Doctor of Philosophy (PhD)
Modeling reactivity in chemical systems has evolved dramatically in line with the capabilities of modern computing. Despite the advances in computational ability, the level in which one can model a system depends on a number of factors including the region of reactivity, size of the system, level of sophistication required in the molecular description, and so on. Electronic structure methods allow for a detailed description of the potential energy surface and inherently include all essential physics required for reactivity to occur, however these methods are limited by their computational expense. On the other hand, force fields allow for an atomistic description of the interactions and drastically reduce the simulation time, yet typical force fields are dependent on a fixed bond topology, and as such, cannot model bond cleavage and formation.
This dissertation addresses modeling reactivity from electronic structure methods to force field development for reactive systems. The first section of the dissertation will focus on the hydrated HCl system. Accurately modeling covalent HCl, as well as ionization and subsequent proton shuttling, is essential in systems such as gas-liquid nucleation in the atmosphere, concentrated acid solutions, and HCl at the air-water interface. The amount of sampling required for gas-liquid nucleation pathways, or simulation time for large system sizes in the case of concentrated acid simulations necessitates an expedient description of the potential energy surface. To this end, a reactive force field has been developed. In order to determine the solvent environment factors required for an accurate force field description, ab initio molecular dynamics and metadynamics have been performed on HCl(H2O)n(n=1-22). These simulations will be discussed in chapter two, while the development and performance of a reactive force field based on the multi-state empirical bond formalism will be described in chapter three. The second section of the dissertation will focus on modeling reactivity with electronic structure methods for two organic systems. The systems range from determining the factors guiding the regioselectivity of silyloxyallyl cations by analyzing reaction profiles, SAPT energy decomposition, and molecular orbital analysis (chapter four), to the formation of an EDA complex and the corresponding charge transfer (chapter five).
Bresnahan, Caitlin Gibson, "Modeling Chemical Reactivity in Aqueous and Organic Systems: From Electronic Structure Methods to Force Field Development" (2019). LSU Doctoral Dissertations. 5031.