Investigating Intramolecular H Atom Transfer Dynamics in ß-Diketones with Ultrafast Infrared Spectroscopies and Theoretical Modeling.

The vibrational signatures and ultrafast dynamics of the intramolecular H-bond in a series of ß-diketones are investigated with 2D IR spectroscopy and computational modeling. The chosen ß-diketones exhibit a range of H atom donor-acceptor distances and asymmetry along the H atom transfer coordinate that tunes the intramolecular H-bond strength. The species with the strongest H-bonds are calculated to have very soft H atom potentials, resulting in highly red-shifted OH stretch fundamental frequencies and dislocation of the H atom upon vibrational excitation. These soft potentials lead to significant coupling to the other normal mode coordinates and give rise to the very broad vibrational signatures observed experimentally. The 2D IR spectra in both the OH and OD stretch regions of the light and deuterated isotopologues reveal broadened and long-lived ground-state bleach signatures of the vibrationally hot molecules. Polarization-sensitive transient absorption measurements in the OH and OD stretch regions reveal notable isotopic differences in orientational dynamics. Orientational relaxation was measured to occur on ∼600 fs and ∼2 ps time scales for the light and deuterated isotopologues, respectively. The orientational dynamics are interpreted in terms of activated H/D atom transfer events driven by collective intramolecular structural rearrangements.

Two-Dimensional Infrared Spectroscopy of Isolated Molecular Ions.

Two-dimensional infrared (2D IR) spectroscopy of mass-selected, cryogenically cooled molecular ions is presented. Nonlinear response pathways, encoded in the time-domain photodissociation action response of weakly bound N2 messenger tags, were isolated using pulse shaping techniques following excitation with four collinear ultrafast IR pulses. 2D IR spectra of Re(CO)3(CH3CN)3+ ions capture off-diagonal cross-peak bleach signals between the asymmetric and symmetric carbonyl stretching transitions. These cross peaks display intensity variations as a function of pump-probe delay time due to coherent coupling between the vibrational modes. Well-resolved 2D IR features in the congested fingerprint region of protonated caffeine (C8H10N4O2H+) are also reported. Importantly, intense cross-peak signals were observed at 3 ps waiting time, indicating that tag-loss dynamics are not competing with the measured nonlinear signals. These demonstrations pave the way for more precise studies of molecular interactions and dynamics that are not easily obtainable with current condensed-phase methodologies.

Ultrafast transient vibrational action spectroscopy of cryogenically cooled ions.

Ultrafast transient vibrational action spectra of cryogenically cooled Re(CO)3(CH3CN)3+ ions are presented. Nonlinear spectra were collected in the time domain by monitoring the photodissociation of a weakly bound N2 messenger tag as a function of delay times and phases between a set of three infrared pulses. Frequency-resolved spectra in the carbonyl stretch region show relatively strong bleaching signals that oscillate at the difference frequency between the two observed vibrational features as a function of the pump-probe waiting time. This observation is consistent with the presence of nonlinear pathways resulting from underlying cross-peak signals between the coupled symmetric-asymmetric C≡O stretch pair. The successful demonstration of frequency-resolved ultrafast transient vibrational action spectroscopy of dilute molecular ion ensembles provides an exciting, new framework for the study of molecular dynamics in isolated, complex molecular ion systems.

Unraveling the Vibrational Spectral Signatures of a Dislocated H Atom in Model Proton-Coupled Electron Transfer Dyad Systems.

Phenol-benzimidazole and phenol-pyridine proton-coupled electron transfer (PCET) dyad systems are computationally investigated to resolve the origins of the asymmetrically broadened H-bonded OH stretch transitions that have been previously reported using cryogenic ion vibrational spectroscopy in the ground electronic state. Two-dimensional (2D) potentials describing the strongly shared H atom are predicted to be very shallow along the H atom transfer coordinate, enabling dislocation of the H atom between the donor and acceptor groups upon excitation of the OH vibrational modes. These soft H atom potentials result in strong coupling between the OH modes, which exhibit significant bend-stretch mixing, and a large number of normal mode coordinates. Vibrational spectra are calculated using a Hamiltonian that linearly and quadratically couples the H atom potentials to over two dozen of the most strongly coupled normal modes treated at the harmonic level. The calculated vibrational spectra qualitatively reproduce the asymmetric shape and breadth of the experimentally observed bands in the 2300-3000 cm-1 range. Interestingly, these transitions fall well above the predicted OH stretch fundamentals, which are computed to be surprisingly red-shifted (<2000 cm-1). Time-dependent calculations predict rapid (<100 fs) relaxation of the excited OH modes and instant response from the lower-frequency normal modes, corroborating the strong coupling predicted by the model Hamiltonian. The results highlight a unique broadening mechanism and complicated anharmonic effects present within these biologically relevant PCET model systems.

