Why study reactions in nanoconfined solvents?
Nanometer-sized cavities and pores can now be routinely generated in sol-gels, supramolecular assemblies, reverse micelles, zeolites, and even proteins, giving strong impetus to improving our understanding of chemistry in confined solvents. These cavities and pores can serve as nanoscale reaction vessels in which a chemical reaction takes place in the small pool of solvent allowed in the restricted space. One ultimate goal is to control the chemistry occurring in these systems by manipulating the properties of the confining framework as well as the species present. This may have important applications for catalysis, sensing, and separations. However, there is currently little understanding about how the confining framework properties affect chemical reactivity, that is
How does a chemical reaction occur differently in a nanoconfined solvent than in a bulk solvent?
We are addressing this question using theoretical and computational approaches, within which the cavity/pore properties can be readily varied and the changes in reactivity directly examined. A key focus is understanding chemical reactions in solvents confined within nanoscale frameworks using both simple models and atomistic models of silica pores. Since charge transfer processes are typically strongly coupled to the solvent and are therefore dramatically affected by the limited number of solvent molecules, geometric constraints, and surface hydrophilicity/hydrophobicity, we are particularly interested in proton transfer reactions and time-dependent fluorescence spectroscopy (which provides insight into charge transfer processes). By understanding how reactivity is connected to the pore characteristics, these studies will assist in the development of design principles for microporous and mesoporous catalysts.
A related issue is
How can one probe the structure and dynamics of a nanoconfined liquid?
Even when the chemistry is dramatically different in confinement compared to the bulk liquid, it can be challenging to understanding the underlying molecular driving forces (e.g., liquid structure, entropic effects, hydrogen bonding). Thus, we are also investigating the molecular-level mechanisms of spectroscopic signatures of nanoconfined liquids, i.e., what information is (or is not) present in the electronic and vibrational spectra of confined liquids. Significant insight can be obtained by comparing simulated spectra, that agree with experimental measurements, to the detailed information in a molecular dynamics simulation. A particular recent focus is on nonlinear spectroscopies for probing liquid dynamics.