Infrared Spectrum of the Protonated Water Dimer in the Gas Phase

Approaching the "Zundel" Limit: Tuning the Vibrational Coupling in N2H+Ng, Ng = {He, Ne, Ar, Kr, Xe, and Rn}

The diazenylium ion (N₂H⁺) is a ubiquitous ion in dense molecular clouds. This ion is often used as a dense gas tracer in outer space. Most of the previous works on diazenylium ion have focused on the shared-proton stretch band, ν_H⁺. In this work, we have performed reduced-dimensional calculations to investigate the vibrational structure of N₂H⁺Ng, Ng = {He, Ne, Ar, Kr, Xe, and Rn}. We demonstrate a few interesting things about this system. First, the vibrational coupling in N₂H⁺ can be tuned to switch on interesting anharmonic effects such as Fermi resonance or combination bands by tagging it with different noble gases. Second, a comparison of the vibrational spectrum from N₂H⁺He to N₂H⁺Rn shows that the ν_H⁺ can be swept from an "Eigen-like" to a "Zundel-like" limiting case. Anharmonic calculations were performed using a multilevel approach, which utilized the MP2 and CCSD(T) levels of theories. Binding energies for the elimination of Ng in N₂H⁺Ng are also reported.

Fragment imaging in the infrared photodissociation of the Ar-tagged protonated water clusters H3O+-Ar and H+(H2O)2-Ar

Infrared photodissociation of protonated water clusters with an Ar atom, namely H₃O⁺-Ar and H⁺(H₂O)₂-Ar, was investigated by an imaging technique for mass-selected ions, to reveal the intra- and intermolecular vibrational dynamics. The presented system has the advantage of achieving fragment ion images with the cluster size- and mode-selective photoexcitation of each OH stretching vibration. Translational energy distributions of photofragments were obtained from the images upon the excitation of the bound (ν_b) and free (ν_f) OH stretching vibrations. The energy fractions in the translational motion were compared between ν_b^I and ν_f^I in H₃O⁺-Ar or between ν_b^II and ν_f^II in H⁺(H₂O)₂-Ar, where the labels "I" and "II" represent H₃O⁺-Ar and H⁺(H₂O)₂-Ar, respectively. In H₃O⁺-Ar, the ν_f^I excitation exhibited a smaller translational energy than ν_b^I. This result can be explained by the higher vibrational energy of ν_f^I, which enabled it to produce bending (ν₄) excited H₃O⁺ fragments that should be favored according to the energy-gap model. In contrast to H₃O⁺-Ar, the ν_b^II excitation of an Ar-tagged H₂O subunit and the ν_f^II excitation of an untagged H₂O subunit resulted in very similar translational energy distributions in H⁺(H₂O)₂-Ar. The similar energy fractions independent of the excited H₂O subunits suggested that the ν_b^II and ν_f^II excited states relaxed into a common intermediate state, in which the vibrational energy was delocalized within the H₂O-H⁺-H₂O moiety. However, the translational energy distributions for H⁺(H₂O)₂-Ar did not agree with a statistical dissociation model, which implied another aspect of the process, that is, Ar dissociation via incomplete energy randomization in the whole H⁺(H₂O)₂-Ar cluster.

Fermi resonance switching in KrH+Rg and XeH+Rg (Rg = Ne, Ar, Kr, and Xe)

Matrix isolation experiments have been successfully employed to extensively study the infrared spectrum of several proton-bound rare gas complexes. Most of these studies have focused on the spectral signature for the H⁺ stretch (ν₃) and its combination bands with the intermolecular stretch coordinate (ν₁). However, little attention has been paid to the Fermi resonance interaction between the H⁺ stretch (ν₃) and H⁺ bend overtone (2ν₂) in the asymmetric proton-bound rare gas dimers, RgH⁺Rg'. In this work, we have investigated this interaction on KrH⁺Rg and XeH⁺Rg with Rg = (Ne, Ar, Kr, and Xe). A multilevel potential energy surface (PES) was used to simulate the vibrational structure of these complexes. This PES is a dual-level comprising of second-order Møller-Plesset perturbation theory and coupled-cluster singles doubles with perturbative triples [CCSD(T)] levels of ab initio theories. We found that when both the combination bands (nν₁ + ν₃) and bend overtone 2ν₂ compete to borrow intensity from the ν₃ band, the latter wins over the former, which then results in the suppression of the nν₁ + ν₃ bands. The current simulations offer new assignments for the ArH⁺Xe and KrH⁺Xe spectra. Complete basis set (CBS) binding energies for these complexes were also calculated at the CCSD(T)/CBS level.

