Proton Delocalization Research Articles

Perfluorosulfonyl Imide versus Perfluorosulfonic Acid Ionomers in Proton-Exchange Membrane Fuel Cells at Low Relative Humidity.

Designing highly conductive ionomers at high temperature and low relative humidity is challenging in proton-exchange membrane fuel cells. Perfluorosulfonyl imide ionomers were believed to achieve this goal, owing to their exceptional acidity and excellent thermal stability. Perfluorosulfonyl imide ionomers are less conductive than the analogous perfluorosulfonic acids despite similar membrane microstructure. In this study, the distinct behavior is rationalized by in situ synchrotron infrared spectroscopy during hydration. The protonation mechanism, formation of the protonic moiety and water clustering are totally different for the two different families of membranes. The ionization mediated by trans-to-cis conformational transition of the perfluorosulfonyl imide ionomer is not accompanied by the formation of hydronium ions. In contrast, Zundel-ion entities were identified as the elementary protonic complex, which is stable over the hydration range. The H-bond network of surrounding water molecules appears to be less connected and the protons remain highly localized and unavailable for efficient structural transport. The delocalization of protons and their mitigated interaction with the surrounding medium are prominent effects that negatively impact conductivity.

Proton delocalization and tunneling in terephthalic acid: Raman spectroscopic study

Raman spectra of terephthalic acid (TPA), regular TPA(H) and deuterated TPA(D), were recorded in the temperature range of 5–300 K. It is shown that temperature dependences of vibration frequencies provide both the data on proton tunneling in tautomeric bonds and the data on phonon-assisted proton jumps. At the same time, the quantum delocalization of proton (deuteron), i.e. penetration of proton (deuteron) density distribution function into the neighboring potential well, affects the temperature dependence of the frequency of translational vibrations O⋅⋅⋅O and is observed at T ≤ 180 K in TPA(H) and at T ≤ 170 K in TPA(D). Proton tunneling through the barrier affects the temperature dependence of the frequency of CO stretchings in carboxyl groups and is observed at T ≤ 100 K. No coordinated proton (deuteron) jumps over the potential barrier with a rate capable to change notably the vibration frequency of CO bonds was observed in TPA over the entire temperature range.

Nuclear Quantum Effects in Hydrophobic Nanoconfinement.

Nuclear quantum effects (NQEs) in water arise due to delocalization, zero-point energy (ZPE), and quantum tunneling of protons. Whereas quantum tunneling is significant only at low temperatures, proton delocalization and ZPE influence the properties of water at normal temperature and pressure (NTP), giving rise to isotope effects. However, the consequences of NQEs for interfaces of water with hydrophobic media, such as perfluorocarbons, have remained largely unexplored. Here, we reveal the existence and signature of NQEs modulating hydrophobic surface forces at NTP. Our experiments demonstrate that the attractive hydrophobic forces between molecularly smooth and rigid perfluorinated surfaces in nanoconfinement are ≈10% higher in H2O than in D2O, even though the contact angles of H2O and D2O on these surfaces are indistinguishable. Our molecular dynamics simulations show that the underlying cause of the difference includes the destabilizing effect of ZPE on the librational motions of interfacial H2O, which experiences larger quantum effects than D2O.

Counter Cations Affect Transport in Aqueous Hydroxide Solutions with Ion Specificity.

The anomalously high mobility of hydroxide and hydronium ions in aqueous solutions is related to proton transfer and structural diffusion. The role of counterions in these solutions, however, is often considered to be negligible. Herein, we explore the impact of alkali metal counter cations on hydroxide solvation and mobility. Impedance measurements demonstrate that hydroxide mobility is attenuated by lithium relative to sodium and potassium. These results are explained by ab initio molecular dynamics simulations and experimental vibrational hydration shell spectroscopy, which reveal substantially stronger ion pairing between OH- and Li+ than with other cations. Hydration shell spectra and theoretical vibrational frequency calculations together imply that lithium and sodium cations have different effects on the delocalization of water protons donating a hydrogen bond to hydroxide. Specifically, lithium leads to enhanced proton delocalization compared with sodium. However, proton delocalization and the overall diffusion process are not necessarily correlated.

Multicomponent Coupled Cluster Singles and Doubles Theory within the Nuclear-Electronic Orbital Framework.

