Abstract
Adiabatic mixed quantum/classical (MQC) molecular dynamics (MD)
simulations were used to generate snapshots of the hydrated electron in
liquid water at 300 K. Water cluster anions that include two complete
solvation shells centered on the hydrated electron were extracted from
the MQC MD simulations and embedded in a roughly 18 angstrom x 18
angstrom x 18 angstrom matrix of fractional point charges designed to
represent the rest of the solvent. Density functional theory (DFT) with
the Becke-Lee-Yang-Parr functional and single-excitation configuration
interaction (CIS) methods were then applied to these embedded clusters.
The salient feature of these hybrid DFT(CIS)/MQC MD calculations is
significant transfer (similar to 18\%) of the excess electron's charge
density into the 2p orbitals of oxygen atoms in OH groups forming the
solvation cavity. We used the results of these calculations to examine
the structure of the singly occupied and the lower unoccupied molecular
orbitals, the density of states, the absorption spectra in the visible
and ultraviolet, the hyperfine coupling (hfcc) tensors, and the
infrared (IR) and Raman spectra of these embedded water cluster anions.
The calculated hfcc tensors were used to compute electron paramagnetic
resonance (EPR) and electron spin echo envelope modulation (ESEEM)
spectra for the hydrated electron that compared favorably to the
experimental spectra of trapped electrons in alkaline ice. The
calculated vibrational spectra of the hydrated electron are consistent
with the red-shifted bending and stretching frequencies observed in
resonance Raman experiments. In addition to reproducing the
visible/near IR absorption spectrum, the hybrid DFT model also accounts
for the hydrated electron's 190-nm absorption band in the ultraviolet.
Thus, our study suggests that to explain several important
experimentally observed properties of the hydrated electron,
many-electron effects must be accounted for: one-electron models that
do not allow for mixing of the excess electron density with the
frontier orbitals of the first-shell solvent molecules cannot explain
the observed magnetic, vibrational, and electronic properties of this
species. Despite the need for multielectron effects to explain these
important properties, the ensemble-averaged radial wavefunctions and
energetics of the highest occupied and three lowest unoccupied orbitals
of the hydrated electrons in our hybrid model are close to the s- and
p-like states obtained in one-electron models. Thus, one-electron
models can provide a remarkably good approximation to the multielectron
picture of the hydrated electron for many applications; indeed, the two
approaches appear to be complementary.
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