Temperature Dependence of the Hydrated Electron's g-Factor and Persistence of a Cavity-Localized Electron at Elevated...

The structure of the aqueous electron, e-(aq), has been the focus of intense scrutiny from both theory and experiment. The temperature dependence of its g-factor, measured via electron paramagnetic resonance and shifted significantly from the free-electron value, has been suggested to be inconsistent with the conventional picture of an electron localized in a solvent void or "cavity". Here, we use ab initio molecular dynamics simulations and quantum chemistry calculations to compute the ensemble-averaged, isotropic g-factor for e-(aq) over the entire temperature range of liquid water. A temperature-dependent shift is obtained, in reasonable agreement with experiment, without any qualitative change in either the localized structure of e-(aq) or its average coordination number. Far from being inconsistent with g-factor measurements, persistence of the cavity motif actually supports the rather mild temperature dependence of the g-factor because the amount of spin density hosted in oxygen orbitals is remarkably insensitive to temperature. These results support a cavity-localized picture of e-(aq) from 0-100 °C at atmospheric pressure. The simulations are also broadly consistent with spectroscopic measurements demonstrating that the molar extinction coefficient is insensitive to temperature and that there are no abrupt temperature-induced changes in the absorption spectrum, even into the supercooled and supercritical water regimes. This analysis further entrenches the cavity-centric picture of e-(aq).