Reactive Systems at the Onset of Microsolvation

Abstract

Proposal: CHE-0132584

PI: Leopold, Kenneth R.

The effect of solvation on molecular structure and reactivity is an important problem in chemistry. The investigation of microsolvated species offers a valuable means of bridging gas phase measurements with studies of condensed matter. Such work offers the possibility of developing an increasingly accurate molecular-level description of solvation. In this project, rotational spectroscopy will be used to examine the effects of microsolvation on reactive acid-base complexes. Molecular structures and dipole moments will be determined from rotational constants and Stark effect data, respectively. Aspects of charge distribution will, in certain cases, also be revealed by nuclear hyperfine structure. Two classes of systems are targeted: (i) partially bound Lewis acid-base complexes, whose structure and bonding change dramatically upon crystallization and (ii) hydrogen bonded systems which undergo proton transfer in solution or in the crystalline phase. These systems are chosen because in both cases, aggregation promotes chemical change and the effects of microsolvation are, therefore, expected to be pronounced. The central question to be addressed in this work is , 'In systems for which aggregation drives chemical change, how big is the effect of the first near neighbor?'. Several systems with higher degrees of solvation are also considered. This work will systematically exploit the hypersensitivity of these reactive acid-base complexes to their near-neighbor interactions for the purpose of investigating 'solvation' effects at the small cluster level.

Much of the chemistry important to human activity involves substances in solution. For example, the chemistry of living things, reactions in contaminated ground waters, and even a host of important industrial processes occur only when molecules are dissolved in a solvent. When such reactions occur, the solvent itself is not a direct participant in the chemistry, though it has become increasingly clear that its presence can play a significant role in mediating the reactions that take place. Thus, in this project, we are interested in studying the role of solvent in mediating chemical processes. Our approach is to gain a basic understanding of the problem by examining the effect of a single solvent molecule close to a pair of reactive molecules. To accomplish this, we isolate two reacting molecules, plus one solvent molecule, and use microwaves to investigate how the solvent affects them. Understanding the influence of a single solvent molecule is an important step, as it is the fundamental building block for understanding the influence of the many solvent molecules present in a real solution. Moreover, the basic knowledge gained from this work may have direct consequences for modeling difficult problems of societal importance, e.g., the formation of particles in the earth's atmosphere, whose presence has wide ranging effects on climate, ozone levels, and human health. In this way, we envision potential practical applications for the fundamental science addressed in this project. Graduate students at the University of Minnesota will participate in this research as part of their training toward the doctoral degree and will learn numerous technical skills applicable to a wide variety of problems.