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Both Zundel and Eigen Isomers Contribute to the IR Spectrum of the Gas-Phase H9O4 + Cluster
journal contribution
posted on 2014-01-09, 00:00 authored by Waldemar Kulig, Noam AgmonThe
“Eigen cation”, H3O+(H2O)3, is the most prevalent protonated water structure
in the liquid phase and the most stable gas-phase isomer of the H+(H2O)4 cluster. Nevertheless, its 50
K argon predissociation vibrational spectrum contains unexplainable
low frequency peak(s). We have simulated the IR spectra of 10 gas-phase
H+(H2O)4 isomers, that include zero
to three argon ligands, using dipole autocorrelation functions from
ab initio molecular dynamics with the CP2K software. We have also
tested the effect of elevated temperature and dispersion correction.
The Eigen isomers describe well the high frequency portion of the
spectrum but do not agree with experiment below 2000 cm–1. Most notably, they completely lack the “proton transfer
bands” observed at 1050 and 1750 cm–1, which
characterize Zundel-type (H5O2
+) isomers. In contrast, linear isomers
with a Zundel core, although not the lowest in energy, show very good
agreement with experiment, particularly at low frequencies. Peak assignments
made with partial velocity autocorrelation functions verify that the
1750 cm–1 band does not originate with the Eigen
isomer but is rather due to coupled proton transfer/water bend in
the Zundel isomer.
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5Oautocorrelation functions50 K argon predissociation vibrational spectrumCP 2K softwareEigen isomerEigen Isomers3OEigen isomersexperimentIR spectracm9OtransferZundel coreZundel isomerpeak assignmentsdispersion correctionfrequency portionprotonated water structureab initioargon ligandsIR Spectrumvelocity autocorrelation functions
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