Moisture-Enabled Electricity Generation: From Physics and Materials to Self-Powered Applications

The exploration of the utilization of sustainable, green energy represents one way in which it is possible to ameliorate the growing threat of the global environmental issues and the crisis in energy. Moisture, which is ubiquitous on Earth, contains a vast reservoir of low-grade energy in the form of gaseous water molecules and water droplets. It has now been found that a number of functionalized materials can generate electricity directly from their interaction with moisture. This suggests that electrical energy can be harvested from atmospheric moisture and enables the creation of a new range of self-powered devices. Herein, the basic mechanisms of moisture-induced electricity generation are discussed, the recent advances in materials (including carbon nanoparticles, graphene materials, metal oxide nanomaterials, biofibers, and polymers) for harvesting electrical energy from moisture are summarized, and some strategies for improving energy conversion efficiency and output power in these devices are provided. The potential applications of moisture electrical generators in self-powered electronics, healthcare, security, information storage, artificial intelligence, and Internet-of-things are also discussed. Some remaining challenges are also considered, together with a number of suggestions for potential new developments of this emerging technology.

Reduced rovibrational coupling Cartesian dynamics for semiclassical calculations: Application to the spectrum of the Zundel cation

We study the vibrational spectrum of the protonated water dimer, by means of a divide-and-conquer semiclassical initial value representation of the quantum propagator, as a first step in the study of larger protonated water clusters. We use the potential energy surface from the work of Huang et al. [J. Chem. Phys. 122, 044308 (2005)]. To tackle such an anharmonic and floppy molecule, we employ fully Cartesian dynamics and carefully reduce the coupling to global rotations in the definition of normal modes. We apply the time-averaging filter and obtain clean power spectra relative to suitable reference states that highlight the spectral peaks corresponding to the fundamental excitations of the system. Our trajectory-based approach allows for the physical interpretation of the very challenging proton transfer modes. We find that it is important, for such a floppy molecule, to selectively avoid initially exciting lower energy modes, in order to obtain cleaner spectra. The estimated vibrational energies display a mean absolute error (MAE) of ∼29 cm^-1 with respect to available multiconfiguration time-dependent Hartree calculations and MAE ∼ 14 cm^-1 when compared to the optically active experimental excitations of the Ne-tagged Zundel cation. The reasonable scaling in the number of trajectories for Monte Carlo convergence is promising for applications to higher dimensional protonated cluster systems.

A theoretical study on the infrared signatures of proton-bound rare gas dimers (Rg-H+-Rg), Rg = {Ne, Ar, Kr, and Xe}

The infrared spectrum of proton-bound rare gas dimers has been extensively studied via matrix isolation spectroscopy. However, little attention has been paid on their spectrum in the gas phase. Most of the Rg₂H⁺ has not been detected outside the matrix environment. Recently, Ar_nH⁺ (n = 3-7) has been first detected in the gas-phase [D. C. McDonald et al., J. Chem. Phys. 145, 231101 (2016)]. In that work, anharmonic theory can reproduce the observed vibrational structure. In this paper, we extend the existing theory to examine the vibrational signatures of Rg₂H⁺, Rg = {Ne, Ar, Kr, and Xe}. The successive binding of Rg to H⁺ was investigated through the calculation of stepwise formation energies. It was found that this binding is anti-cooperative. High-level full-dimensional potential energy surfaces at the CCSD(T)/aug-cc-pVQZ//MP2/aug-cc-pVQZ were constructed and used in the anharmonic calculation via discrete variable representation. We found that the potential coupling between the symmetric and asymmetric Rg-H⁺ stretch (ν₁ and ν₃ respectively) causes a series of bright n₁ν₁ + ν₃ progressions. From Ne₂H⁺ to Xe₂H⁺, an enhancement of intensities for these bands was observed.

Near-Infrared Spectroscopy and Anharmonic Theory of Protonated Water Clusters: Higher Elevations in the Hydrogen Bonding Landscape

Near-infrared spectroscopy measurements are presented for protonated water clusters, H⁺(H₂O) _n, in the size range of n = 1-8. Clusters are produced in a pulsed-discharge supersonic expansion, mass selected, and studied with infrared laser photodissociation spectroscopy in the regions of 3600-4550 and 4850-7350 cm^-1. Although there is some variation with cluster size, the main features of these spectra are a broad absorption near 5300 cm^-1, a sharp doublet near 7200 cm^-1, as well as a structured absorption near 4100 cm^-1 for n ≥ 2. The vibrational patterns measured for the hydronium, Zundel, and Eigen ions are compared to those predicted by different forms of anharmonic theory. Second-order vibrational perturbation theory (VPT2) and a local mode treatment of the OH stretches both capture key aspects of the spectra but suffer understandable deficiencies in the quantitative description of band positions and intensities.

