Nonequilibrium Solvent Effects during Photodissociation in Liquids: Dynamical Energy Surfaces, Caging, and Chemical Identity

Abstract

In the gas phase, potential energy surfaces can be used to provide insight into the details of photochemical reaction dynamics. In solution, however, it is unclear what potential energy surfaces, if any, can be used to describe even simple chemical reactions such as the photodissociation of a diatomic solute. In this paper, we use mixed quantum/classical (MQC) molecular dynamics (MD) to study the photodissociation of Na 2 + in both liquid Ar and liquid tetrahydrofuran (THF). We examine both the gas-phase potential surfaces and potentials of mean force (PMF), which assume that the solvent remains at equilibrium with the solute throughout the photodissociation process and show that neither resemble a nonequilibrium dynamical energy surface that is generated by taking the time integral of work. For the photodissociation of 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. The results show that nonequilibrium effects play an important role even in simple solution-phase reactions, requiring the use of dynamical energy surface to understand such chemical events.

How Water–Ion Interactions Control the Formation of Hydrated Electron:Sodium Cation Contact Pairs

“…Solvated electrons are interesting objects because their properties are entirely determined by their interaction with the surrounding solvent. A solvated electron–sodium cation contact pair, however, has a behavior somewhere between that of a solvated neutral sodium atom and a solvated electron, as exemplified by previous experiments and simulations studying solvated electron–Na + contact pairs in liquid tetrahydrofuran (THF). − The question we explore in this section is for aqueous sodium cation–hydrated electron contact pairs: how do changes in the cation–water interactions affect the pair’s electronic properties?…”

Solvent Control of Chemical Identity Can Change Photodissociation into Photoisomerization

“…For example, first-shell solvent molecules can “cage” the products of photodissociation reactions, inhibiting separation of the photofragments and promoting recombination. − Additionally, photoreaction pathways and photofragment relaxation time scales can differ depending on solvent polarity − or viscosity . Moreover, solvent interactions can alter the potential energy surface on which reactions take place, changing them significantly from what they were for an isolated gas-phase solute. − Previously, we have shown that Pauli repulsion interactions from surrounding solvent molecules can compress a solute’s bonding electrons, raising a solute’s bond vibrational frequency . We have also shown that modest locally specific solute–solvent interactions, with energetics similar to those of a hydrogen bond, can change the chemical identity of a solute. , In such cases, the chemical species must be thought of as a solute–solvent complex rather than a gas-phase solute perturbed by solvent interactions. , …”

“…We simulate photoexcitation of the solute by taking uncorrelated equilibrium configurations of the ground-state system and placing the bonding electron onto its first excited state at time zero, then propagating dynamics adiabatically to generate a nonequilibrium ensemble of 20 trajectories. We chose this system because it is readily amenable to study via MQC MD: the electronic structure of gas-phase Na 2 + is fairly easy to describe because of the relative lack of exchange and correlation contributions between the valence bonding electron and the core electrons, and the necessary electron–Na + core and electron–THF pseudopotentials have already been developed and thoroughly benchmarked. − The methods we employ here are similar to those in our previous work, ,, and they reproduce the gas-phase quantum chemistry of Na 2 + , and experimental properties of Na + :solvated electron tight contact pairs in liquid THF quite well. , Further details are given in Methods and the Supporting Information.…”

“…As the molecule separates, half the bonding electron is associated with each Na + core, as there is nothing in the gas phase to break the symmetry to localize the electron and create the eventual Na 0 + Na + photoproducts. We , and Gervais and co-workers , have also explored how Na 2 + behaves in Ar following photoexcitation. Gervais and co-workers also noticed an inversion of electronic structure character with Li 2 + in Ne n clusters, although from diabatic crossing …”

“…The fact that the initial photoexcitation produces an excited-state wave function with more π character than σ* character suggests that there is little driving force to separate the Na + ions in the Franck–Condon region: in other words, the presence of the datively bonded THF solvents means that photoexcitation is not initially dissociative . Yet, if trajectories are run for a sufficiently long time, i.e., a time much longer than necessary for the gas-phase photodissociation reaction, the Na + ions in liquid THF eventually do separate.…”