Origins of the diffuse shared proton vibrational signatures in proton-coupled electron transfer model dyad complexes.

Phenol-benzimidazole and phenol-pyridine dyad complexes have served as popular model systems for the study of proton-coupled electron transfer (PCET) kinetics in solution-phase experiments. Interpretation of measured PCET rates in terms of key structural parameters, such as the H-bond donor-acceptor distance, however, remains challenging. Herein, we report vibrational spectra in the electronic ground state for a series of phenol-benzimidazole and phenol-pyridine complexes isolated and cryogenically cooled in an ion trap. The four models studied each display highly red-shifted and broadened OH stretching transitions that arise from strong H-bonding interactions between the phenol OH group and the basic N site on benzimidazole/pyridine rings. The OH stretch transition in each model displays relatively strong absorption onsets near 2500 cm-1 with broad shoulders that extend asymmetrically to higher frequencies over hundreds of wavenumbers. In contrast, the deuterated isotopologues yield much weaker OD stretch transitions that appear symmetrically broadened. The spectral breadth and shape of the OD stretch transitions are ascribed to variations in OD stretch frequencies that arise from zero-point distributions in the proton donor-acceptor low-frequency soft mode vibration. The asymmetric structure of the OH stretch transitions is attributed to a set of combination bands between the OH stretch and a series of low-frequency H-bond soft modes. The spectra and modeling highlight the importance of OH stretch-soft mode couplings, which are thought to play important roles in PCET and proton transfer dynamics.

Vibrational Dynamics of the Intramolecular H-Bond in Acetylacetone Investigated with Transient and 2D IR Spectroscopy.

Acetylacetone (AcAc) has proven to be a fruitful but highly challenging model system for the experimental and computational interrogation of strong intramolecular hydrogen bonds. Key questions remain, however, regarding the identity of the minimum-energy structure of AcAc and the dynamics of intramolecular proton transfer. Here, we investigate the OH/OD stretch and bend regions of the enol tautomer of AcAc and its deuterated isotopologue with transient absorption and 2D IR spectroscopy. The OH bend region reveals a single dominant diagonal transition near 1625 cm-1 with intense cross peaks to lower-frequency modes, demonstrating highly mixed fingerprint transitions that contain OH bend character. The anharmonic coupling of the OH bend results in a highly elongated OH bend excited-state absorption transition that indicates a large manifold of OH bend overtone/combination bands in the OH stretch region that leads to strong bend-stretch Fermi resonance interactions. The OH and OD stretch regions consist of broad ground-state bleach signals, but there is no clear evidence of ω21 excited-state absorptions due to rapid population relaxation arising from strong intramolecular coupling to bending, fingerprint, and low-frequency H-bond modes. Orientational relaxation dynamics persist for timescales longer than the vibrational lifetimes, with polarization anisotropy components decaying within approximately 2 and 10 periods of the O-O oscillation for the OH and OD stretch, respectively. The significant isotopic dependence of the orientational dynamics is discussed in the context of intramolecular mode coupling, diffusional processes, and contributions from proton/deuteron transfer dynamics.

Time-Domain Vibrational Action Spectroscopy of Cryogenically Cooled, Messenger-Tagged Ions Using Ultrafast IR Pulses.

