Proton Exchange Process Research Articles

15N NMR Spectroscopy, X-ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases

Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}C5H4N (I) and 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2), and [II-H2][BPh4]2 (3). Solution-state (1)H and (15)N NMR spectroscopy shows a symmetrical cation in 2, indicating a facile proton-exchange process in solution. Solid-state (15)N NMR data differentiates between the two groups, indicating a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for 1a by single-crystal neutron diffraction. The crystal structure of 1a shows association of two [I-H](+) cations within a cage of [BPh4](-) anions. Computational analysis performed in the gas phase and in MeCN solution shows that the free energy barrier to transfer a proton between imino centers in [II-H](+) is 1 order of magnitude lower in MeCN than in the gas phase. The results provide evidence that linking hpp groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished pKa(MeCN) values. Spectrophotometrically measured pKa values are in excellent agreement with calculated values and confirm that I and II are superbases in solution.

Electro-optic mode switch based on lithium-niobate Mach-Zehnder interferometer.

We propose an electro-optic mode switch based on an optical waveguide Mach-Zehnder interferometer fabricated with x-cut lithium niobate by the annealed proton exchange process. The device can switch between the fundamental mode and the higher-order mode with a low driving voltage. Our typical fabricated device, which has a total length of ∼24 mm, shows a mode extinction ratio of ∼35 dB and a 20-dB bandwidth of ∼12 nm at the wavelength 1552nm, when driven at a voltage of 1.7V at 26°C. High performance can be obtained at any wavelength in the C+L band with a driving voltage varying by no more than 3V. The proposed mode switch is easy to fabricate and could find applications in reconfigurable mode-division-multiplexing systems.

Periodic domain patterning by electron beam of proton exchanged waveguides in lithium niobate

Formation of domain structure by electron beam irradiation in congruent lithium niobate covered by surface dielectric layer with planar and channel waveguides produced by Soft Proton Exchange (SPE) process has been studied. Formation of domains with arbitrary shapes as a result of discrete switching has been revealed. The fact was attributed to ineffective screening of depolarization field in the crystals with a surface layer modified by SPE process. The dependences of the domain sizes on the dose and the distance between irradiated areas have been revealed. Finally, we have demonstrated that electron beam irradiation of lithium niobate crystals with surface resist layer can produce high quality periodical domain patterns after channel waveguide fabrication. Second harmonic generation with normalized nonlinear conversion efficiency up to 48%/(W cm2) has been achieved in such waveguides.

Fabrication of Optical Modulator Based on Proton Exchange

<p>For the investigation of optical modulator, the optical wave-guide was fabricated on x-cut LiNbO<sub>3</sub> substrate using proton exchange method with self-aligned electrode. The electrode pattern was designed using a self-aligned thin film electrode method. After proton exchange process, the wave-guide could be prepared by annealing process to control the width and depth of the optical wave-guide. The initial crossover state of the fabricated 1´2 optical switch was observed with controlling the annealing process variables and the structure of self-aligned thin film electrodes. As the results in the present work, the measured cross talk and minimum detectable switching voltage were obtained at the values of -29.5dB and 8.0V, respectively, with good merits.</p>

Unlocking the observation of different proton populations in fluorinated polymers by solid-state 1H and 19F double resonance NMR spectroscopy.

Nafion proton exchange membranes (PEMs) for fuel cell applications are extensively studied and commercially applied, but their unique proton conduction capabilities are still somewhat unexplained. For studying proton dynamics in situ, molecular level spectroscopic techniques have been of limited utility so far. By solid-state 1H and 19F double resonance nuclear magnetic resonance (NMR) spectroscopy using the recently revived multiple contact cross-polarization (MC-CP) pulse sequence along with double-quantum 1H-1H filtering, high resolution proton populations distinct from the dominant water resonance were observed in Nafion for the first time. This methodology quenches signal decay due to spin-lattice relaxation in the rotating frame and enables magnetization transfer between the relatively mobile 1H and 19F spin baths in Nafion. Further studies of these previously unrevealed proton populations will lead to a better understanding of the Nafion proton conduction mechanism and proton exchange processes in general.

Channel waveguides and y-junctions in x-cut single-crystal lithium niobate thin film.