The nuclear-electronic orbital (NEO) method treats all electrons and specified nuclei, typically protons, quantum mechanically on the same level with molecular orbital techniques. This approach directly includes nuclear delocalization, anharmonicity, and zero point energy contributions of the quantum nuclei in the self-consistent-field procedure for solving the time-independent Schrödinger equation. Herein the multicomponent wave function based methods configuration interaction singles and doubles (CISD) and coupled cluster singles and doubles (CCSD) are implemented within the NEO framework and are applied to molecular systems. In contrast to the NEO-HF (Hartree-Fock) and NEO-CISD methods, which produce proton densities that are much too localized, the NEO-CCSD method produces accurate proton densities in reasonable agreement with a grid-based reference. Moreover, the NEO-CCSD method also predicts accurate proton affinities in agreement with experimental measurements for a set of 12 molecules. An advantage of the NEO-CCSD method is its ability to include nuclear quantum effects, such as proton delocalization and zero point energy, during geometry optimizations and nuclear dynamics simulations. The NEO-CCSD method is a promising, parameter free approach for including nuclear quantum effects in high-level electronic structure calculations of molecular systems.

N-oxide Derivatives: Car-Parrinello Molecular Dynamics and Electron Localization Function Study on Intramolecular Hydrogen Bonds.

First-principle molecular dynamics simulations (CPMD) and electron localization function (ELF) topological analysis were performed for two N-oxides, 2-[( N, N-dimethylamino- N-oxy)methyl]-4,6-dimethylphenol (I) and 5,5'-dibromo-3-[(diethylamino)methyl]-2,2'-biphenol N-oxide (II). Special attention was paid to hydrogen bond dynamics and spectroscopic features. Simulations were carried out in vacuo and in the crystalline phase. It was found that for compound I proton transfer phenomena do not occur spontaneously. An opposite situation was noticed for one of the two intramolecular hydrogen bonds of compound II. The influence of the neighboring molecules (including microsolvation) and the crystal field on the hydrogen bond properties cannot be neglected. Hydrogen bond spectroscopic signatures were quite well reproduced by power spectra of atomic velocity. Metric and spectroscopic theoretical findings were compared with experimental data available, especially in a view of the fact that experimental X-ray diffraction and FT-IR data of compound II indicate strong delocalization of the N-oxide bridge proton. ELF analysis showed that for compound II the N-oxide bridge belongs to the topological category of strong hydrogen bonds with the separate ELF basin for the bridged hydrogen atom.

Inelastic and deep inelastic neutron spectroscopy of water molecules under ultra-confinement

Water confined within sub-nanometer channels of silicate minerals presents an extreme case of confinement, where the restricted molecules are situated in channels whose diameter is not much larger than the water molecule itself. Recently, we discovered a new quantum tunneling state of the water molecule confined in 5 Å channels in the mineral beryl, characterized by extended proton and electron delocalization. Several peaks were observed in the inelastic neutron scattering (INS) spectra which were uniquely assigned to water quantum tunnelling. In addition, the water proton momentum distribution measured with deep inelastic neutron scattering (DINS) at 4.3 K directly showed coherent delocalization of the water protons in the ground state. The obtained average kinetic energy (EK) of the water protons was found to be ~30% less than it is in bulk liquid water and ice phases. In the current work we present INS and DINS study of water in single crystal beryl in wider temperature range, T=5–260 K, where we observed significant increase of EK of the confined water protons with temperature increase. The obtained INS data also indicate that with increasing temperature water molecules are progressively involved in hydrogen bonding (HB) with the beryl cage, while HB is almost absent at low temperatures.

Enhanced chemical activity and wettability at adjacent Brønsted acid sites in HZSM-5

The adjustable acid sites in zeolites and its well-defined pore structure allows for a fine-tuning of the catalytic performance. The activity and selectivity of several reactions have been shown to be dependent on the location and distribution of the acid sites in the zeolite. However, the underlying mechanisms responsible for this dependence remain to be explored. Here, we report density functional theory calculations, through which we investigate the impact of proximity of Brønsted acid sites in the HZSM-5. We find that Brønsted sites with close spatial proximity can significantly strengthen the adsorption of water, which is used as a molecular probe for the local activity. We find that a water molecule can form H-bonds with two adjacent sites with increased adsorption energy. This increase is attributed to enhanced polarization of the water molecule through pronounced interfacial charge transfer. In some case, we observe that the proton becomes detached from the zeolite framework and forms a hydronium ion. We further show that hybrid functional calculations are essential for accurately describing the structural stability when two sites are located in close proximity of each other. Both, the enhanced polarization and proton delocalization may affect activity and selectivity for zeolite-catalyzed reactions.

