Ab Initio Investigation of the Resonance Raman Spectrum of the Hydrated Electron

Abstract

According to the conventional picture, the aqueous or “hydrated” electron, e –(aq), occupies an excluded volume (cavity) in the structure of liquid water. However, simulations with certain one-electron models predict a more delocalized spin density for the unpaired electron, with no distinct cavity structure. It has been suggested that only the latter (non-cavity) structure can explain the hydrated electron’s resonance Raman spectrum, although this suggestion is based on calculations using empirical frequency maps developed for neat liquid water, not for e –(aq). All-electron ab initio calculations presented here demonstrate that both cavity and non-cavity models of e –(aq) afford significant red-shifts in the O–H stretching region. This effect is nonspecific and arises due to electron penetration into frontier orbitals of the water molecules. Only the conventional cavity model, however, reproduces the splitting of the H–O–D bend (in isotopically mixed water) that is observed experimentally and arises due to the asymmetric environments of the hydroxyl moieties in the electron’s first solvation shell. We conclude that the cavity model of e –(aq) is more consistent with the measured resonance Raman spectrum than is the delocalized, non-cavity model, despite previous suggestions to the contrary. Furthermore, calculations with hybrid density functionals and with Hartree–Fock theory predict that non-cavity liquid geometries afford only unbound (continuum) states for an extra electron, whereas in reality this energy level should lie more than 3 eV below vacuum level. As such, the non-cavity model of e –(aq) appears to be inconsistent with available vibrational spectroscopy, photoelectron spectroscopy, and quantum chemistry.

Role of hemibonding in the structure and ultraviolet spectroscopy of the aqueous hydroxyl radical

“…76 Periodic boundary conditions were implemented using the QM/MM-Ewald technique that we have previously described 77,78 and applied to systems such as e − (aq). 78,79 Periodic images of the QM atoms are represented using ChElPG charges, [80][81][82] derived from the QM electrostatic potential. For efficiency, we use an implementation of the ChElPG algorithm based on atom-centered Lebedev grids, 77 with 50 angular points per radial shell.…”

Simulating the Ghost: Quantum Dynamics of the Solvated Electron

“…The spectrum of the solvated electron gives rise to an unusual vibrational feature, containing a doublet in the bending region. This can only arise from HOD molecules close to the cavity: one peak corresponds to those that have the O-H oriented towards the electron, the other to those having the O-D oriented towards it [30]. We predict a peak splitting with two maxima at 1338, 1399 cm −1 , which are assigned to H-O-D-e − and D-O-H-e − bending modes, which corresponds closely to the experimentally-observed splitting of 60 cm −1 , although the absolute peak positions are redshifted.…”

Can Hydrated Electrons be Produced from Water with Visible Light?

“…[7][8][9][10][11][12][13][14] The main-stream opinion favours the so-called cavity model, that is, a localized electron cloud confined in a cavity of the hydrogen-bonded network of water. [5][6][7]12,15,16] Alternative models with more molecular flavour, such as H 3 OÀ OH À complexes, were proposed early by Robinson and co-workers [17] and Tuttle and Golden. [18] The hydrated hydronium, H 3 OÀ (H 2 O) n , model of the solvated electron is supported by more recent ab initio calculations for finite-size clusters.…”

“…While the properties and the reactivity of the hydrated electron were extensively investigated since decades, [2,3] the structure of this defect of liquid water at the atomic level is still a matter of considerable debate [7–14] . The main‐stream opinion favours the so‐called cavity model, that is, a localized electron cloud confined in a cavity of the hydrogen‐bonded network of water [5–7,12,15,16] . Alternative models with more molecular flavour, such as H 3 O−OH − complexes, were proposed early by Robinson and co‐workers [17] and Tuttle and Golden [18] .…”