Proton exchanged channel waveguides in x-cut single-crystal lithium niobate thin film could avoid optical leakage loss which existed in the z-cut case. Indicated by simulations, the mechanism and condition of the optical leakage loss were studied. The light energy in the exchanged layer and the mode sizes were calculated to optimize the parameters for fabrication. By a very short time (3 minutes) proton exchange process without anneal, the channel waveguide with 2 μm width and 0.16 μm exchanged depth in the x-cut lithium niobate thin film had a propagation loss as low as 0.2 dB/cm at 1.55 μm. Furthermore, the Y-junctions based on the low-loss waveguide were designed and fabricated. For a Y-junction based on the 3 μm wide channel waveguide with 8000 μm bending radius, the total transmission could reach 85% ~90% and the splitting ratio maintained at a stable level around 1:1. The total length was smaller than 1 mm, much shorter than the conventional Ti-diffused and proton exchanged Y-junctions in bulk lithium niobate.

Low-loss waveguides in a single-crystal lithium niobate thin film.

We report low-loss channel waveguides in a single-crystal LiNbO(3) thin film achieved using the annealed proton exchange process. The simulation indicated that the mode size of the α phase channel waveguide could be as small as 1.2 μm(2). Waveguides with several different widths were fabricated, and the 4 μm-wide channel waveguide exhibited a mode size of 4.6 μm(2). Its propagation loss was accurately evaluated to be as low as 0.6 dB/cm at 1.55 μm. The single-crystal lattice structure in the LiNbO(3) thin film was preserved by a moderate annealed proton exchange process (5 min of proton exchange at 200°C, followed by 3 h annealing at 350°C), as revealed by measuring the extraordinary refractive index change and x ray rocking curve. A longer proton exchange time followed by stronger annealing would destroy the crystal structure and induce a high loss in the channel waveguides.

Dynamics of proton exchange in a model phosphonic acid-functionalized polymer

The dynamics and mechanism of proton exchange in phosphonic acid-functionalized polymers were studied using poly(vinyl-phosphonic acid) (PVPA) as a model system along with quantum chemical calculations and Born–Oppenheimer molecular dynamics (BOMD) simulations at the B3LYP/TZVP level as model calculations. This theoretical study began with searching for the smallest, most active polymer segments and their intermediate conformations which could be involved in the local proton-exchange process. The B3LYP/TZVP results confirmed that a low local dielectric environment and excess proton conditions are required to generate the intermediate conformations, and the shapes of the potential energy curves of the proton exchange between the two phosphonic acid functional groups are sensitive to the local conformational changes. In contrast, a high local dielectric environment increases the energy barriers, thereby preventing the proton from returning to the original functional group. Based on the static results, a mechanism for the proton exchange between the two functional groups involving fluctuations in the local dielectric environment and a local conformational change was proposed. The BOMD results confirmed the proposed mechanism by showing that the activation energies for the proton exchange in the hydrogen bond between two immobilized phosphonic acid moieties, obtained from the exponential relaxation behaviors of the envelopes of the velocity autocorrelation functions and the 1H Nuclear Magnetic Resonance (NMR) line-shape analyses, are too low to be the rate-determining process. Instead, coupled librational motion in the backbone which leads to the interconversion between the two intermediate conformations possesses higher activation energy, and therefore represents one of the most important rate-determining processes. These findings suggested that the rate of the proton exchange in the model phosphonic acid-functionalized polymer is determined by the polymer mobility which, in this case, is the large-amplitude librational motion of the vinyl backbone. © 2015 Wiley Periodicals, Inc.

Lithium-Niobate Mach-Zehnder Interferometer With Enhanced Index Contrast by SiO<sub>2</sub> Film

An electro-optic tunable lithium-niobate (LN) waveguide Mach-Zehnder interferometer (MZI) is designed and fabricated with the annealed proton-exchange process. The MZI consists of a straight waveguide arm and a bent waveguide arm with a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film deposited on the two sides. The residual stress generated by the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film can enhance the index contrast of the bent waveguide arm and thus reduce the bending loss of the MZI. A comparison of several experimental devices fabricated on the same chip size of 50 mm x 7 mm confirms significant reduction of the insertion loss by incorporating the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film. For our best sample, which functions as a tunable comb filter with a channel spacing of 0.75 nm in the C-band, the insertion losses of the two output ports are 13.5 and 6 dB, respectively, and the corresponding maximum extinction ratios are ~23 and ~1.5 dB. The device has an electrical wavelength-tuning sensitivity of -.0.2 nm/V and a temperature sensitivity of ~0.1 nm/°C. Our design approach can significantly improve the quality of LN devices that require waveguide bends.