Herein, we present the initial steps toward developing a framework that will enable the characterization of photoinitiated dynamics within large molecular ions in the gas phase with temporal and energy resolution. We combine the established techniques of tag-loss action spectroscopy on cryogenically trapped molecular ions with ultrafast vibrational spectroscopy by measuring the linear action spectrum of N2-tagged protonated diglycine (GlyGlyH+·N2) with an ultrafast infrared (IR) pulse pair. The presented time-domain data demonstrate that the excited-state vibrational populations in the tagged parent ions are modulated by the ultrafast IR pulse pair and encoded through the messenger tag-loss action response. The Fourier transform of the time-domain action interferograms yields the linear frequency-domain vibrational spectrum of the ion ensemble, and we show that this spectrum matches the linear spectrum collected in a traditional manner using a frequency-resolved IR laser. Time- and frequency-domain interpretations of the data are considered and discussed. Finally, we demonstrate the acquisition of nonlinear signals through cross-polarization pump-probe experiments. These results validate the prerequisite first steps of combining tag-loss action spectroscopy with two-dimensional IR spectroscopy for probing dynamics in gas-phase molecular ions.

Probing Hydrogen-Bonding Interactions within Phenol-Benzimidazole Proton-Coupled Electron Transfer Model Complexes with Cryogenic Ion Vibrational Spectroscopy.

Hydrogen-bonding interactions within a series of phenol-benzimidazole model proton-coupled electron transfer (PCET) dyad complexes are characterized using cryogenic ion vibrational spectroscopy. A highly red-shifted and surprisingly broad (>1000 cm-1) transition is observed in one of the models and assigned to the phenolic OH stretch strongly H-bonded to the N(3) benzimidazole atom. The breadth is attributed to a combination of anharmonic Fermi-resonance coupling between the OH stretch and background doorway states involving OH bending modes and strong coupling of the OH stretch frequency to structural deformations along the proton-transfer coordinate accessible at the vibrational zero-point level. The other models show unexpected protonation of the benzimidazole group upon electrospray ionization instead of at more basic remote amine/amide groups. This leads to the formation of HO-+HN(3) H-bond motifs that are much weaker than the OH-N(3) H-bond arrangement. H-bonding between the N(1)H+ benzimidazole group and the carbonyl on the tyrosine backbone is the stronger and preferred interaction in these complexes. The results show that conjugation effects, secondary H-bond interactions, and H-bond soft modes strongly influence the OH-N(3) interaction and highlight the importance of the direct monitoring of proton stretch transitions in characterizing the proton-transfer reaction coordinate in PCET systems.

Entropic barriers in the kinetics of aqueous proton transfer.

Aqueous proton transport is uniquely rapid among aqueous processes, mediated by fluctuating hydrogen bond reorganization in liquid water. In a process known as Grotthuss diffusion, the excess charge diffuses primarily by sequential proton transfers between water molecules rather than standard Brownian motion, which explains the anomalously high electrical conductivity of acidic solutions. Employing ultrafast IR spectroscopy, we use the orientational anisotropy decay of the bending vibrations of the hydrated proton complex to study the picosecond aqueous proton transfer kinetics as a function of temperature, concentration, and counterion. We find that the orientational anisotropy decay exhibits Arrhenius behavior, with an apparent activation energy of 2.4 kcal/mol in 1M and 2M HCl. Interestingly, acidic solutions at high concentration with longer proton transfer time scales display corresponding decreases in activation energy. We interpret this counterintuitive trend by considering the entropic and enthalpic contributions to the activation free energy for proton transfer. Halide counteranions at high concentrations impose entropic barriers to proton transfer in the form of constraints on the solution's collective H-bond fluctuations and obstruction of potential proton transfer pathways. The corresponding proton transfer barrier decreases due to weaker water-halide H-bonds in close proximity to the excess proton, but the entropic effects dominate and result in a net reduction in the proton transfer rate. We estimate the activation free energy for proton transfer as ∼1.0 kcal/mol at 280 K.

Direct Observation of Ion Pairing in Aqueous Nitric Acid Using 2D Infrared Spectroscopy.

Ion-ion interactions and ion pairing play an important role in the properties of concentrated electrolyte solutions, yet remain difficult to study due to the heterogeneous and highly dynamic behavior of these systems. In concentrated acid solutions, these questions take on a further level of complexity because the structure of the aqueous proton itself is uncertain and may be influenced by the counterion. Here, we address these questions by studying the IR spectra of nitric acid as a function of concentration in H2O and comparing these to the spectra of several alkali nitrate salts. We show how the close proximity between cations and NO3- ions in solution at high concentration affect the IR spectra and therefore the molecular structures. Using two-dimensional IR spectroscopy, we demonstrate the formation of contracted ion pair configurations in nitric acid solutions between NO3- ions and H+(aq) via the observation of a distinct anisotropic intermolecular crosspeak between these species. By studying the concentration dependence of this spectral feature, we show that this ion-paired configuration exists in solution at concentrations as low as 2 M and suggests that the structure of H+(aq) solvation complex in these ion pairs differs from the structure in bulk solution.

