Local Solvent Environment Can Define Solute Chemical Identity, Dynamics, and Reactivity
Is it acceptable to assume that when a molecule is placed in solution, it retains the chemical identity and general behavior of its gas-phase counterpart? In this thesis, I explore this question through mixed quantum classical (MQC) molecular dynamics (MD) simulations of sodium dimer (Na2) and sodium dimer cation (Na2+) in liquid tetrahydrofuran (THF). Although most chemical reactions, particularly those relevant to biological systems, take place in the condensed phase, the solvent is generally thought of as a mere medium that holds the reactants and allows them to encounter each other via diffusion. Of course, there are scenarios where the solvent is known to influence the chemistry of the solute, but these cases are usually straightforward and are only thought about for a small subset of chemical reactions. However, no studies have yet described the local solvent environment as part of the chemical identity of the solute. In this thesis, I show that when there are even modest local specific interactions between a solute and solvent, the solvent controls the chemical identity of the solute, entirely changing the types of chemistry that can take place.
In the specific case of an Na2 or Na2+ solute in liquid THF, I show that local solvent molecules actually integrate as part of the the solute's identity, thus, stabilizing the solute in multiple states that differ only in the number of solvent molecules associated with the solute's identity. These stable states, which can interconvert only by surmounting a large free energy barrier, behave as chemical species distinct not only from their gas-phase counterpart but also from each other. In addition, solvent interactions can also affect the dynamics of chemical reactions, such as photodissociation. Because the lowest energy excited state of gas-phase Na2+ is dissociative, this molecule makes an ideal basis for studies of photodissociation in the condensed phase, an important probe for understanding complex reaction dynamics. In this thesis, I show that when Na2+is photoexcited in liquid THF, the initial shape of the bonding electron is completely different than that of excited state Na2+ in the gas phase. This means that unlike its gas-phase counterpart, upon photoexcitation, the bond of Na2+ solvated in liquid THF does not immediately break. Instead, the electron must dynamically rotate into an orientation more favorable for dissociation.
To investigate the dynamics of this process, I first pose a question fundamental to theoretical studies of the condensed phase: can a nonequilibrium system be understood through observation of the fluctuations of its equilibrium dynamics? This approximation, known as linear response (LR), is commonly assumed for condensed phase systems, but in this thesis I show that LR breaks down for the photodissociation of Na2+ in THF precisely because the local solvent environment experienced by the molecule varies between the equilibrium and nonequilibrium dynamics. In particular, I show that the solvent molecules associated with the solute's identity must shift from their preferred ground state positions to facilitate the rotation of the solute bonding electron into the position favorable for dissociation. Furthermore, in THF, the chemical identity of the solute can change during dissociation via the integration of new THF molecule's into the solute's identity. These processes consume most of the solute's dissociation energy, thus hindering its ability to fully dissociate.
Thus, one cannot simply assume that a solute's chemical identity is retained in solution. In fact, when there are even modest solute--solvent interactions present, the local solvent environment actually controls the solute's chemical identity, and thus also its dynamics and reactivity.