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

Abstract

Predissociation spectra of the H(5)O(2) (+)RG(n)(RG = Ar,Ne) cluster ions are reported in energy regions corresponding to both the OH stretching (3350-3850 cm(-1)) and shared proton (850-1950 cm(-1)) vibrations. The two free OH stretching bands displayed by the Ne complex are quite close to the band origins identified earlier in bare H(5)O(2) (+) [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 H(5)O(2) (+) "Zundel" ion remains largely intact in H(5)O(2) (+)Ne. The low-energy spectrum of the Ne complex is simpler than that observed previously for H(5)O(2) (+)Ar, and is dominated by two sharp transitions at 928 and 1047 cm(-1), with a weaker feature at 1763 cm(-1). The H(5)O(2) (+)Ar(n),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 H(5)O(2) (+) 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 H(5)O(2) (+). These results are consistent with assignment of the strong low-energy bands in the H(5)O(2) (+)Ne spectrum to the vibration of the shared proton mostly along the O-O axis, with the 1763 cm(-1) band traced primarily to the out-of-phase, intramolecular bending vibrations of the two water molecules.

The vibrational spectra of protonated water clusters: A benchmark for self-consistent-charge density-functional tight binding

“…The peaks around 700-1900 cm −1 correspond to the out-of-phase bending vibrations of the flanking water molecules. 37,54,55 For the protonated water clusters, the OH stretching peaks have a lower intensity in the FT-DAC spectrum compared to the FT-VAC spectrum, which has also been observed by Iyengar et al 40,42 The FT-VAC spectra show a peak around 1450 cm −1 , which is also present in NMA with a relatively low intensity. The component FT-VAC analyses indicate that this peak is mainly due to the oscillation of the shared proton perpendicular to the O-O axis.…”

“…The component FT-VAC analyses indicate that this peak is mainly due to the oscillation of the shared proton perpendicular to the O-O axis. However, this band is nearly absent in both the FT-DAC and experimental spectra, 36,37,54,55 indicating that the corresponding mode does not significantly change the dipole moment, i.e., not IR active.…”

“…In this study, we carry out SCC-DFTB calculations for the IR spectra of various protonated water clusters in the gas phase and compare the results to experimental spectra, which only became available very recently. [30][31][32][33][34][35][36][37] The experimental spectra exhibit different signatures for protonated water clusters of different sizes as well as for their neutral counterparts. Therefore, these characteristic signatures can be used to probe the possible existence and structure of a ͑protonated/ neutral͒ water network in the active site of enzymes.…”

Proton transfer in liquid water confined inside graphene slabs

“…Protonated water clusters also provide very valuable information to understand proton and water properties at interfaces. First, characteristic signatures of Zundel and Eigen species have been revealed by photoevaporation of weakly bound argon in photofragmentation mass spectroscopy and compared to ab-initio data at MP2 level [33,34], with reasonably good agreement in most cases. Second, infrared spectroscopy of protonated benzene-water nanoclusters [35] indicated a local ordering of the first water shell around the proton induced by the interface.…”

“…The results are shown in Fig.7. We have chosen to show roughly the full frequency range, since although specific spectral signatures of proton vibrations are located between 1400 and 3000 wavenumbers [48,72], some authors like Hammer et al [33] include the range between 850 and 1400 as relevant for shared proton motions.…”