Spectroscopy of Proton Coordination with Ethylenediamine

Protonated ethylenediamine monomer, dimer, and trimer were produced in the gas phase by an electrical discharge/supersonic expansion of argon seeded with ethylenediamine (C₂H₈N₂, en) vapor. Infrared spectra of these ions were measured in the region from 1000 to 4000 cm^-1 using laser photodissociation and argon tagging. Computations at the CBS-QB3 level were performed to explore possible isomers and understand the infrared spectra. The protonated monomer exhibits a gauche conformation and an intramolecular hydrogen bond. Its parallel shared proton vibration occurs as a broad band around 2785 cm^-1, despite the formally equivalent proton affinities of the two amino groups involved, which usually leads to low frequency bands. The barrier to intramolecular proton transfer is 2.2 kcal mol^-1 and does not vanish upon addition of the zero-point energy, unlike the related protonated ammonia dimer. The structure of the dimer is formed by chelation of the monomer's NH₃⁺ group, thereby localizing the excess proton and increasing the frequency of the intramolecular shared proton vibration to 3157 cm^-1. Other highly fluxional dimer structures with facile intermolecular proton transfer and concomitant structural reorganization were computed to lie within 2 kcal mol^-1 of the experimentally observed structure. The spectrum of the trimer is rather diffuse, and a clear assignment is not possible. However, an isomer with an intramolecular proton transfer like that of the monomer is most consistent with the experimental spectrum.

Infrared Multiphoton Dissociation Spectroscopy with Free-Electron Lasers: On the Road from Small Molecules to Biomolecules

Infrared multiphoton dissociation (IRMPD) spectroscopy is commonly used to determine the structure of isolated, mass-selected ions in the gas phase. This method has been widely used since it became available at free-electron laser (FEL) user facilities. Thus, in this Minireview, we examine the use of IRMPD/FEL spectroscopy for investigating ions derived from small molecules, metal complexes, organometallic compounds and biorelevant ions. Furthermore, we outline new applications of IRMPD spectroscopy to study biomolecules.

IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters: Enol-Keto Tautomers and Ion→Solvent Proton Transfer

Protonated ions of acetylacetone, H⁺(Hacac), and their argon-tagged analogues are produced via a pulsed discharge and cooled in a supersonic expansion. These ions are mass analyzed, selected in a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy using the method of rare-gas atom tagging. Computational studies at the DFT/B3LYP level are employed to elucidate the structures and spectra of these ions, which are expected to exist as either enol- or keto-based tautomers. The protonated acetylacetone ion is found to form a single enol-based isomer. Adding one or two water molecules to this ion, for example, H⁺(Hacac)(H₂O)_1,2, produces primarily enol-based structures, although a small concentration of keto structures also contribute to the spectra. The vibrational patterns resulting from hydrogen bonding in these systems are not well-described by theory. Addition of a third water molecule to form the H⁺(Hacac)(H₂O)₃ ion causes a significant change in the spectroscopy, attributed to proton transfer from the H⁺(Hacac) ion into the water solvent.

MP4 Study of the Anharmonic Coupling of the Shared Proton Stretching Vibration of the Protonated Water Dimer in Equilibrium and Transition States

The structure and harmonic and anharmonic IR spectra of the protonated water dimer (PWD) were calculated in C₁, C₂, and C_s symmetry at the MP4/acc-pVTZ level of theory. We found that structure and IR spectra are practically identical in C₂ and C₁ symmetry, demonstrating that an equilibrium C₁ configuration of the PWD is not realized. Anharmonic coupling of the shared proton stretching vibration with all other modes in the PWD in C₂ and C_s symmetry was the focus of this investigation. For this purpose, 28 two-dimensional potential energy surfaces (2D PES) were built at the MP4/acc-pVTZ level of theory and the corresponding vibrational Schrödinger equations were solved using the DVR method. Differences in the coupling of the investigated mode with other modes in the C₂ and C_s configurations, along with some factors that determine the red- or blue-shift of the stretching vibration frequency, were analyzed. We obtained a rather reasonable value of the stretching frequency of the bridging proton (1058.4 cm^-1) unperturbed by Fermi resonance. The Fermi resonance between the fundamental vibration ν₇ and the combined vibration ν₂ + ν₆ of the same symmetry was analyzed through anharmonic second-order perturbation theory calculations, as well as by 3D PES constructed using Q₂, Q₆, and Q₇ as normal coordinates. A significant (up to 50%) transfer of intensity from the fundamental vibration to the combined one was found. We have estimated the frequency of the bridging proton stretching vibration in the C_s configuration of the PWD based on calculations of the intrinsic anharmonicity and anharmonic double modes interactions at the MP4/acc-pVTZ level of theory (1261 cm^-1).