Broadband 2D IR spectroscopy reveals dominant asymmetric H5O2+ proton hydration structures in acid solutions.

Given the critical role of the aqueous excess proton in redox chemistry, determining its structure and the mechanism of its transport in water are intense areas of experimental and theoretical research. The ultrafast dynamics of the proton's hydration structure has made it extremely challenging to study experimentally. Using ultrafast broadband two-dimensional infrared spectroscopy, we show that the vibrational spectrum of the aqueous proton is fully consistent with a protonated water complex broadly defined as a Zundel-like H5O2+ motif. Analysis of the inhomogeneously broadened proton stretch two-dimensional lineshape indicates an intrinsically asymmetric, low-barrier O-H+-O potential that exhibits surprisingly persistent distributions in both its asymmetry and O-O distance. This structural characterization has direct implications for the extent of delocalization exhibited by a proton's excess charge and for the possible mechanisms of proton transport in water.

Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast IR Spectroscopy.

Aqueous proton transport involves the ultrafast interconversion of hydrated proton species that are closely linked to the hydrogen bond dynamics of water, which has been a long-standing challenge to experiments. In this study, we use ultrafast IR spectroscopy to investigate the distinct vibrational transition centered at 1750 cm-1 in strong acid solutions, which arises from bending vibrations of the hydrated proton complex. Broadband ultrafast two-dimensional IR spectroscopy and transient absorption are used to measure vibrational relaxation, spectral diffusion, and orientational relaxation dynamics. The hydrated proton bend displays fast vibrational relaxation and spectral diffusion timescales of 200-300 fs; however, the transient absorption anisotropy decays on a remarkably long 2.5 ps timescale, which matches the timescale for hydrogen bond reorganization in liquid water. These observations are indications that the bending vibration of the aqueous proton complex is relatively localized, with an orientation that is insensitive to fast hydrogen bonding fluctuations and dependent on collective structural relaxation of the liquid to reorient. We conclude that the orientational relaxation is a result of proton transfer between configurations that are well described by a Zundel-like proton shared between two flanking water molecules.

Delocalization and stretch-bend mixing of the HOH bend in liquid water.

Liquid water's rich sub-picosecond vibrational dynamics arise from the interplay of different high- and low-frequency modes evolving in a strong yet fluctuating hydrogen bond network. Recent studies of the OH stretching excitations of H2O indicate that they are delocalized over several molecules, raising questions about whether the bending vibrations are similarly delocalized. In this paper, we take advantage of an improved 50 fs time-resolution and broadband infrared (IR) spectroscopy to interrogate the 2D IR lineshape and spectral dynamics of the HOH bending vibration of liquid H2O. Indications of strong bend-stretch coupling are observed in early time 2D IR spectra through a broad excited state absorption that extends from 1500 cm-1 to beyond 1900 cm-1, which corresponds to transitions from the bend to the bend overtone and OH stretching band between 3150 and 3550 cm-1. Pump-probe measurements reveal a fast 180 fs vibrational relaxation time, which results in a hot-ground state spectrum that is the same as observed for water IR excitation at any other frequency. The fastest dynamical time scale is 80 fs for the polarization anisotropy decay, providing evidence for the delocalized or excitonic character of the bend. Normal mode analysis conducted on water clusters extracted from molecular dynamics simulations corroborate significant stretch-bend mixing and indicate delocalization of δHOH on 2-7 water molecules.

IR spectral assignments for the hydrated excess proton in liquid water.