Para-Selectivity of silicalite-1 coated MFI type galloaluminosilicate in aromatization of light alkanes

MFI type galloaluminosilicates (GaAlMFI) were coated with inert silicalite-1 layers. The silicalite-1 coating could improve the para-selectivity for xylene production in aromatization of propane. In the silicalite-1 coating step, low alkali concentration in the synthesis solution enhanced para-selectivity. A proton-exchange procedure also had an impact on the para-selectivity. Significant decrease of para-selectivity was observed over the silicalite-1 coated GaAlMFI with repeated proton-exchange. From the 71Ga and 27Al MAS NMR measurements, demetalation occurred during the proton-exchange process, and those metal species is likely to become catalytically active sites without para-selectivity. From the aromatization of propane and n-butane, para-selectivity in n-butane aromatization was found to be higher than that in propane aromatization. From the product distribution, naphthalene and its derivatives were easier to be produced in propane aromatization compared to n-butane aromatization. The formation of naphthalene and its derivatives may relate to decrease in the para-selectivity in propane aromatization.

Water Effect on Proton Exchange of X-cut Lithium Niobate in the Melt of Benzoic Acid

The influence of controllable amount of water admixture (up to 0.5 w.%) in benzoic acid on the proton exchange process of X-cut congruent lithium niobate was studied in this paper by means of mode spectroscopy, optical microscopy, optical profilometry, atomic force microscopy, XRD, and chemical etching. It has been determined that addition of 0.5 w.% Н2О to benzoic acid increases proton exchange rate and changes phase composition and number of defects of proton exchanged layer in lithium niobate.

Correlation of the antimicrobial activity of salicylaldehydes with broadening of the NMR signal of the hydroxyl proton. Possible involvement of proton exchange processes in the antimicrobial activity

Certain substituted salicylaldehydes are potent antibacterial and antifungal agents and some of them merit consideration as potential chemotherapeutic agents against Candida infections, but their mechanism of action has remained obscure. We report here a distinct correlation between broadening of the NMR signal of the hydroxyl proton of salicylaldehydes and their activity against several types of bacteria and fungi. When proton NMR spectra of the compounds were determined using hexadeuterodimethylsulfoxide as solvent and the height of the OH proton signal was measured, using the signal of the aldehyde proton as an internal standard, it was discovered that a prerequisite of potent antimicrobial activity is that the proton signal is either unobservable or relatively very low, i.e. that it is extremely broadened. Thus, none of the congeners whose OH proton signal was high were potent antimicrobial agents. Some congeners that gave a very low OH signal were, however, essentially inactive against the microbes, indicating that although drastic broadening of the OH signal appears to be a prerequisite, also other (so far unknown) factors are needed for high antimicrobial activity. Because broadening of the hydroxyl proton signal is related to the speed of the proton exchange process(es) involving that proton, proton exchange may be involved in the mechanism of action of the compounds. Further studies are needed to analyze the relative importance of different factors (such as electronic effects, strength of the internal hydrogen bond, co-planarity of the ring and the formyl group) that determine the rates of those processes.

Potentiometric Titration and Out-Of-Equilibrium pH Response of the Biotite-Water System

Minerals in soils contribute significantly to the capacity of soils to buffer pH disturbance. In this paper, we present the pH buffering capacity of a common soil phyllosilicate mineral, biotite. We performed equilibrium potentiometric titrations and we also monitored the pH response kinetics of the mineral-water system in order to develop an out-of-equilibrium model able to capture the interactions between minerals and the surrounding aqueous fluid. During titrations, after each addition of titrant solution from pH 11 to ca. 3, the pH response patterns were monitored over time until reaching a pseudo-equilibrium pH value. Based on the potentiometric dataset Best7, equilibrium calculations were performed to obtain the concentrations and the equilibrium protonation constants of each deprotonable component that fit best our titration curve. In parallel, the out-of-equilibrium pH responses over time at each point of the titration were used in a simple first-order kinetic approach that allow for the determination of “slow” proton exchange [H+ex]t0(mol L-1) and the associated rate constants for the exchange reactions, k (s-1). Our results show a maximum in “slow” proton exchange [H+ex]t0 associated to a minimal value of k at pH < 5 while at neutral and basic pH exhibits the opposite, i.e. , fast rate constant for a minimum value of the “slow” proton exchange. Expressing the observed “slow” proton exchange processes in terms of entropy production, our result demonstrate that the maximum resilience stability of biotite-water system to pH perturbation is in acidic pH, probably due to the consumption of proton associated with biotite dissolution reactions.