How the Zundel (H5O2+) Potential Can Be Used to Predict the Proton Stretch and Bend Frequencies of Larger Protonated Water Clusters

From a series of seminal experiments on the IR spectra of protonated water clusters and associated theoretical analyses, it is clear that the energies and spectral features of the proton stretch and bend modes are very sensitive functions of the cluster size. Here we show that this dynamic range can be understood by examining the sensitivity of these modes in the potential of the Zundel cation, H₅O₂⁺, as the separation of the two water monomers is varied. As this distance increases, the proton increasingly localizes on a monomer, and this is encoded in the IR spectrum of the proton vibrational modes. The quantitative predictions from this simple correlation are verified for the H₇O₃⁺ and H₉O₄⁺ clusters, for which new benchmark harmonic frequencies are reported. The predictions are also in good accord with trends seen experimentally and previous calculations for these and five other clusters, including H⁺(H₂O)₂₁.

Ab initio and density functional theory (DFT) studies on triflic acid with water and protonated water clusters

The structure, stability and infrared spectral signatures of triflic acid (TA) with water clusters (W_n) and protonated water clusters (TAH⁺W_n, n = 1 - 6) were computed using DFT and MP2 methods. Our calculations show that a minimum of three water molecules are necessary to stabilize the dissociated zwitterionic form of TA. It can be seen from the results that there is no significant movement of protons in smaller (n = 1 and 2) and linear (n = 1 - 6) types of water clusters. Further, the geometries of TAW_n clusters first form a neutral pair (NP) to contact ion pair (CIP), then form a solvent separated ion pair (SSIP) in a water hexamer. These findings reveal that proton transfer may take place through NP to CIP and then CIP to SSIP. The calculated binding energies (BEs) of ion pair clusters is always higher than that of NP clusters (i.e., more stable than the NP). Existing excess proton linear chain clusters transfer a proton to adjacent water molecules via a Grotthuss mechanism, whereas the same isomers in the branched motifs do not conduct protons. Examination of geometrical parameters and infrared frequencies reveals hydronium ion (H₃O⁺ also called Eigen cation) formation in both TAW_n and protonated TAW_n clusters. The stability of Eigen water clusters is three times higher than that of other non-Eigen water clusters. Our study shows clearly that formation of ion pairs in TAW_n and TAH⁺W_n clusters greatly favors proton transfer to neighboring water molecules and also enhances the stability of these complexes.

DFT Study of Hydrogen-Bonding Interaction, Solvation Effect, and Electric-Field Effect on Raman Spectra of Hydrated Proton

Strong hydrogen-bonding interaction and Raman spectra of hydrated proton have been investigated using hybrid density functional theory method B3LYP. The solvation model of density (SMD) approach is employed in the present calculation to simulate hydrated protons in aqueous solution. Focusing on the different hydrogen-bonded Eigen-water and Zundel-water interactions, we present a better assignment of Raman signals of hydrated proton on the basis of vibrational analysis in different environments. Our results showed that B3LYP calculations could give a good prediction for characteristic vibrational frequencies of Eigen and Zundel isomers in liquid phase. The O-H stretching vibrational frequencies from Eigen and Zundel units are very sensitive to hydrogen-bonding interaction with solvent water molecules. Moreover, the solvation effect and the external electric-field effect lead to the proton deviating from the central position of Zundel structure and finally resulting in a transition to Eigen one in aqueous solution. Furthermore, by combining theoretical prediction and Raman scattering theory, we calculate absolute Raman intensities of characteristic signals based on the polarizability tensor derivatives of hydrated proton clusters. This is very helpful to infer the microstructure of hydrated protons in aqueous solution by using Raman measurements.