The local environmental sensitivity of infrared (IR) spectroscopy to a hydrogen-bonding structure makes it a powerful tool for investigating the structure and dynamics of excess protons in water. Although of significant interest, the line broadening that results from the ultrafast evolution of different solvated proton-water structures makes the assignment of liquid-phase IR spectra a challenging task. In this work, we apply a normal mode analysis using density functional theory of thousands of proton-water clusters taken from reactive molecular dynamics trajectories of the latest generation multistate empirical valence bond proton model (MS-EVB 3.2). These calculations are used to obtain a vibrational density of states and IR spectral density, which are decomposed on the basis of solvated proton structure and the frequency dependent mode character. Decompositions are presented on the basis of the proton sharing parameter δ, often used to distinguish Eigen and Zundel species, the stretch and bend character of the modes, the mode delocalization, and the vibrational mode symmetry. We find there is a wide distribution of vibrational frequencies spanning 1200-3000 cm-1 for every local proton configuration, with the region 2000-2600 cm-1 being mostly governed by the distorted Eigen-like configuration. We find a continuous red shift of the special-pair O⋯H+⋯O stretching frequency, and an increase in the flanking water bending intensity with decreasing δ. Also, we find that the flanking water stretch mode of the Zundel-like species is strongly mixed with the flanking water bend, and the special pair proton oscillation band is strongly coupled with the bend modes of the central H5O2+moiety.

Spectroscopic snapshots of the proton-transfer mechanism in water.

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.

Anharmonic exciton dynamics and energy dissipation in liquid water from two-dimensional infrared spectroscopy.

Water's extended hydrogen-bond network results in rich and complex dynamics on the sub-picosecond time scale. In this paper, we present a comprehensive analysis of the two-dimensional infrared (2D IR) spectrum of O-H stretching vibrations in liquid H2O and their interactions with bending and intermolecular vibrations. By exploring the dependence of the spectrum on waiting time, temperature, and laser polarization, we refine our molecular picture of water's complex ultrafast dynamics. The spectral evolution following excitation of the O-H stretching resonance reveals vibrational dynamics on the 50-300 fs time scale that are dominated by intermolecular delocalization. These O-H stretch excitons are a result of the anharmonicity of the nuclear potential energy surface that arises from the hydrogen-bonding interaction. The extent of O-H stretching excitons is characterized through 2D depolarization measurements that show spectrally dependent delocalization in agreement with theoretical predictions. Furthermore, we show that these dynamics are insensitive to temperature, indicating that the exciton dynamics alone set the important time scales in the system. Finally, we study the evolution of the O-H stretching mode, which shows highly non-adiabatic dynamics suggestive of vibrational conical intersections. We argue that the so-called heating, commonly observed within ∼1 ps in nonlinear IR spectroscopy of water, is a nonequilibrium state better described by a kinetic temperature rather than a Boltzmann distribution. Our conclusions imply that the collective nature of water vibrations should be considered in describing aqueous solvation.

Interplay of Ion-Water and Water-Water Interactions within the Hydration Shells of Nitrate and Carbonate Directly Probed with 2D IR Spectroscopy.

The long-range influence of ions in solution on the water hydrogen-bond (H-bond) network remains a topic of vigorous debate. Recent spectroscopic and theoretical studies have, for the most part, reached the consensus that weakly coordinating ions only affect water molecules in the first hydration shell. Here, we apply ultrafast broadband two-dimensional infrared (2D IR) spectroscopy to aqueous nitrate and carbonate in neat H2O to study the solvation structure and dynamics of ions on opposite ends of the Hofmeister series. By exciting both the water OH stretches and ion stretches and probing the associated cross-peaks between them, we are afforded a comprehensive view into the complex nature of ion hydration. We show in aqueous nitrate that weak ion-water H-bonding leads to water-water interactions in the ion solvation shells dominating the dynamics. In contrast, the carbonate CO stretches show significant mixing with the water OH stretches due to strong ion-water H-bonding such that the water and ion modes are intimately correlated. Further, the excitonic nature of vibrations in neat H2O, which spans multiple water molecules, is an important factor in describing ion hydration. We attribute these complex dynamics to the likely presence of intermediate-range effects influenced by waters beyond the first solvation shell.

Isotopomer-selective spectra of a single intact H2O molecule in the Cs(+)(D2O)5H2O isotopologue: Going beyond pattern recognition to harvest the structural information encoded in vibrational spectra.

