Proton Dynamics Research Articles

Spin‐lattice NMR relaxation and second moment of NMR line in solids containing CH3 groups

Complex proton dynamics in solids containing methyl groups are studied over a wide temperature range. Temperature dependences of the proton T1 relaxation time and the second moment of NMR lines are analyzed. General rules describing the temperature dependencies have been formulated from an analysis of the data. The second moments, M2, are correlated with those based on T1 measurements. The C3 jumps over the barrier cause a minimum in T1 below the liquid nitrogen temperature in the presence of tunnelling splitting, or a little above 77 K when tunnelling splitting equals zero. The slope of the high‐ temperature side of this minimum permits evaluation of the activation energy, which can be used to characterize the tunnelling correlation time. The T1 is temperature independent in the lowest temperature range, indicating that the tunnelling correlation time assumes a constant value. The tunnelling splitting was estimated as the best fit parameter from the T1 temperature dependence. The high tunnelling splitting is responsible for T1 values which are high and independent from the Larmor frequency. The tunnelling jumps reduce the rigid lattice second moment at zero Kelvin. The second reduction is caused by over the barrier C3 jumps of methyl protons. The tunnelling jumps cease above a temperature Ttun. The occurrence of the minimum T1 above liquid nitrogen temperature, as well as the reduction of the second moment M2 are due to slowest motion of protons, which can be the isotropic motion of molecule. This minimum is well described by the Woessner method of calculating the correlation function of complex motion. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 44A: 214–225, 2015.

Ferroelectric-Paraelectric Phase Transition of CsH2PO4studied by Static NMR and MAS NMR

The microscopic dynamics of CsH2PO4, with two distinct hydrogen bond lengths, are studied by static nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR. The proton dynamics of the two crystallographically inequivalent hydrogen sites were discussed in terms of the 1 H

Car-Parrinello Molecular Dynamics Simulations of Infrared Spectra of Crystalline Vitamin C with Analysis of Double Minimum Proton Potentials for Medium-Strong Hydrogen Bonds.

We studied proton dynamics of a hydrogen bonds of the crystalline l-ascorbic acid. Our approach was based on the Car-Parrinello molecular dynamics. The focal point of our study was simulation of the infrared spectra of l-ascorbic acid associated with the O-H stretching modes that are very sensitive to the strength of hydrogen bonding. In the l-ascorbic acid there are four kinds of hydrogen bonds. We calculated their spectra by using anharmonic approximation and the time course of the dipole moment function as obtained from the Car-Parrinello simulation. The quantization of the nuclear motion of the protons was made to perform detailed analysis of strength and properties of hydrogen bonds. We presented double minimum proton potentials with small value of barriers for medium-strong hydrogen bonds. We have also shown the difference character of medium-strong hydrogen bonds compared to weaker hydrogen bonds in the l-ascorbic acid.

Dielectric spectrum of water as a proton dynamics response

Broadband spectra of ordinary water, heavy water, and ice are analyzed in terms of the dynamic conductivity. It is shown that the spectrum below 1015 Hz satisfies the sum rule for the oscillator strength and is related to the dynamics of the proton/deuteron subsystem in the quasilattice of low-mobility oxygen atoms. Debye relaxation andOHband are represented as a result of the same mechanism, i.e., the proton motion in the harmonic potential averaged over different observation periods.

Proton dynamics in the hydrogen bonds of 4-amino-3,5-dihalogenobenzoic acid

On the polycrystalline sample of 4-amino-3,5-dihalogenobenzoic acid, 4-NH2-3,5-X2C6H2COOH, which has a symmetric dimer structure in the crystal, the proton tunneling in the hydrogen bonds has been investigated by NQR and NMR spin–lattice relaxation times T1 measurements. Two 35Cl NQR lines of the X=Cl derivative show the existence of two crystallographically inequivalent chlorine atoms in the high-temperature phase, in consistency with the reported crystal structure. Below 138K, each splits into a doublet indicating the symmetry breaking of the benzoic acid dimer. The proton dynamics was analyzed by a coherent and incoherent tunneling models, for the high- and low-temperature phases, respectively. The temperature dependence of the correlation time of proton translation was estimated. As for the X=I derivative, the proton dynamics was discussed similarly by 1H NMR T1 data by assuming occurrence of a phase transition at low-temperature.