The Hydrated Excess Proton in the Zundel Cation H5 O2 (+) : The Role of Ultrafast Solvent Fluctuations

The nature of the excess proton in liquid water has remained elusive after decades of extensive research. In view of ultrafast structural fluctuations of bulk water scrambling the structural motifs of excess protons in water, we selectively probe prototypical protonated water solvates in acetonitrile on the femtosecond time scale. Focusing on the Zundel cation H5 O2 (+) prepared in room-temperature acetonitrile, we unravel the distinct character of its vibrational absorption continuum and separate it from OH stretching and bending excitations in transient pump-probe spectra. The infrared absorption continuum originates from a strong ultrafast frequency modulation of the H(+) transfer vibration and its combination and overtones. Vibrational lifetimes of H5 O2 (+) are found to be in the sub-100 fs range, much shorter than those of unprotonated water. Theoretical results support a picture of proton hydration where fluctuating electrical interactions with the solvent and stochastic thermal excitations of low-frequency modes continuously modify the proton binding site while affecting its motions.

A closer examination of the coupling between ionic hydrogen bond (IHB) stretching and flanking group motions in (CH3OH)2H+: the strong isotope effects

The intermode coupling between shared proton (O–H⁺–O) fundamental stretching and flanking modes in (CH₃OH)₂H⁺ was revisited in the following contexts: (1) evaluation of Hamiltonian matrix elements represented in a “pure state” (PS) basis and (2) tuning of coupling strengths using H/D isotopic substitution.

Tuning the vibrational coupling of H3O+ by changing its solvation environment

This study demonstrates how the intermode coupling in the hydronium ion (H₃O⁺) is modulated by the composition of the first solvation shell.

Driven molecular dynamics studies of the shared proton motion in the H5O2+·Ar cluster: the effect of argon tagging and deuteration on vibrational spectra

We report IR spectra of H5O2(+) and H5O2(+)·Ar and their deuterium isotopologues using ab initio molecular dynamics. The trajectories were propagated as microcanonical (NVE) ensembles at energies corresponding to temperatures 50 and 100 K. The potential energy surface is calculated on-the-fly at the MP2/aug-cc-pVDZ level of theory. The calculations show that adding an argon atom to H5O2(+) introduces symmetry breaking in the Zundel core ion, causes blueshift in the shared proton vibration by about 200 cm(-1), and leads to the splitting of the OH stretch vibrations into four bands. Driven molecular dynamics (DMD) method is used to assign the spectrum by coupling the dipole moment to an external electric field oscillating at frequency ω. The broad feature at 1100 cm(-1) in the H5O2(+)·Ar spectrum is ascribed to the large amplitude shared proton vibration coupled with torsion and wag modes. MD MP2 simulations predict the H/D redshift in the shared proton vibration and water bending vibration to be about 280 and 460 cm(-1), respectively, in good agreement with experimental observations.

Deciphering the infrared spectrum of the protonated water pentamer and the hybrid Eigen-Zundel cation

Traditionally, infrared band assignment for the protonated water clusters, such as H(+)(H2O)5, is based on their lowest energy isomer. Recent experiments extend the observation spectral window to lower frequencies, for which such assignment appears to be inadequate. Because this hydrogen-bonded system is highly anharmonic, harmonic spectral calculations are insufficient for reliable interpretation. Consequently, we have calculated the IR spectrum of several isomers of the protonated water pentamer using an inherently anharmonic methodology, utilizing dipole and velocity autocorrelation functions computed from ab initio molecular dynamic trajectories. While the spectrum of H(+)(H2O)5 is universally assumed to represent the branched Eigen isomer, we find a better agreement for a mixture of a ring and linear isomers. The first has an Eigen core and contributes at high frequencies, whereas the latter accounts for all prominent low-frequency bands. Interestingly, its core is neither a classical Eigen nor a Zundel cation, but rather has hybrid geometry. Such an isomer may play a role in proton conductance along short proton wires.

Hydrogen-bond structure and anharmonicity in croconic acid

Layered structure of croconic acid and radial distributions from large scale MD simulations, highlighting a distinct broadening even at 300 K where the material remains ferroelectric.

Structures of biomolecular ions in the gas phase probed by infrared light sources

Infrared (IR) spectroscopy of biomolecular ions combines mass spectrometry's high sensitivity and ability to analyze complex mixtures with the enhanced structural information available from vibrational spectroscopy. IR spectroscopy is in principle well placed to distinguish isomers and allow chemical classification of unknown molecules. This review gives an outline of current instrumentation, spectroscopic approaches, and potential bottlenecks. We discuss the most promising applications in bioanalytical mass spectrometry in view of recent experimental results, as well as future applications based on bioinformatics.

