Site-specific vibrational spectral signatures of water molecules in the magic H ₃ O ⁺ (H ₂ O) ₂₀ and Cs ⁺ (H ₂ O) ₂₀ clusters

Abstract

Theoretical models of proton hydration with tens of water molecules indicate that the excess proton is embedded on the surface of clathrate-like cage structures with one or two water molecules in the interior. The evidence for these structures has been indirect, however, because the experimental spectra in the critical H-bonding region of the OH stretching vibrations have been too diffuse to provide band patterns that distinguish between candidate structures predicted theoretically. Here we exploit the slow cooling afforded by cryogenic ion trapping, along with isotopic substitution, to quench water clusters attached to the H 3 O + and Cs + ions into structures that yield well-resolved vibrational bands over the entire 215-to 3,800-cm −1 range. The magic H 3 O + (H 2 O) 20 cluster yields particularly clear spectral signatures that can, with the aid of ab initio predictions, be traced to specific classes of network sites in the predicted pentagonal dodecahedron H-bonded cage with the hydronium ion residing on the surface.water clusters | cryogenic vibrational spectroscopy | hydrogen bonding O ver the last decade, the cooperative mechanics underlying the microhydration of simple ions has undergone a renaissance due to rapid advances in experimental and theoretical methods. On the experimental side, it is routine to capture and study size-selected species cooled to cryogenic temperatures (1-5), and theoretical techniques are now capable of handling tens of atoms with all-electron, "supermolecule" approaches, where complex hydration networks are treated in the ansatz of polyatomic molecular physics (6). A dramatic example of the new insights afforded by this combined approach is the recent elucidation of the spectral signature associated with the hydronium ion when it is accommodated on the surface of a pentagonal dodecahedral cage formed by the "magic" H 3 O + (H 2 O) 20 cluster (7). The theoretical structure proposed earlier (8) (denoted I) is illustrated in Fig. 1, and the recently reported D 2 predissociation spectrum of the cryogenically cooled, D 2 tagged H 3 O + (H 2 O) 20 cluster is compared with that observed for the H 3 O + (H 2 O) 3 Eigen cation (9, 10) in Fig. 1 A and B, respectively. The key bands derived from the OH stretching motions of the surface-embedded hydronium (denoted ν a H3O +) were assigned (7) to the broad features in the experimental spectrum about 500 cm −1 below the corresponding bands in the free Eigen cation. The intramolecular HOH bending and OH stretching modes of the surface water molecules are not strongly shifted by introduction of the ion, however, where the latter appear as a broad envelope spanning the 3,000-to 3,700-cm −1 range typical of liquid water. An interesting aspect of the D 2 tagged spectrum obtained with cryogenic cooling (as opposed to that reported on the same ion generated under the more rapid quenching conditions at play in a supersonic jet ion source) (8, 11) is that distinct features begin to emerge above the continuous background absorption in the OH stretchin...

Evidence of Polaron Excitations in Low Temperature Raman Spectra of Oxalic Acid Dihydrate

“…Fournier et al studied H3O + (H2O)20 and Cs + (H2O)20 clusters and reported the umbrella mode of H3O + at 1200 cm -1 , while the asymmetric bending of the H3O + was observed around 2000 cm -1 43 . Interestingly, the intensity of the corresponding umbrella mode for D3O + around 900 cm -1 and asymmetric stretching in the interval 1500-1700 cm -1 were much weaker than in the protonated case 43 . This would explain lower intenstiy of the observed bands in the Raman spectra of (COOD)2·D2O…”

Hidden role of intermolecular proton transfer in the anomalously diffuse vibrational spectrum of a trapped hydronium ion

“…This suggests that strong nuclear quantum effects act to broaden the spectrum of the light isotopologue as well as introduce extra features arising from overtones and combination bands (12). In this context, the behavior of the closely related H 3 O + (TMA) 3 [trimethylammonium (TMA) = N(CH 3 ) 3 ] cluster reported by Fujii and coworkers (13) (reproduced in Fig.…”

Snapshots of Proton Accommodation at a Microscopic Water Surface: Understanding the Vibrational Spectral Signatures of the Charge Defect in Cryogenically Cooled H+(H2O)n=2–28 Clusters

“…31−33 This trend is illustrated by the minimum energy structures for selected clusters displayed in Figure 1. In this article, we combine recent new results on the larger, three-dimensional clusters 34,35 with earlier work on the smaller sizes to reveal the trends in the breadth, envelope shape, and overall location of the bands attributed to the proton defect. We then analyze these trends in the context of the cluster structures as well as the deformations of the electron distributions on the nearby water molecules to understand the factors that drive the large, size-dependent frequency shifts of these spectroscopic signatures.…”