Ab Initio Studies of Hydrated Electron/Cation Contact Pairs: Hydrated Electrons Simulated with Density Functional Theory Are Too Kosmotropic.

We have performed the first DFT-based ab initio MD simulations of a hydrated electron ( e aq - ) in the presence of Na + , a system chosen because ion-pairing behavior in water depends sensitively on the local hydration structure. Experiments show that e aq - 's interact weakly with Na + ; the e aq - 's spectrum blue shifts by only a few tens of meV upon ion pairing without changing shape. We find that the spectrum of the DFT-simulated e aq - red shifts and changes shape upon interaction with Na + , in contrast with experiment. We show that this is because the hydration structure of the DFT-simulated e aq - is too ordered or kosmotropic. Conversely, simulations that produce e aq - 's with a less ordered or chaotropic hydration structure form weaker ion pairs with Na + , yielding predicted spectral blue shifts in better agreement with experiment. Thus, ab initio simulations based on hybrid GGA DFT functionals fail to produce the correct solvation structure for the hydrated electron.