Reinvestigation of the Infrared Spectrum of the Gas-Phase Protonated Water Tetramer

Gas-phase H9O4+ has been considered
an archetypal Eigen cation,
H3O+(H2O)3. Yet ab initio molecular dynamics (AIMD) suggested that its infrared
spectrum is explained by a linear-chain Zundel isomer, alone or in
a mixture with the Eigen cation. Recently, hole-burning experiments
suggested a single isomer, with a second-order vibrational perturbation
theory (VPT2) spectrum agreeing with the Eigen cation. To resolve
this discrepancy, we have extended both calculations to more advanced
DFT functionals, better basis sets, and dispersion correction. For
Zundel-isomers, we find VPT2 anharmonic frequencies for four low-frequency
modes involving the excess proton unreliable, including the 1750 cm–1 band that is pivotal for differentiating between
Zundel and Eigen isomers. Because the analogous bands of the H5O2+ cation
show little effect of anharmonicity, we utilize the harmonic frequencies
for these modes. With this caveat, both AIMD and VPT2 agree on the
spectrum as originating from a Zundel isomer. VPT2 also shows that
both isomers have the same spectrum in the high frequency
region, so that the hole burning experiments should be extended to
lower frequencies.