Relaxation Pathways of Photoexcited Iodide–Methanol Clusters: A Computational Investigation

Solvent Control of Chemical Identity Can Change Photodissociation into Photoisomerization

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.

Time-resolved radiation chemistry: dynamics of electron attachment to uracil following UV excitation of iodide-uracil complexes

Electron attachment to uracil was investigated by applying time-resolved photoelectron imaging to iodide-uracil (I(-)U) complexes. In these studies, an ultraviolet pump pulse initiated charge transfer from the iodide to the uracil, and the resulting dynamics of the uracil temporary negative ion were probed. Five different excitation energies were used, 4.00 eV, 4.07 eV, 4.14 eV, 4.21 eV, and 4.66 eV. At the four lowest excitation energies, which lie near the vertical detachment energy of the I(-)U complex (4.11 eV), signatures of both the dipole bound (DB) as well as the valence bound (VB) anion of uracil were observed. In contrast, only the VB anion was observed at 4.66 eV, in agreement with previous experiments in this higher energy range. The early-time dynamics of both states were highly excitation energy dependent. The rise time of the DB anion signal was ∼250 fs at 4.00 eV and 4.07 eV, ∼120 fs at 4.14 eV and cross-correlation limited at 4.21 eV. The VB anion rise time also changed with excitation energy, ranging from 200 to 300 fs for excitation energies 4.00-4.21 eV, to a cross-correlation limited time at 4.66 eV. The results suggest that the DB state acts as a "doorway" state to the VB anion at 4.00-4.21 eV, while direct attachment to the VB anion occurs at 4.66 eV.