We report the vibrational signatures of a single H2O molecule occupying distinct sites of the hydration network in the Cs(+)(H2O)6 cluster. This is accomplished using isotopomer-selective IR-IR hole-burning on the Cs(+)(D2O)5(H2O) clusters formed by gas-phase exchange of a single, intact H2O molecule for D2O in the Cs(+)(D2O)6 ion. The OH stretching pattern of the Cs(+)(H2O)6 isotopologue is accurately recovered by superposition of the isotopomer spectra, thus establishing that the H2O incorporation is random and that the OH stretching manifold is largely due to contributions from decoupled water molecules. This behavior enables a powerful new way to extract structural information from vibrational spectra of size-selected clusters by explicitly identifying the local environments responsible for specific infrared features. The Cs(+)(H2O)6 structure was unambiguously assigned to the 4.1.1 isomer (a homodromic water tetramer with two additional flanking water molecules) from the fact that its computed IR spectrum matches the observed overall pattern and recovers the embedded correlations in the two OH stretching bands of the water molecule in the Cs(+)(D2O)5(H2O) isotopomers. The 4.1.1 isomer is the lowest in energy among other candidate networks at advanced (e.g., CCSD(T)) levels of theoretical treatment after corrections for (anharmonic) zero-point energy. With the structure in hand, we then explore the mechanical origin of the various band locations using a local electric field formalism. This approach promises to provide a transferrable scheme for the prediction of the OH stretching fundamentals displayed by water networks in close proximity to solute ions.

Vibrational Signatures of Solvent-Mediated Deformation of the Ternary Core Ion in Size-Selected [MgSO4Mg(H2O)n=4-11](2+) Clusters.

Elucidation of the molecular-level mechanics underlying the dissolution of salts is one of the long-standing, fundamental problems in electrolyte chemistry. Here we follow the incremental structural changes that occur when water molecules are sequentially added to the ternary [MgSO4Mg](2+) ionic assembly using cryogenic vibrational predissociation spectroscopy of the cold, mass-selected [MgSO4Mg(H2O)n=4-11](2+) cluster ions. Although the bare [MgSO4Mg](2+) ion could not be prepared experimentally, its calculated minimum energy structure corresponds to a configuration where the two Mg(2+) ions attach on opposite sides of the central SO4(2-) ion in a bifurcated fashion to yield a D2d symmetry arrangement. Analysis of the observed spectral patterns indicate that water molecules preferentially attach to the flanking Mg(2+) ions for the n ≤ 7 hydrates, which results in an incremental weakening of the interaction between the ions. Water molecules begin to interact with the sequestered SO4(2-) anion promptly at n = 8, where changes in the band pattern clearly demonstrate that the intrinsic bifurcated binding motif among the ions evolves into quasilinear Mg(2+)-O-S arrangements as water molecules H-bond to the now free SO groups. Although condensed-phase MgSO4 occurs with a stable hexahydrate in which water molecules lie between the ion pairs, addition of a sixth water molecule to one of the Mg(2+) ions in the n = 11 cluster occurs with the onset of the second hydration shell such that the cation remains coordinated to one of the SO4(2-) oxygen atoms.

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.

We review the role that gas-phase, size-selected protonated water clusters, H(+)(H2O)n, have played in unraveling the microscopic mechanics responsible for the spectroscopic behavior of the excess proton in bulk water. Because the larger (n ≥ 10) assemblies are formed with three-dimensional cage morphologies that more closely mimic the bulk environment, we report the spectra of cryogenically cooled (10 K) clusters over the size range 2 ≤ n ≤ 28, over which the structures evolve from two-dimensional arrangements to cages at around n = 10. The clusters that feature a complete second solvation shell around a surface-embedded hydronium ion yield spectral signatures of the proton defect similar to those observed in dilute acids. The origins of the large observed shifts in the proton vibrational signature upon cluster growth were explored with two types of theoretical analyses. First, we calculate the cubic and semidiagonal quartic force constants and use these in vibrational perturbation theory calculations to establish the couplings responsible for the large anharmonic red shifts. We then investigate how the extended electronic wave functions that are responsible for the shapes of the potential surfaces depend on the nature of the H-bonded networks surrounding the charge defect. These considerations indicate that, in addition to the sizable anharmonic couplings, the position of the OH stretch most associated with the excess proton can be traced to large increases in the electric fields exerted on the embedded hydronium ion upon formation of the first and second solvation shells. The correlation between the underlying local structure and the observed spectral features is quantified using a model based on Badger's rule as well as via the examination of the electric fields obtained from electronic structure calculations.