Solvent Control of Chemical Identity Can Change Photodissociation into Photoisomerization

Abstract

In solution-phase chemistry, the solvent is often considered to be merely a medium that allows reacting solutes to encounter each other. In this work, however, we show that moderate locally specific solute–solvent interactions can affect not only the nature of the solute but also the types of reactive chemistry. We use quantum simulation methods to explore how solvent participation in solute chemical identity alters reactions involving the breaking of chemical bonds. In particular, we explore the photoexcitation dynamics of Na2 + dissolved in liquid tetrahydrofuran. In the gas phase, excitation of Na2 + directly leads to dissociation, but in solution, photoexcitation leads to an isomerization reaction involving rearrangement of the first-shell solvent molecules; this isomerization must go to completion before the solute can dissociate. Despite the complexity, the solution-phase reaction dynamics can be captured by a two-dimensional energy surface where one dimension involves only the isomerization of the first-shell solvent molecules.

How Solvation Alters the Thermodynamics of Asymmetric Bond-Breaking: Quantum Simulation of NaK+ in Liquid Tetrahydrofuran

“…This means that at each bond distance and on each potential surface the solvent has had essentially infinite time to relax. Upon nonequilibrium photoexcitation of NaK + in solution, however, changes in the local THF dative bonding environment may not be able to keep up with the changing internuclear separation, especially if the datively bonded THF molecules need to isomerize prior to bond separation . Thus, in addition to the thermodynamics that we explore here, there may be kinetic limitations that cause the dissociating NaK + molecule to follow the diabatic PMFs more than the adiabatic surfaces.…”

“…In previous work, we investigated the effect of solvation on the PESs and reactivity of simple diatomic solutes like Na 2 and Na 2 + . − In weakly interacting, nonpolar solvents like liquid argon, the solvent compresses the density of the bonding electron(s), altering the PES by decreasing the equilibrium bond distance and increasing the bond vibrational frequency. ,, Even though both Ar and Na 2 are nonpolar, collisions between the solvent and solute can induce instantaneous solute dipoles due to Pauli repulsive interactions that displace the bonding electron density, changing vibrational selection rules. ,, We also saw that during photodissociation of Na 2 + , asynchronous collisions between Ar solvent atoms and the Na + photofragments induce decoherence of the bonding electron, localizing it onto a single Na + and thus breaking the symmetry and determining the dissociation products …”

“…The Na 2 + molecule undergoes similar effects when solvated in THF, exhibiting (4,5) and (5,5) coordination states . Moreover, although Na 2 + readily photodissociates in the gas phase, in THF the solvent complexes must undergo a solvent-related photoisomerization reaction prior to dissociation, requiring a 2-D effective energy surface to describe the basic reactivity of the solution-phase system …”

“…To date, the most detailed investigations of solvent effects on small molecules have explored symmetric diatomics, where there is only a single set of possible reaction products. − ,− This leads to the question of what happens when solvents interact with small molecules that are asymmetric: can the local asymmetry of a solvent environment enhance or even reverse the inherent asymmetry of a solute? To address this question, we extend our previous simulation studies to examine the behavior of NaK + in THF solution.…”

Using Machine Learning to Understand the Causes of Quantum Decoherence in Solution-Phase Bond-Breaking Reactions

“…Our simulations use mixed quantum/classical (MQC) molecular dynamics (MD) simulations, where the bonding electron is described quantum mechanically and the solvent motions are described classically. Interactions between the bonding electron and classical particles are treated through previously developed pseudopotentials. − The details of the methods are the same as those used in our previous work ,,− and can also be found in the Supporting Information. Briefly, the system is composed of a single Na 2 + solute and 1600 Ar atoms.…”