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Nonequilibrium energy surfaces and discrete solvent environments in solution-phase chemical bond breaking


Solvent dynamics are varied, complex, and can even change during the course of a chemical reaction. At the same time, they can be important for understanding the physics of solution-phase chemistry, thus requiring a framework for which to think about solvent effects. On one side, energy surfaces capture the average behavior of the reaction dynamics, but it is not clear how energy surfaces for solution-phase reactions ought to be constructed. On the other side, response functions capture the solvent fluctuations and provide information on how the solvent responds to changes of reacting solutes. This thesis explores the nature of solvent dynamics during the course of a bond-breaking reaction using the simulated photodissociation dynamics of Na$_2^+$ in liquid Ar and tetrahydrofuran (THF).

Following the introduction, Chapter 2, reprinted with permission from Andy Vong, Devon R. Widmer, and Benjamin J. Schwartz ``Nonequilibrium Solvent Effects During Photodissociation in Liquids: Dynamical Energy Surfaces, Caging and Chemical Identity" \textit{J. Phys. Chem. Lett.} \textbf{2020}, \textit{11}, 9230--9238, identifies key photodissociation dynamics in Ar and THF and how energy surfaces for solution-phase reactions may be constructed. The potential energy surfaces of solution-phase reactions are generally inherited from gas-phase potentials or calculated by assuming that the solvent is in equilibrium with the solute, commonly referred to as the potential of mean force. For photodissociation reactions, which are molecularly ``violent", it is unlikely for the solvent to remain at equilibrium with the dissociating solute. Alternatively, a time-integral of work expression can directly capture the nonequilibrium dynamics to create a dynamical, nonequilibrium energy surface. For Na$_2^+$ in liquid Ar, the dynamical energy surface shows clear signatures of solvent caging, and the degree of caging is directly related to the mass of the solvent atoms. For Na$_2^+$ in liquid THF, local specific interactions between the solute and solvent lead to changes in chemical identity that create a kinetic trap that effectively prevents the molecule from dissociating. For both systems, this time-integral of work expression captures the key nonequilibrium effects during bond breaking, providing an example of how solution-phase energy surfaces may be constructed and indicating how both a gas-phase energy surface and potential of mean force are inadequate for describing solution-phase dynamics.

In Chapter 3, reprinted with permission from Andy Vong and Benjamin J. Schwartz ``Bond-Breaking Reactions Encounter Distinct Solvent Environments Causing Breakdown of Linear Response" \textit{J. Phys. Chem. Lett.} \textbf{2022}, \textit{13}, 6783--6791, the nature of the solvent dynamics during the photodissociation of Na$_2^+$ in liquid Ar are followed along the bond-length coordinate. Surprisingly, we find that the solute experiences a small number of solvent environments that change in a discrete fashion as the bond lengthens. We also test a common assumption about nonequilibrium solvent fluctuations, the linear response approximation, and find that linear response fails by all measures, even when nonstationarity of solvent dynamics is considered. The observation of distinct solvent response environments with a solvent that can undergo only translational motions highlights the complexity of solute-solvent interactions, but that there are only a few environments give hope to the idea that solvation dynamics can be understood for solution-phase reactions that explore a wide configuration space, such as photodissociation.

In Chapter 4, 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" \textit{J. Phys. Chem. Lett.} \textbf{2022}, \textit{13}, 7931--7938, we improve upon the dynamical energy surface of Na$_2^+$ in THF by explicitly considering the motion of neighboring solvent molecules. Moderate locally-specific solute-solvent interactions can make it more appropriate to think of neighboring solvent molecules as a part of the solute's chemical identity. By focusing on the dynamics of a Na$_2$(THF)$_n^+$ complex, rather than just Na$_2^+$, we identify a second reaction coordinate and formulate a two-dimensional dynamical energy surface. This new energy surface highlights how solvent effects changes what would be a strictly dissociative reaction in the gas phase into a two-step, sequential reaction with the first step similar to a photoisomerization reaction, and the second step being a weakly dissociative step.

Overall, this work serves as a reference point for developing a framework for thinking about solution-phase chemistry by considering how energy surfaces might be constructed for these reactions and detailing how the solute can experience discrete changes in solvent dynamics and environments.

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