Physical Properties and CO2 Reaction Pathway of 1-Ethyl-3-Methylimidazolium Ionic Liquids with Aprotic Heterocyclic Anions.

Ionic liquids (ILs) with aprotic heterocyclic anions (AHA) are attractive candidates for CO(2) capture technologies. In this study, a series of AHA ILs with 1-ethyl-3-methylimidazolium ([emim](+)) cations were synthesized, and their physical properties (density, viscosity, and ionic conductivity) were measured. In addition, CO(2) solubility in each IL was determined at room temperature using a volumetric method at pressures between 0 and 1 bar. The AHAs are basic anions that are capable of reacting stoichiometrically with CO(2) to form carbamate species. An interesting CO(2) uptake isotherm behavior was observed, and this may be attributed to a parallel, equilibrium proton exchange process between the imidazolium cation and the basic AHA in the presence of CO(2), followed by the formation of "transient" carbene species that react rapidly with CO(2). The presence of the imidazolium-carboxylate species and carbamate anion species was verified using (1)H and (13)C NMR spectroscopy. While the reaction between CO(2) and the proposed transient carbene resulted in cation-CO(2) binding that is stronger than the anion-CO(2) reaction, the reactions of the imidazolium AHA ILs were fully reversible upon regeneration at 80 °C with nitrogen purging. The presence of water decreased the CO(2) uptake due to the inhibiting effect of the neutral species (protonated form of AHA) that is formed.

Highly Confining Proton Exchanged Waveguides on Z-Cut LiNbO&lt;sub&gt;3&lt;/sub&gt; With Preserved Nonlinear Coefficient

We propose a new proton exchange process to reproducibly realize on lithium niobate highly confining (δn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> = 0.1) planar waveguides without degrading the nonlinear coefficient of the crystal. Waveguides were fabricated with different acidic baths and the crystallographic changes, the index profiles and the local SHG response were measured. We show that for certain fabrication conditions, characteristic of the so-called high-index soft proton exchange process, the high-index modification can be obtained without degrading the crystal nonlinear coefficient. This result should open the way to the fabrication of very efficient nonlinear devices.

Cationic Ring Opening polymerization of ε-caprolactam by a Montmorillonite Clay Catalyst

The ring opening bulk polymerization of ε-caprolactam catalyzed by Maghnite-H+ was reported. Maghnite-H+ is a montmorillonite silicate sheet clay was prepared through a straight forward proton exchange process. The effect of the amount of catalyst, and temperature was studied. Increasing Maghnite-H+ proportion and temperature produced the increase in ε-caprolactam conversion. The kinetics indicated that the polymerization rate is first order with respect to monomer concentration. Mechanism studies showed that monomer inserted into the growing chains with the acyl–oxygen bond scission rather than the break of alkyl–oxygen bond. © 2014 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

Theoretical study on collision dynamics of H+ + CH4 at low energies

In this work we make an investigation on collision dynamics of H(+) + CH4 at 30 eV by using time-dependent density functional theory coupled with molecular dynamics approach. All possible reactions are presented based on 9 incident orientations. The calculated fragment intensity is in nice agreement with experimental results. The mechanism of reaction transition for dissociation and proton exchange processes is explained by the intra-molecule energy transfer. However, the energy loss of the proton is in poor agreement with experimental results. The discrepancy is attributed to the mean-field treatment of potential surface. We also studied the dependence on initial velocity of both proton and methane. In addition, we find that for dynamical evolution a different self-interaction correction (SIC) may lead to different results, but with respect to the position of rainbow angle, average-density SIC seems to have reasonable correction.

Hydrogen bonding induced proton exchange reactions in dense D2-NH3 and D2-CH4 mixtures.