Ultrafast Intramolecular Electron and Proton Transfer in Bis(imino)isoindole Derivatives.

Concerted motion of electrons and protons in the excited state is pertinent to a wide range of chemical phenomena, including those relevant for solar-to-fuel light harvesting. The excited state dynamics of small proton-bearing molecules are expected to serve as models for better understanding such phenomena. In particular, for designing the next generation of multielectron and multiproton redox catalysts, understanding the dynamics of more than one proton in the excited state is important. Toward this goal, we have measured the ultrafast dynamics of intramolecular excited state proton transfer in a recently synthesized dye with two equivalent transferable protons. We have used a visible ultrafast pump to initiate the proton transfer in the excited state, and have probed the transient absorption of the molecule over a wide bandwidth in the visible range. The measurement shows that the signal which is characteristic of proton transfer emerges within ∼710 fs. To identify whether both protons were transferred in the excited state, we have measured the ultrafast dynamics of a related derivative, where only a single proton was available for transfer. The measured proton transfer time in that molecule was ∼427 fs. The observed dynamics in both cases were reasonably fit with single exponentials. Supported by the ultrafast observations, steady-state fluorescence, and preliminary computations of the relaxed excited states, we argue that the doubly protonated derivative most likely transfers only one of its two protons in the excited state. We have performed calculations of the frontier molecular orbitals in the Franck-Condon region. The calculations show that in both derivatives, the excitation is primarily from the HOMO to LUMO causing a large rearrangement of the electronic charge density immediately after photoexcitation. In particular, charge density is shifted away from the phenolic protons and toward the proton acceptor nitrogens. The proton transfer is hypothesized to occur both due to enhanced acidity of the phenolic proton and enhanced basicity of the nitrogen in the excited state. We hope this study can provide insight for better understanding of the general class of excited state concerted electron-proton dynamics.

(Invited) Water and Proton Dynamics in Perfluorinated Surfactants and Membranes by QENS

The proton transport in fuel cell ionomer membranes is intimately related to the dynamical properties of water molecules confined in the interconnected network of ionic domains. The confinement of a fluid at the nanoscale profoundly affects the molecular dynamics and results in important deviations with respect to the bulk. The rotational and translational motions can be uniquely investigated by means of Quasi Elastic Neutron Scattering experiments, especially in the case of protonated systems due to the high incoherent cross section of hydrogen. Insights into the nature of the motions, i.e. diffusive or confined, can be gained by analysing the scattering function S(Q,w) measured on the ps-ns time-scale, Q being the momentum transfer and w the energy transfer of the neutrons. We will present here a comprehensive QENS study of water/proton dynamics in PFSA membranes [1,2] and perfluorinated surfactants used as a model soft confining system [3.4]. The Gaussian model for localized translational diffusion [5] was used to obtain quantitative parameters as the relaxation times, confinement sizes and local/nanometric diffusion coefficients of protons and water molecules. The comparison between state-of-the art fuel cell membranes and self-assembled well-defined surfactant phases as host confining matrices will be used to highlight the subtle interplay between spatial confinement and local interactions, resulting in the presence of slow hydronium motions and faster water motions localized in nanometric droplets. Additionally, we will show that the QENS technique can be invaluably combined with PFG-NMR to obtain a multi-scale comprehensive description of the ionic diffusion and water behaviour [1,2]. These experimental results are also cross-fertilized by Molecular Dynamics simulations performed on the same materials using a coarse-grained backbone and explicit solvent molecules [6]. [1] J-C. Perrin, S. Lyonnard and F. Volino; Quasielastic neutron scattering study of water dynamics in hydrated nafion membranes, Journal of Physical Chemistry C, 111 (2007), 3393-3404.[2] S. Lyonnard and G. Gebel, Neutrons for Fuel Cells Membranes: structure, sorption and transport properties, European Physical Journal 213 (1) (2012), 195-211;S. Lyonnard, Structure and Transport Properties in Polymer Electrolyte Membranes Probed at Microscopic Scales, Springer-Verlag, New Energies, Ed. German Antonio Ferreira, 2013. [3] S. Lyonnard, Q. Berrod, B-A. Bruning, G. Gebel, A. Guillermo, H. Ftouni, J. Ollivier and B. Frick, Perfluorinated surfactants as model charged systems for understanding the effect of confinement on proton transport and water mobility in fuel cell membranes. A study by QENS., Eur. Phys. Journal Special Topics, 189 (1), 205-216 (2010). [4] Q. Berrod; S. Lyonnard ; A. Guillermo.;J.Ollivier.; B. Frick; G.Gebel, QENS investigation of proton confined motions in hydrated perfluorinated sulfonic membranes and self-assembled surfactants, The European Physical Journal 2014, in press. [5] F. Volino, J-C. Perrin and S. Lyonnard; Gaussian Model for Localized Translational Motion: Application to Incoherent Neutron Scattering, Journal of Physical Chemistry B, 110 (2006), 11217-11223. [6] S. Hanot, S. Lyonnard and S. Mossa, Water confined in self-assembled ionic surfactants nanostructures, under review (2014).

