Both Zundel and Eigen isomers contribute to the IR spectrum of the gas-phase H9O4+ cluster.

The "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.