How to Probe Hydrated Dielectrons Experimentally: Ab Initio Simulations of the Absorption Spectra of Aqueous Dielectrons, Electron Pairs, and Hydride.

In the radiation chemistry of water, two hydrated electrons ( e hyd - ) can react to form H 2 and OH - . Experiments and simulations suggest that this reaction occurs through a mechanism involving colocalization of two e hyd - 's into the same solvent cavity, forming a hydrated dielectron ( ( e h y d ) 2 2 - ) intermediate, with aqueous hydride (H - ) as a subintermediate. However, there has been no direct experimental observation of either ( e h y d ) 2 2 - or H - . Here, we present TD-DFT-based predictions for the absorption spectrum of open-shell-singlet and triplet e hyd - pairs, ( e h y d ) 2 2 - , and H - . We find that relative to e hyd - , triplet and open-shell singlet electron pairs show spectral shifts to the blue and red, respectively. Additionally, we find that ( e h y d ) 2 2 - absorbs even further to the red, and that H - has a charge-transfer-to-solvent-like transition at wavelengths several eV to the blue, providing a direct experimental handle with which to probe these species. We propose a three-pulse transient absorption experiment that should allow detection of ( e h y d ) 2 2 - and H - .