Anharmonic Rice-Ramsperger-Kassel-Marcus (RRKM) and product branching ratio calculations for the partially deuterated protonated water dimers: dissociation and isomerization

Partially deuterated protonated water dimers, H2O·H(+)·D2O, H2O·D(+)·HDO, and HDO·H(+)·HDO, as important intermediates of isotopic labeled reaction of H3O(+) + D2O, undergo direct dissociation and indirect dissociation, i.e., isomerization before the dissociation. With Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations, we have computed their dissociation and isomerization rate constants separately under the harmonic and anharmonic oscillator models. On the basis of the dissociation and isomerization rate constants, branching ratios of two primary products, [HD2O(+)]∕[H2DO(+)], are predicted under various kinetics models with the harmonic or anharmonic approximation included. The feasible kinetics model accounting for experimental results is shown to include anharmonic effect in describing dissociation, while adopting harmonic approximation for isomerization. Thus, the anharmonic effect is found to play important roles affecting the dissociation reaction, while isomerization rates are shown to be insensitive to whether the anharmonic or harmonic oscillator model is being applied.

Natural polarizability and flexibility via explicit valency: the case of water

As the dominant physiological solvent, water drives the folding of biological macromolecules, influences conformational changes, determines the ionization states of surface groups, actively participates in catalytic events, and provides "wires" for long-range proton transfer. Elucidation of all these roles calls for atomistic simulations. However, currently available methods do not lend themselves to efficient simulation of proton transfer events, or even polarizability and flexibility. Here, we report that an explicit account of valency can provide a unified description for the polarizability, flexibility, and dissociability of water in one intuitive and efficient setting. We call this approach LEWIS, after the chemical theory that inspires the use of valence electron pairs. In this paper, we provide details of the method, the choice of the training set, and predictions for the neat ambient liquid, with emphasis on structure, dynamics, and polarization. LEWIS water provides a good description of bulk properties, and dipolar and quadrupolar responses.

Dynamics and mechanism of structural diffusion in linear hydrogen bond

Dynamics and mechanism of proton transfer in a protonated hydrogen bond (H-bond) chain were studied, using the CH(3)OH(2)(+)(CH(3)OH)(n) complexes, n = 1-4, as model systems. The present investigations used B3LYP/TZVP calculations and Born-Oppenheimer MD (BOMD) simulations at 350 K to obtain characteristic H-bond structures, energetic and IR spectra of the transferring protons in the gas phase and continuum liquid. The static and dynamic results were compared with the H(3)O(+)(H(2)O)(n) and CH(3)OH(2)(+)(H(2)O)(n) complexes, n = 1-4. It was found that the H-bond chains with n = 1 and 3 represent the most active intermediate states and the CH(3)OH(2)(+)(CH(3)OH)(n) complexes possess the lowest threshold frequency of proton transfer. The IR spectra obtained from BOMD simulations revealed that the thermal energy fluctuation and dynamics help promote proton transfer in the shared-proton structure with n = 3 by lowering the vibrational energy for the interconversion between the oscillatory shuttling and structural diffusion motions, leading to a higher population of the structural diffusion motion than in the shared-proton structure with n = 1. Additional explanation on the previously proposed mechanisms was introduced, with the emphases on the energetic of the transferring proton, the fluctuation of the number of the CH(3)OH molecules in the H-bond chain, and the quasi-dynamic equilibriums between the shared-proton structure (n = 3) and the close-contact structures (n ≥ 4). The latter prohibits proton transfer reaction in the H-bond chain from being concerted, since the rate of the structural diffusion depends upon the lifetime of the shared-proton intermediate state.

Lewis-inspired representation of dissociable water in clusters and Grotthuss chains

Proton transfer to and from water is critical to the function of water in many settings. However, it has been challenging to model. Here, we present proof-of-principle for an efficient yet robust model based on Lewis-inspired submolecular particles with interactions that deviate from Coulombic at short distances to take quantum effects into account. This "LEWIS" model provides excellent correspondence with experimental structures for water molecules and water clusters in their neutral, protonated and deprotonated forms; reasonable values for the proton affinities of water and hydroxide; a good value for the strength of the hydrogen bond in the water dimer; the correct order of magnitude for the stretch and bend force constants of water; and the expected time course for Grotthuss transport in water chains.