate, and strong anharmonic couplings of the proton to many degrees of freedom of the complex. Given the importance of proton-transfer reactions and the difficulties associated with accurate modeling of their chemical dynamics, there is a critical need for a ground-electronic-state, experimental model system in which the kinetics and dynamics of the proton transfer can be measured directly. The goal of this project is to develop such a model system and to use that system to test theoretical predictions about the factors that control the reaction dynamics. This work is yielding insights into proton-transfer reaction dynamics that will improve the models that scientists use to think about these reactions and how to control them. The project is divided into three stages: 1. Development of a model system for studying the kinetics of proton-transfer reactions. Preliminary data suggest that complexes of deuterated formic acid with tertiary amines have the necessary spectroscopic and chemical properties to access the proton-transfer reaction kinetics via 2D IR chemical-exchange measurements. 2. Determining the factors that control the kinetic properties of the model proton-transfer system. A working hypothesis is that measurements of the primary kinetic isotope effect, the temperature dependence of the rate and the isotope effect, and the variation of the rate with the solvent properties will reveal the factors that control the proton-transfer reaction dynamics and will serve as a benchmark for computational methods. 3. Understanding the effect of vibrational excitation on the proton-transfer equilibrium. A working hypothesis here is that vibrational relaxation via solvation and nonadiabatic transitions perturbs the proton-transfer equilibrium as predicted by some theoretical models.