The vibrational predissociation spectra of the $H5O2+∙RGn(RG=Ar,Ne)$ clusters: Correlation of the solvent perturbations in the free OH and shared proton transitions of the Zundel ion

 Predissociation spectra of the $H5O2+∙RGn(RG=Ar,Ne)$ cluster ions are reported in energy regions corresponding to both the OH stretching $(3350–3850cm−1)$ and shared proton $(850–1950cm−1)$ vibrations. The two free OH stretching bands displayed by the Ne complex are quite close to the band origins identified earlier in bare $H5O2+$ [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)], indicating that the symmetrical $H5O2+$ “Zundel” ion remains largely intact in $H5O2+∙Ne$⁠. The low-energy spectrum of the Ne complex is simpler than that observed previously for $H5O2+∙Ar$⁠, and is dominated by two sharp transitions at 928 and $1047cm−1$⁠, with a weaker feature at $1763cm−1$⁠. The $H5O2+∙Arn,n=1–5$ spectra generally exhibit complex band structures reflecting solvent-induced symmetry breaking of the Zundel core ion. The extent of solvent perturbation is evaluated with electronic structure calculations, which predict that the rare gas atoms should attach to the spectator OH groups of $H5O2+$ rather than to the shared proton. In the asymmetric complexes, the shared proton resides closer to the more heavily solvated water molecule, leading to redshifts in the rare gas atom-solvated OH stretches and to blueshifts in the shared proton vibrations. The experimental spectra are compared with recent full-dimensional vibrational calculations (diffusion Monte Carlo and multimode/vibrational configuration interaction) on $H5O2+$⁠. These results are consistent with assignment of the strong low-energy bands in the $H5O2+∙Ne$ spectrum to the vibration of the shared proton mostly along the O–O axis, with the $1763cm−1$ band traced primarily to the out-of-phase, intramolecular bending vibrations of the two water molecules.