Direct Observation of Polarization in Short Hydrogen Bonds due to Proton Delocalization

Based on their relatively shallow potential energy surfaces (PESs), short hydrogen bonds with heavy atom distances about or less than 2.5 Å are believed to exhibit proton delocalization, which has been argued to be beneficial to biological catalysis1. While the degree of delocalization is usually characterized by neutron diffraction or inferred from broad bands in the IR spectra, these approaches can result in ambiguities2 and information on energetics cannot be directly obtained. Here we apply an external electric field to perturb the proton equilibrium position of a short hydrogen bond in previously characterized GFP S65T/H148D mutants with various degrees of proton affinity mismatch (ΔpKα) between the chromophore and aspartate3. The chromophore serves as a spectral probe of the proton position, allowing us to perform electronic Stark spectroscopy. The local curvature of the PES is reflected by the ease of proton polarization, and the resulting proton displacement effectively amplifies the electric field experienced by the chromophore, leading to anomalously large Stark tuning rates as readouts. These Stark tuning rates are sensitive to the presence of the short hydrogen bond, proton position, ΔpKα and deuteration, in accordance with large hydrogen bond polarizations predicted by Zundel4. The results reinforce our previous arguments on the absence of low barrier hydrogen bonds in these variants3: the general shape of PESs is double-welled with a rather high barrier in between and the proton is delocalized within each well. [1] Perrin, C. L.; Nielson, J. B. Annu. Rev. Phys. Chem. 1997, 48, 511-544. [2] Graen, T. et al. J. Am. Chem. Soc. 2016, 138, 16620-16631. [3] Oltrogge, L. M.; Boxer, S. G. ACS Cent. Sci. 2015, 1, 148-156. [4] Janoshek, R. et al. J. Am. Chem. Soc. 1972, 94, 2387-2396.

Solvent Dependence of Double Proton Transfer in the Formic Acid-Formamidine Complex: Path Integral Molecular Dynamics Investigation.

Solvent dependence of double proton transfer in the formic acid-formamidine (FA-FN) complex at room temperature was investigated by means of ab initio path integral molecular dynamics (AIPIMD) simulation with taking nuclear quantum and thermal effects into account. The conductor-like screening model (COSMO) was applied for solvent effect. In comparison with gas phase, double proton delocalization between two heavy atoms (O and N) in FA-FN were observed with reduced proton transfer barrier height in low dielectric constant medium (<4.8). For dielectric constant medium at 4.8, the chance of finding these two protons are more pronounced due to the solvent effect which completely washes out the proton transfer barrier. In the case of higher dielectric constant medium (>4.8), the ionic species becomes more stable than the neutral ones and the formate anion and formamidium cation are thermodynamically stable. For ab initio molecular dynamics simulation, in low dielectric constant medium (<4.8) a reduction of proton transfer barrier with solvent effect is found to be less pronounced than the AIPIMD due to the absence of nuclear quantum effect. Moreover, the motions of FA-FN complex are significantly different with increasing dielectric constant medium. Such a difference is revealed in detail by the principal component analysis.

Ionization and Conformational Equilibria of Citric Acid: Delocalized Proton Binding in Solution.

The microspeciation of citric acid is studied by analyzing NMR titration data. When the site binding (SB) model, which assumes fully localized proton binding to the carboxylic groups, is used to obtain microscopic energy parameters (dissociation constants, pair and triplet interaction energies between charged carboxylate groups), contradictory results are obtained. The resulting macroscopic constants are in very good agreement with the values reported in the literature using potentiometry. However, the found pair interaction energy between the terminal carboxylates and the triplet interaction energy are physically meaningless. To solve this apparent contradiction, we consider the possibility of delocalized proton binding, so that the proton can be exchanged at high velocity in the NMR time scale through short, strong, low-barrier (SSLB) hydrogen bonds. With this aim, ab initio MP2 calculations using the SMD polarizable continuum model for the solvent were performed and the fully roto-microspeciation elucidated. First, fully localized proton binding was assumed, and the resulting microstate probabilities are in reasonable agreement with those reported in previous works that use selective blocking of the carboxylic groups. They are, however, in clear disagreement with the microstate probabilities derived from the NMR titration data, which predict, within a very narrow confidence interval, a unique microspecies for the symmetric di-ionized form. Moreover, counterintuitively, the interaction between terminal charged groups is much larger than that between central and terminal groups. As a consequence, we have explored the possibility of delocalized proton binding by calculating the energy of intermediate proton positions between two carbolxylic groups. The results reveal that the exchange of the proton through the hydrogen bonds is in some cases produced without energetic barrier. This effect is specially relevant in the di-ionized form, with all the most stable conformations forming a SSLB, which together would constitute the only microstate detected by NMR. An alternative reaction scheme for the ionization process, based on proton delocalization, is proposed.