We have investigated high-pressure behaviors of simple binary mixtures of NH3 and D2 to 50 GPa and CH4 and D2 to 30 GPa using confocal micro-Raman spectroscopy. The spectral data indicate strong proton exchange reactions occur in dense D2-NH3 mixture, producing different isotopes of ammonia such as NH3, NH2D, NHD2, and ND3. In contrast, the proton exchange process in dense D2-CH4 mixture is highly limited, and no vibration feature is apparent for deuterated methane. The vibrational modes of H2 isotopes in D2-NH3 are blue shifted from those of pure H2 isotopes, whereas the modes of D2-CH4 show overall agreement with those in pure D2 and CH4. In turn, this result advocates the presence of strong repulsion and thereby internal pressure in D2-NH3 mixture, which are absent in D2-CH4. In fact, the bond length of hydrogen molecules in D2-NH3, calculated from the present spectral data, is shorter than that observed in pure hydrogen - supporting the enhanced intermolecular interaction in the mixture. Comparing the present spectral results with those previously observed in D2-H2O mixtures further suggests that the strength of repulsive interaction or the magnitude of internal pressure in the mixtures is proportional to the strength of hydrogen bonding in H2O, NH3, and CH4 in decreasing order. Hence, we suggest that the proton exchange is assisted by hydrogen bonding in these molecules.

Molecular Interactions, Proton Exchange, and Photoinduced Processes Prompted by an Inclusion Process and a [2]Pseudorotaxane Formation

Appropriate design of the host and guest components allows formation of a novel [2]pseudorotaxane complex with an interrupted photoinduced electron transfer (PET)-coupled fluorescence resonance energy transfer (FRET) response. This is the first example of an inclusion complex with NO6-based azacrown ether as the host unit (H). Different guest molecules (G1, G2, G3, and G4) with varying stopper size are used for the studies. Unlike G1, G2, and G3, G4 with a relatively bulkier stopper fails to form a [2]pseudorotaxane complex. Isothermal titration microcalorimetry measurements reveal a systematic increase in the association constant for H·G1, H·G2, and H·G3 with a change in the stopper size. Thermodynamic data suggest that the formation of H·G1/H·G2/H·G3 is exclusively driven by a large positive entropic gain (TΔS = 19.69/26.80/21.81 kJ·mol(-1)), while the enthalpy change is slightly negative for H·G1/H·G3 (-2.61/-1.97 kJ·mol(-1)) and slightly positive for H·G2 (ΔH = 5.98 kJ·mol(-1)). For these three inclusion complexes, an interrupted PET-coupled FRET response is observed with varying efficiency, which is attributed to the subtle differences in acidity of the NH2(+) unit of the guest molecules and thus the proton exchange ability between the host and respective guest. This is substantiated by the results of the computational studies.

The C-terminal extension of bacterial flavodoxin-reductases: Involvement in the hydride transfer mechanism from the coenzyme

To study the role of the mobile C-terminal extension present in bacterial class of plant type NADP(H):ferredoxin reductases during catalysis, we generated a series of mutants of the Rhodobacter capsulatus enzyme (RcFPR). Deletion of the six C-terminal amino acids beyond alanine 266 was combined with the replacement A266Y, emulating the structure present in plastidic versions of this flavoenzyme. Analysis of absorbance and fluorescence spectra suggests that deletion does not modify the general geometry of FAD itself, but increases exposure of the flavin to the solvent, prevents a productive geometry of FAD:NADP(H) complex and decreases the protein thermal stability. Although the replacement A266Y partially coats the isoalloxazine from solvent and slightly restores protein stability, this single change does not allow formation of active charge-transfer complexes commonly present in the wild-type FPR, probably due to restraints of C-terminus pliability. A proton exchange process is deduced from ITC measurements during coenzyme binding. All studied RcFPR variants display higher affinity for NADP+ than wild-type, evidencing the contribution of the C-terminus in tempering a non-productive strong (rigid) interaction with the coenzyme. The decreased catalytic rate parameters confirm that the hydride transfer from NADPH to the flavin ring is considerably hampered in the mutants. Although the involvement of the C-terminal extension from bacterial FPRs in stabilizing overall folding and bent-FAD geometry has been stated, the most relevant contributions to catalysis are modulation of coenzyme entrance and affinity, promotion of the optimal geometry of an active complex and supply of a proton acceptor acting during coenzyme binding.