Study of Morphological Variation on SiO2/Nafion Composite Membrane Under Different Hydration Condition

Passive air breathing proton exchange membranes fuel cells (AB-PEMFC) have a unique advantage such as remarkably reducing stack weight by minimizing balance of plant (BOP). However, the performance of the AB-PEMFC is worse than general PEMFC because it is difficult to controlling humidification and stack temperature. Thus, the proton exchange membranes (PEMs) which are used to AB-PEMFC have to stably work under various humidification and temperature. A lot of research groups are reported that inorganic fillers (SiO2, TiO2, ZrO2) and Nafion composite membrane show high performance under low relative humidity by increasing water retention on the membrane. In this study, we synthesize SiO2/Nafion composite membrane for compensating the weakness of AB-PEMFC. The performance of membrane electrode assembly (MEA) with SiO2/Nafion composite membrane is tested under different hydration and temperature conditions. For more detailed characterization, morphological variations of the composite membrane under hydration conditions are studied by using dynamic mode atomic force microscopy (DMAFM). DMAFM can map amplitude and phase distribution on the specimen surface. The phase lag, which is mechanical vibration changing of a cantilever, is resulting of tip-sample interaction, which is induced by microscopic mechanical property of sample. Thus, the DMAFM is best tool for characterizing heterogeneous materials such as composite membrane. The morphological structure variation on the composite membrane by water retention elevation is systematically studied by using phase lag changing under different humidification condition. References C. A. Hayden, and G. R. Goward, Proton Dynamics of Nafion and Nafion/SiO2 Composites by Solid tate NMR and Pulse Field Gradient NMR Macromolecules 40, (2007), 1529-1537 Bella Nicotera, Apostolos Enotiadis, Kristina Angjeli, Luigi Coppola, Giuseppe A. Ranieri, and imitrios Gournis, Effective Improvement of Water-Retention in Nanocomposite embranes Using Novel Organo-Modified Clays as Fillers or High Temperature PEMFCs, Journal of Physical Chemistry B 115, (2011), 9087–9097 Figure 1

Ion Dynamics and Mechanical Properties of Sulfonated Polybenzimidazole Membranes for High-Temperature Proton Exchange Membrane Fuel Cells

Polybenzimidazole (PBI)-based membranes are one of the systems of choice for polymer electrolyte fuel cells. Monomer sulphonation is one of the strategies suggested to improve proton transport in these membranes. We report a NMR and dynamic mechanical study aiming to investigate the effect of the sulphonation on the proton dynamics and the mechanical properties of the membranes. The analyses of 1H self-diffusion coefficients and 1H and 31P spectra versus temperature show that sulphonation causes the formation of interchain cross-links, which involve phosphoric acid molecules and the sulfonic groups. This, in turn, reduces the proton mobility and, consequently, the ionic conductivity. The increase of the membrane stiffness with sulphonation is confirmed by dynamic mechanical analysis through the behavior of the storage modulus.