Unravelling the influence of quantum proton delocalization on electronic charge transfer through the hydrogen bond

Upon hydrogen bond formation, electronic charge density is transferred between the donor and acceptor, impacting processes ranging from hydration to spectroscopy. Here we use ab initio path integral simulations to elucidate the role of nuclear quantum effects in determining the charge transfer in a range of hydrogen bonded species in the gas and liquid phase. We show that the quantization of the nuclei gives rise to large changes in the magnitude of the charge transfer as well as its temperature dependence. We then explain how a single geometric parameter determines the charge transfer through the hydrogen bond. These results thus demonstrate that nuclear quantum effects are vital for the accurate description of charge transfer and offer a physically transparent way to understand how hydrogen bonding gives rise to it.

Tuning the Ultrafast Dynamics of Photoinduced Proton-Coupled Electron Transfer in Energy Conversion Processes

Photoinduced proton-coupled electron transfer (PCET) is essential for a wide range of energy conversion processes in chemical and biological systems. Understanding the underlying principles of photoinduced PCET at a level that allows tuning and control of the ultrafast dynamics is crucial for designing renewable and sustainable energy sources such as artificial photosynthesis devices and photoelectrochemical cells. This Perspective discusses fundamental aspects of photoinduced PCET, including the characterization of different types of excited electronic states, as well as the roles of solute and solvent dynamics, nonadiabatic transitions, proton delocalization, and vibrational relaxation. It also presents strategies for tuning and controlling the charge transfer dynamics and relaxation processes by altering the nature and positions of molecular substituents, the distance associated with electron transfer, the proton transfer interface, and the solvent properties. These insights, in conjunction with furthe...

Short Carboxylic Acid-Carboxylate Hydrogen Bonds Can Have Fully Localized Protons.

Short hydrogen bonds (H-bonds) have been proposed to play key functional roles in several proteins. The location of the proton in short H-bonds is of central importance, as proton delocalization is a defining feature of low-barrier hydrogen bonds (LBHBs). Experimentally determining proton location in H-bonds is challenging. Here, bond length analysis of atomic (1.15-0.98 Å) resolution X-ray crystal structures of the human protein DJ-1 and its bacterial homologue, YajL, was used to determine the protonation states of H-bonded carboxylic acids. DJ-1 contains a buried, dimer-spanning 2.49 Å H-bond between Glu15 and Asp24 that satisfies standard donor-acceptor distance criteria for a LBHB. Bond length analysis indicates that the proton is localized on Asp24, excluding a LBHB at this location. However, similar analysis of the Escherichia coli homologue YajL shows both residues may be protonated at the H-bonded oxygen atoms, potentially consistent with a LBHB. A Protein Data Bank-wide screen identifies candidate carboxylic acid H-bonds in approximately 14% of proteins, which are typically short [⟨dO-O⟩ = 2.542(2) Å]. Chemically similar H-bonds between hydroxylated residues (Ser/Thr/Tyr) and carboxylates show a trend of lengthening O-O distance with increasing H-bond donor pKa. This trend suggests that conventional electronic effects provide an adequate explanation for short, charge-assisted carboxylic acid-carboxylate H-bonds in proteins, without the need to invoke LBHBs in general. This study demonstrates that bond length analysis of atomic resolution X-ray crystal structures provides a useful experimental test of certain candidate LBHBs.

On the tunneling splitting in a cyclic water trimer

We propose an alternative explanation of the “bifurcation” splittings observed for the water trimer in the VRT experiments of Saykally’s group [Chem. Rev. 103 (2003) 2533]. In our interpretation, the splittings originate from the quantum delocalization of hydrogen bonded protons in the mean field potential between two oxygen neighbors. The pattern and the order of our calculated splittings is in the range of experimentally observed values. Consequently, quantum delocalization of protons should be considered seriously as the origin of experimentally observed fine splittings. The presented model can be extended to a water pentamer and, hopefully, advance our understanding of liquid water.

