The Grotthuss mechanism explains the anomalously high proton mobility in water as a sequence of proton transfers along a hydrogen-bonded (H-bonded) network. However, the vibrational spectroscopic ...signatures of this process are masked by the diffuse nature of the key bands in bulk water. Here we report how the much simpler vibrational spectra of cold, composition-selected heavy water clusters, D⁺(D₂O)n, can be exploited to capture clear markers that encode the collective reaction coordinate along the proton-transfer event. By complexing the solvated hydronium "Eigen" cluster D₃O⁺(D₂O)₃ with increasingly strong H-bond acceptor molecules (D₂, N₂, CO, and D₂O), we are able to track the frequency of every O-D stretch vibration in the complex as the transferring hydron is incrementally pulled from the central hydronium to a neighboring water molecule.
The properties of hydrogen ions in aqueous solution are governed by the ability of water to incorporate ions in a dynamical hydrogen bond network, characterized by a structural variability that has ...complicated the development of a consistent molecular level description of H+(aq). Isolated protonated water clusters, H+(H2O) n , serve as finite model systems for H+(aq), which are amenable to highly sensitive and selective gas phase spectroscopic techniques. Here, we isolate and assign the infrared (IR) signatures of the Zundel-type and Eigen-type isomers of H+(H2O)6, the smallest protonated water cluster for which both of these characteristic binding motifs coexist, down into the terahertz spectral region. We use isomer-selective double-resonance population labeling spectroscopy on messenger-tagged H+(H2O)6·H2 complexes from 260 to 3900 cm–1. Ab initio molecular dynamics calculations qualitatively recover the IR spectra of the two isomers and allow attributing the increased width of IR bands associated with H-bonded moieties to anharmonicities rather than excited state lifetime broadening. Characteristic hydrogen-bond stretching bands are observed below 400 cm–1.
We use cryogenic ion vibrational spectroscopy to characterize the structure and fluxionality of the magic number boron cluster B
. The infrared photodissociation (IRPD) spectrum of the D
-tagged all-
...B isotopologue of B
is reported in the spectral range from 435 to 1790 cm
and unambiguously assigned to a planar boron double wheel structure based on a comparison to simulated IR spectra of low energy isomers from density-functional-theory (DFT) computations. Born-Oppenheimer DFT molecular dynamics simulations show that B
exhibits internal quasi-rotation already at 100 K. Vibrational spectra derived from these simulations allow extracting the first spectroscopic evidence from the IRPD spectrum for the exceptional fluxionality of B
.
CO2 adsorption on Ti3O6 –, which serves as a model for an oxygen vacancy on a titania surface, is studied using infrared photodissociation (IRPD) spectroscopy in combination with density functional ...theory (DFT) and coupled cluster computations, as well as a recently developed multicomponent artificial force induced reaction method. The IRPD spectra of D2-tagged (Ti3O6)(CO2) n –, with n = 1, 2, are reported in the spectral window of 450–2400 cm–1 and assigned based on a comparison to harmonic IR spectra from the DFT calculations. We find that CO2 binding leaves the unpaired electron largely unperturbed. The first two CO2 molecules adsorb chemically to Ti3O6 – by incorporating a formally doubly negatively charged, either doubly or triply coordinated O atom to form a bidentate or tridentate bridging carbonate dianion (CO3 2–), respectively. The latter binding motif exhibits a characteristic IR signature in the form of an intense doublet of peaks near 1400 cm–1 stemming from two antisymmetric carbonate stretching modes.
Significance Understanding the mechanics underlying the diffuse OH stretching spectrum of water is a grand challenge for contemporary physical chemistry. Water clusters play an increasingly important ...role in this endeavor, as they allow one to freeze and isolate the spectral behavior of relatively large assemblies with well-defined network morphologies. We exploit recently developed, hybrid instruments that integrate laser spectroscopy with cryogenic ion trap mass spectrometry to capture the H ₃O ⁺ and Cs ⁺ ions in cage structures formed by 20 water molecules. Their infrared spectra reveal a pattern of distinct transitions that is unprecedented for water networks in this size range. Theoretical analysis of these patterns then reveals the intramolecular distortions associated with water molecules at various sites in the 3D cages.
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 ₃O ⁺ and Cs ⁺ ions into structures that yield well-resolved vibrational bands over the entire 215- to 3,800-cm ⁻¹ range. The magic H ₃O ⁺(H ₂O) 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.
We use cryogenic ion trap vibrational spectroscopy in combination with density functional theory (DFT) to study the adsorption of up to four water molecules on Al3O4 +. The infrared photodissociation ...spectra of Al3O4(D2O)1–4+ are measured in the O–D stretching (3000–2000 cm–1) as well as the fingerprint spectral region (1300–400 cm–1) and are assigned based on a comparison with simulated harmonic infrared spectra for global minimum-energy structures obtained with DFT. We find that dissociative water adsorption is favored in all cases. The unambiguous assignment of the vibrational spectra of these gas phase model systems allows identifying characteristic spectral regions for O–D and O–H stretching modes of terminal (μ1) and bridging (μ2) hydroxyl groups in aluminum oxide/water systems, which sheds new light on controversial assignments for solid Al2O3 phases.
We use cryogenic ion trap vibrational spectroscopy to study the structure of the protonated water pentamer, H
(H
O)
, and its fully deuterated isotopologue, D
(D
O)
, over nearly the complete ...infrared spectral range (220-4000 cm
) in combination with harmonic and anharmonic electronic structure calculations as well as RRKM modelling. Isomer-selective IR-IR double-resonance measurements on the H
(H
O)
isotopologue establish that the spectrum is due to a single constitutional isomer, thus discounting the recent analysis of the band pattern in the context of two isomers based on AIMD simulations 〈W. Kulig and N. Agmon, Phys. Chem. Chem. Phys., 2014, 16, 4933-4941〉. The evolution of the persistent bands in the D
(D
O)
cluster allows the assignment of the fundamentals in the spectra of both isotopologues, and the simpler pattern displayed by the heavier isotopologue is consistent with the calculated spectrum for the branched, Eigen-based structure originally proposed 〈J.-C. Jiang, et al., J. Am. Chem. Soc., 2000, 122, 1398-1410〉. This pattern persists in the vibrational spectra of H
(H
O)
in the temperature range from 13 K up to 250 K. The present study also underscores the importance of considering nuclear quantum effects in predicting the kinetic stability of these isomers at low temperatures.
The mechanism of dissociative D2 adsorption on Ti2O4-, which serves as a model for an oxygen vacancy on a titania surface, is studied using infrared photodissociation spectroscopy in combination with ...density functional theory calculations and a recently developed single-component artificial force induced reaction method. Ti2O4- readily reacts with D2 under multiple collision conditions in a gas-filled ion trap held at 16 K forming a global minimum-energy structure (DO-Ti-(O)2-Ti(D)-O)-. The highly exergonic reaction proceeds quasi barrier-free via several intermediate species, involving heterolytic D2-bond cleavage followed by D-atom migration. We show that, compared to neutral Ti2O4, the excess negative charge in Ti2O4- is responsible for the substantial lowering of the D2 dissociation barrier, but does not affect the molecular D2 adsorption energy in the initial physisorption step.
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