Composite polymer electrolyte membranes based on Mg–Al layered double hydroxide (LDH) platelets for H2/air-fed fuel cells

Layered double hydroxides (LDH) were incorporated into Nafion polymer for the development of innovative hybrid nanocomposites exploitable in the high temperature PEM fuel cells. Mg2+/Al3+ LDHs (at two metal ratios, 2:1 and 3:1) with different countervailing anions in the interlayer space (CO32−, ClO4−, NO3−) were synthesized, while fully exfoliated composite membranes were prepared by solution intercalation method. NMR technique (diffusion and relaxation) was used to investigate the transport properties of water confined in membranes. The protons dynamics appears to be influenced from the Mg2+/Al3+ metal ratios of the nanoadditives, while the type of anion affects the electrochemical measurements. Compared to filler-free Nafion, both water absorption and diffusion in the hybrid membranes are increased with a noteworthy behavior at high temperature, proving the exceptional water retention property of these materials together with superior mechanical properties. The proton conductivity reaches 2.7×10−2Scm−1 at 100°C and 50% RH, as the best value for the composite based on Naf-LDH-Mg2+/Al3+(2/1)-NO3−.

Dielectric Response of Holmium Formate Crystallohydrate at 100mHz–10MHz

The dielectric response of Holmium formate crystallohydrate was studied in the frequency range of 10−1–107 Hz at 20–70°C. Experimental data were analyzed within formalisms of dielectric permittivity, conductivity and dielectric modulus. Specific features of dielectric properties and conductivity which were detected are typical for crystallohydrates and ferroelectrics with hydrogen bonds. It is assumed that such anomalies are caused by changes in the dynamics of protons that participate in hydrogen bonds in the crystal structure.

Bifurcations and Exact Solutions in a Model of Hydrogen-Bonded-Chains

A model of dynamics of protons in hydrogen-bonded quasi-one-dimensional networks was derived, which is a singular system of the second kind with three parameters and two singular straight lines. In this paper, we use the method of dynamical systems to discuss the bifurcations of phase portraits of the vector fields defined by the singular system. Corresponding to the phase orbits of the system in different parameter conditions, we compute all possible exact parametric representations of solutions. It is shown that in given parameter conditions, there exist solitary wave solutions, kink wave solutions and periodic wave solutions. The mentioned system has no peakon solution.

Model for the interpretation of nuclear magnetic resonance relaxometry of hydrated porous silicate materials.

Nuclear magnetic resonance (NMR) relaxation experimentation is an effective technique for probing the dynamics of proton spins in porous media, but interpretation requires the application of appropriate spin-diffusion models. Molecular dynamics (MD) simulations of porous silicate-based systems containing a quasi-two-dimensional water-filled pore are presented. The MD simulations suggest that the residency time of the water on the pore surface is in the range 0.03-12 ns, typically 2-5 orders of magnitude less than values determined from fits to experimental NMR measurements using the established surface-layer (SL) diffusion models of Korb and co-workers [Phys. Rev. E 56, 1934 (1997)]. Instead, MD identifies four distinct water layers in a tobermorite-based pore containing surface Ca2+ ions. Three highly structured water layers exist within 1 nm of the surface and the central region of the pore contains a homogeneous region of bulklike water. These regions are referred to as layer 1 and 2 (L1, L2), transition layer (TL), and bulk (B), respectively. Guided by the MD simulations, a two-layer (2L) spin-diffusion NMR relaxation model is proposed comprising two two-dimensional layers of slow- and fast-moving water associated with L2 and layers TL+B, respectively. The 2L model provides an improved fit to NMR relaxation times obtained from cementitious material compared to the SL model, yields diffusion correlation times in the range 18-75 ns and 28-40 ps in good agreement with MD, and resolves the surface residency time discrepancy. The 2L model, coupled with NMR relaxation experimentation, provides a simple yet powerful method of characterizing the dynamical properties of proton-bearing porous silicate-based systems such as porous glasses, cementitious materials, and oil-bearing rocks.