Surface Charge Layer of Amorphous Solid Water with Adsorbed Acid or Base: Asymmetric Depth Distributions of H+ and OH– Ions

The charge at the surface of water and the resultant surface voltage play an important role in many natural phenomena and technological applications. However, the relationship between surface charge and the interfacial distribution of H+ and OH– ions remains unclear. We measured the surface voltage produced by an ionized acid or a base at the surface of amorphous solid water (ASW) using a Kelvin work-function probe and studied the depth distributions of H+ and OH– ions. H+ ions were distributed over a thicker region from the surface than OH– ions, although both ions reside preferentially at the surface. This difference led to the formation of opposite surface charges in the presence of the acid or base. The deeper penetration of H+ ions is attributed to efficient proton transport dynamics in the lattice and the resultant dynamic delocalization of protons. The study demonstrates that the asymmetric H+ and OH– distributions may be important to understand the electrical and acid–base properties of ASW and cr...

Quantum Tunneling of Water in Beryl: A New State of the Water Molecule.

Using neutron scattering and abinitio simulations, we document the discovery of a new "quantum tunneling state" of the water molecule confined in 5Å channels in the mineral beryl, characterized by extended proton and electron delocalization. We observed a number of peaks in the inelastic neutron scattering spectra that were uniquely assigned to water quantum tunneling. In addition, the water proton momentum distribution was measured with deep inelastic neutron scattering, which directly revealed coherent delocalization of the protons in the ground state.

Ab initio molecular dynamics investigation of proton delocalization in crown ether complexes with H3O+ and NH4 +

The simple structure of crown ethers has found a wide range of applications in macrocyclic chemistry, largely due to its ability to capture positively charged ions either in atomic or molecular state. In the present work, ab initio molecular dynamics calculations are performed to characterize the host–guest interactions and conformational dynamics of the complexes formed by the H3O+ and NH4 + cations with the native crown ethers 15-crown-5 and 18-crown-6. A particular focus is set on the proton delocalization effects unveiled in recent spectroscopy experiments for some of these complexes under well-defined isolated (solvent-less) conditions. The calculations confirm that the infrared signature of these benchmark crown ether complexes provides a sensitive probe for proton delocalization. The ab initio study also shows that proton delocalization occurs for the H3O+ host (and not for NH4 +), provided that the size of the crown ring allows for tight H-bonding interactions between the cationic guest and the crown ether, leading short H2O···H+···O (crown) bonds.

ELECTRICAL CONDUCTIVITY OF THE KAOLIN CLAY MINERAL DEPENDING ON THE TEMPERATURE

The aim is to study temperature dependence of the electrical conductivity of the kaolin mineral on crystalchemical factors and thermal transformations. Methods. We have applied the method of investigation of geo-electric properties of high-resistivity materials. Results. We gained the results of the study of the temperature dependence of electrical conductivity in the range of 100-1000 °C of the kaolin clay mineral, which is of great scientific and practical importance. We have proved the existence of the general laws of the nature of the change in the electrical conductivity within the temperature range for all the samples studied, due to the existence of associated complexes of elementary defects in the crystal lattice of the mineral, which play an essential role in the kinetic nature in the geosphere of the Earth. Availability of the spectrum values of the activation energy in the extrinsic region characterizes the stepped nature of dehydroxylation and delocalization of protons of hydroxyl groups of non-equivalent positions of power in the crystal lattice of the mineral, which characterizes the formation of fluid regime of sedimentary strata in dehydration. Conclusions. It was found that the behavior of the electrical conductivity is interconnected with the processes caused by the existence of elementary defects associated in complexes in the crystal lattice of the mineral. The range of values of the activation energy of conductivity reflects the process of dehydroxylation and delocalization of protons and hydroxyl groups.

A Quasi-Elastic Neutron Scattering Study of the Dynamics of Electrically Constrained Water.

We have measured the quasi-elastic neutron scattering (QENS) of an electrohydrodynamic liquid bridge formed between two beakers of pure water when a high voltage is applied, a setup allowing to investigate water under high-voltage without high currents. From this experiment two proton populations were distinguished: one consisting of protons strongly bound to oxygen atoms (immobile population, elastic component) and a second one of quasi-free protons (mobile population, inelastic component) both detected by QENS. The diffusion coefficient of the quasi-free protons was found to be D = (26 ± 10) × 10(-5) cm(2) s(-1) with a jump length lav ∼ 3 Å and an average residence time of τ0 = 0.55 ± 0.08 ps. The associated proton mobility in the proton channel of the bridge is ∼9.34 × 10(-7) m(2) V(-1) s(-1), twice as fast as diffusion-based proton mobility in bulk water. It also matches the so-called electrohydrodynamic or "apparent" charge mobility, an experimental quantity which so far has lacked molecular interpretation. These results further corroborate the proton channel model for liquid water under high voltage and give new insights into the molecular mechanisms behind electrohydrodynamic charge transport phenomena and delocalization of protons in liquid water.