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The Structure of the Hydrated Electron. Part 2. A Mixed Quantum/Classical Molecular Dynamics Embedded Cluster Density Functional Theory: Single−Excitation Configuration Interaction Study
journal contribution
posted on 2007-06-21, 00:00 authored by Ilya A. Shkrob, William J. Glover, Ross E. Larsen, Benjamin J. SchwartzAdiabatic 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 Å × 18 Å × 18 Å 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 (∼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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Keywords
electron densityHydrated ElectronMQC MD simulationsoxygen atomspoint chargessolvation cavityRaman spectramultielectron pictureEPRDFT modelOH groups300 Kcharge densitywater cluster anionsresonance Raman experimentsESEEMPart 2.hfcc tensorsmultielectron effectsCISecho envelope modulationvibrational spectra2 p orbitalsIRabsorption spectrasolvation shellsfrontier orbitals
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