Electric response in the antiferroelectric crystal of 4,4′-di-t-butyl-2,2′-bipyridyl with chloranilic acid

The electric response was analyzed in the vicinity of the structural phase transition at 412K in the single crystals of 2,5-dichloro-3,6-dihydroxy-p-benzoquinone (chloranilic acid, CLA) with 4,4′-di-t-butyl-2,2′-bipyridyl (dtBBP). The dielectric permittivity of the complex measured along the b direction between 300 and 440K and at frequencies ranging from 500Hz to 2MHz indicates two phenomena. At low frequencies, dielectric losses are ascribed to the electric conductivity of the crystal, while at high frequencies, to the dielectric relaxation described by means of the Cole–Cole relationship. The parameters of the dielectric response: the relaxation time, τ, the dielectric increment, εο−ε∞, and the distribution parameter of the relaxation time, α, were estimated and analyzed. The low-temperature structure of the crystals indicates the antiferroelectric arrangement of the supramolecular hydrogen bonds. The dielectric results also presented for the deuterated crystals of dtBBP·CLA proved that the dynamics of protons in the hydrogen bonds are responsible for the mechanism of phase transition.

Proton Dynamics on Goethite Nanoparticles and Coupling to Electron Transport.

The surface chemistry of metal oxide particles is governed by the charge that develops at the interface with aqueous solution. Mineral transformation, biogeochemical reactions, remediation, and sorption dynamics are profoundly affected in response. Here we report implementation of replica-exchange constant-pH molecular dynamics simulations that use classical molecular dynamics for exploring configurational space and Metropolis Monte Carlo walking through protonation space with a simulated annealing escape route from metastable configurations. By examining the archetypal metal oxide, goethite (α-FeOOH), we find that electrostatic potential gradients spontaneously arise between intersecting low-index crystal faces and across explicitly treated oxide nanoparticles at a magnitude exceeding the Johnson-Nyquist voltage fluctuation. Fluctuations in adsorbed proton density continuously repolarize the surface potential bias between edge-sharing crystal faces, at a rate slower than the reported electron-polaron hopping rate in goethite interiors. This suggests that these spontaneous surface potential fluctuations will control the net movement of charge carriers in the lattice.

Direct visualization of concerted proton tunnelling in a water nanocluster

Many-body tunnelling is a complex but important phenomenon. Scanning tunnelling microscopy experiments with a Cl-terminated tip on a cyclic cluster of hydrogen-bonded water molecules now demonstrate controllable concerted tunnelling of four protons. Proton transfer through hydrogen bonds plays a fundamental role in many physical, chemical and biological processes1,2,3,4,5. Proton dynamics is susceptible to quantum tunnelling, which typically involves many hydrogen bonds simultaneously, leading to correlated many-body tunnelling6,7,8,9. In contrast to the well-studied incoherent single-particle tunnelling, our understanding of many-body tunnelling is still in its infancy. Here we report the real-space observation of concerted proton tunnelling in a cyclic water tetramer using a cryogenic scanning tunnelling microscope. This is achieved by monitoring the reversible interconversion of the hydrogen-bonding chirality of the water tetramer with a chlorine-terminated scanning tunnelling microscope tip. We found that the presence of the Cl anion at the tip apex may either enhance or suppress the concerted tunnelling process, depending on the details of the coupling symmetry between the Cl ion and the protons. Our work opens up the possibility of controlling the quantum states of protons with atomic-scale precision.

