UCLA

Although theoretical treatments of gas phase chemical reactions provide an intuitive representation how such reaction should proceed, it is questionable whether similar approaches can be applied in the condensed phase. Most reactions of interest do not take place in vacuum but within a solvent medium; indeed, some solutes, such as hydrated electrons, have no gas phase counterparts. Thus to understand chemical reactivity in solution, the role of solvent molecules and their effects on chemical reactions need to be explored. In this thesis, I present theoretical simulations of how explicit solvent molecules alter solution-phase reactions relative to their gas-phase counterparts - demonstrating how solvents qualitatively alter the nature of chemical reactions.

Chapter 2 presents work, reprinted with permission from Kenneth J. Mei, William R. Borrelli, Andy Vong, and Benjamin J. Schwartz. ”Using Machine Learning to Understand the Causes of Quantum Decoherence in Solution-Phase Bond-Breaking Reactions” J. Phys. Chem. Lett. 2024, 15, 903-911, doi.org/10.1021/acs.jpclett.3c03474, investigating how a simple solvent, such as liquid argon, affects the photodissociation products of Na+2 . In the gas phase, theoretical predictions suggest that the single bonding electron of Na+2 remains in a superposition of positional quantum states, each centered on one of the Na+ cores, indefinitely. In solution, the local solvent environment breaks the symmetry and causes collapse, or decoherence, of the bonding electron wavefunction onto one the two Na+ photofragments. We find that the solvent motions underlying this decoherence event is high-dimensional, requiring Machine Learning (ML) to adequately predict which Na+ fragment the electron localizes onto. ML identifies the key features behind this process to be a minimal degree of photofragment separation and the presence of out-of-phase solute-solvent collisions.

In Chapter 3, reprinted with permission from Andy Vong, Kenneth J. Mei, Devon R. Widmer, and Benjamin J. Schwartz. ”Solvent Control of Chemical Identity Can Change Photodissociation into Photoisomerization” J. Phys. Chem. Lett. 2022, 13, 7931-7938. doi.org/10.1021/acs.jpclett.2c01955. Here, we perform simulations of Na+2 in a moderately interacting solvent, liquid tetrahydrofuran (THF). THF makes locally-specific solute-solvent dative bonds that can alter the solute, so that the first-shell THF solvent molecules must be thought of as part of the solute molecule. Rather than observing a clean photodissociation reaction, as with Na+2 in the gas phase, the Na2(THF)+n complex undergoes a photoinduced isomerization of the datively-bound THFs before photodissociation can occur. In this system, solvation qualitatively alters the nature of the reaction from photodissociation in the gas phase to a two-step photoisomerization and photodissociation reaction in solution.

The contents of Chapter 4 are reprinted with permission from Kenneth J. Mei and Benjamin J. Schwartz. ”How Solvation Alters the Thermodynamics of Asymmetric Bond- Breaking: Quantum Simulation of NaK+ in Liquid Tetrahydrofuran” J. Phys. Chem. Lett. 2024, 15, 8187-8195. doi.org/10.1021/acs.jpclett.4c01636. Here, we further investigate the role of solvents by looking at the dissociation of a heteronuclear molecule, NaK+. In the gas phase, the products of dissociation are Na0+K+ on the electronic ground state and Na++K0 on the first excited state, a result on the higher electron affinity of Na+. However, we find that solvation in liquid THF, switches the ground- and excited-state dissociation products, making the Na++K0 products more thermodynamically stable. In turn, the switching of ground and excited state products induces a crossing of the ground- and excited-state free energy surfaces, suggesting the presence of a long-range electron transfer reaction that must be modulated by solvent motions.

Chapter 5 investigates the spectral signatures of a hydrogen evolution reaction involving two hydrated electrons, reproduced with permission from Kenneth J. Mei, William R. Borrelli, Jose L. Guardado Sandoval, Benjamin J. Schwartz. ”How to Probe Hydrated Dielectrons Experimentally: Ab Initio Simulations of the Absorption Spectra of Aqueous Dielectrons, Electron Pairs, and Hydride” J. Phys. Chem. Lett. 2024, 15, 9557-9565. doi.org/10.1021/acs.jpclett.4c02404. For the past few decades, this reaction has been speculated to involve the hydrated dielectron and aqueous hydride as intermediates. However, these intermediates have eluded direct experimental detection to date. In another publication, William R. Borrelli, Jose L. Guardado Sandoval, Kenneth J. Mei, and Benjamin J. Schwartz. ”The Roles of H-Bonding and Hydride Solvation in the Reaction of Hydrated (Di)electrons with Water to create H2 and OH−” J. Chem. Theory Comput. 2024, 20, 16,7337-7346, doi.org/10.1021/acs.jctc.4c00780, we found that the water hydrogen bond network is necessary to initiate the hydrogen evolution reaction through shuttling hydroxide away from the reaction center through a Grotthus-type proton hopping mechanism. In this Chapter, solvent fluctuations that bring separate hydrated electrons closer together exhibit either a blue-shift or red-shift of their absorption spectrum depending on their relative spin states. Additionally, I present the spectral signatures of the hydrated dielectron and aqueous hydride intermediates, providing possible guidelines for an experiment to directly measure.

Overall, this body of work demonstrates that solvents are not necessarily inert mediabut can play an integral role in chemical reactions, in some cases, qualitatively altering the nature of a chemical reaction or significantly changing the products.