An analysis of hydrated proton diffusion in ab initio molecular dynamics

A detailed understanding of the inherently multiscale proton transport process raises a number of scientifically challenging questions. For example, there remain many (partially addressed) questions on the molecular mechanism for long-range proton migration and the potential for the formation of long-lived traps giving rise to burst-and-rest proton dynamics. Using results from a sizeable collection of ab initio molecular dynamics (AIMD) simulations (totaling ∼2.7 ns) with various density functional approximations (Becke-Lee-Yang-Parr (BLYP), BLYP-D3, Hamprecht-Cohen-Tozer-Handy, B3LYP) and temperatures (300-330 K), equilibrium and dynamical properties of one excess proton and 128 water molecules are studied. Two features in particular (concerted hops and weak hydrogen-bond donors) are investigated to identify modes in the system that are strongly correlated with the onset of periods of burst-and-rest dynamics. The question of concerted hops seeks to identify those time scales over which long-range proton transport can be classified as a series of sequential water hopping events or as a near-simultaneous concerted process along compressed water wires. The coupling of the observed burst-and-rest dynamics with motions of a fourth neighboring water molecule (a weak hydrogen-bond donor) solvating the protonated water molecule is also investigated. The presence (absence) of hydrogen bonds involving this fourth water molecule before and after successful proton hopping events is found to be strongly correlated with periods of burst (rest) dynamics (and consistent with pre-solvation concepts). By analyzing several realizations of the AIMD trajectories on the 100-ps time scale, convergence of statistics can be assessed. For instance, it was observed that the probability for a fourth water molecule to approach the hydronium, if not already proximal at the beginning of the lifetime of the hydronium, is very low, indicative of the formation of stable void regions. Furthermore, the correlations of the neighboring water atoms are identified as the fourth water approaches the hydronium. Finally, the temperature effects on structural and dynamical properties are studied.

Mechanisms and dynamics of protonation and lithiation of ferrocene.

By elucidating the mechanism of the simplest electrophilic substitution reaction of ferrocene, it was found that the verification of the protonation reaction has been a difficulty. In the work reported here, ab initio chemical dynamics simulations were performed at B3LYP/DZVP level of theory to understand the atomic level mechanisms of protonation and lithiation of ferrocene. Protonation of ferrocene resulted in the agostic and metal-protonated forms. Trajectory calculations revealed that protonation of ferrocene occurs by exo and endo mechanisms, with exo being the major path. H(+) was found to be mobile and hopped from the Cp ring to the metal center and vice versa after the initial attack on ferrocene, with the metal-complex having a shorter lifetime. These results remove the ambiguity surrounding the mechanism, as proposed in earlier experimental and computational studies. Lithiation of ferrocene resulted in the formation of cation-π and metal-lithiated complexes. Similar to protonation, trajectory results revealed that both exo and endo paths were followed, with the exo path being the major one. In addition, lithiated-ferrocene exhibited planetary motion. The major path (exo) followed in the protonation and lithiation of ferrocene is consistent with the observations in earlier experimental studies for other hard electrophiles.

Proton dynamics in bacterial spores, a neutron scattering investigation

Results from first neutron scattering experiments on bacterial spores are reported. The elastic intensities and mean square displacements have a non-linear behaviour as function of temperature, which is in agreement with a model presenting more pronounced variations at around 330 K (57 ∘C) and 400 K (127 ∘C). Based on the available literature on thermal properties of bacterial spores, mainly referring to differential scanning calorimetry, they are suggested to be associated to main endothermic transitions induced by coat and/or core bacterial response to heat treatment.

Proton dynamics in oxides: insight into the mechanics of proton conduction from quasielastic neutron scattering.

This article is concerned with the use of quasielastic neutron scattering as a technique for investigation of the dynamical properties of proton conducting oxides. Currently, the main interest in these materials comes from their promise as electrolytes in future electrochemical devices and particularly through their use as electrolytes in next-generation, intermediate-temperature, fuel cells. However, the realization of such devices depends critically on the development of new, more highly proton conducting oxides. Such a development depends on increasing the current understanding of proton conduction in oxides and for this purpose quasielastic neutron scattering is an important mean. The aim of this article is to introduce the non-specialist reader to the basic principles of quasielastic neutron scattering, its advantages and disadvantages, to summarize the work that has been done on proton conducting oxides using this technique, as well as to discuss future opportunities within this field of research.