Hydroxyl Proton Research Articles

Dissolution of bamboo hemicellulose in 1-butyl-3-methylimidazolium halide-based ionic liquids

Understanding the dissolution of hemicellulose in ionic liquids (ILs) is important in order to explore efficient utilization of ILs for fractionating lignocellulose and extracting hemicellulose. In this study, three ILs, namely 1-butyl-3-methylimidazolium chloride (BmimCl), 1-butyl-3-methylimidazol-ium bromide (BmimBr), and 1-butyl-3-methylimidazolium iodide (BmimI), were used as solvents to dissolve bamboo hemicellulose over the temperature range of 80 °C to 150 °C. Representative hemicellulose with a solubility higher than 9.0 g/100 g IL was regenerated and characterized. The interaction between the hemicellulose and ILs was evaluated using carbon-13 nuclear magnetic resonance. The results showed that the solubility of hemicellulose in the ILs was in the following order: BmimCl > BmimBr > BmimI. Though moderate degradation of the hemicellulose possibly occurred during dissolution, the main chain of the hemicellulose was almost unchanged. The enhanced solubility of the hemicellulose was because of the formation of hydrogen bonds between the hydroxyl proton of the hemicellulose and anion of the ILs.

Monitoring the mechanism and kinetics of a transesterification reaction for the biodiesel production with low field 1H NMR spectroscopy

The production of bio-fuels as a substitute for crude oil is steadily increasing. The reactions producing bio-fuels need detailed analysis concerning mechanism and reaction kinetics to increase yield. Standard analytical methods provide a way to monitor the reaction in real-time but detailing the mechanism through chemical shift fingerprinting is not possible using these methods. The reaction kinetics during biofuel production can be followed in real-time by nuclear magnetic resonance (NMR) spectroscopy by passing reaction mixture through the magnet. The present work reports the use of desktop NMR spectroscopy for a real-time study of the transesterification of triglycerides (vegetable oil) with methanol for formation of methyl esters (biodiesel) to detail catalytic activity, reaction mechanism and kinetics. The reaction was investigated for different catalyst concentrations, different molar ratios of reactants, and different temperatures. The changes in the chemical shift of the hydroxyl protons in the reaction mixture resulting from changing catalyst concentration and temperature provide information about the role of the catalyst in the aqueous and organic phases. The reaction was determined to be mass transfer controlled in the initial stage and kinetically controlled at later stage depending upon the reaction conditions. Analysis of the reaction for different molar ratios of oil and biodiesel and for increasing methanol concentration suggests the formation of dimers. The time variation of the methyl ester (bio-diesel) concentration was determined by partial least squares regression (PLS-R) using high-field NMR spectroscopy as reference. To obtain rate constants for each reaction the kinetics were modeled assuming fatty acid methyl esters as major product, and mono and diglycerides as intermediates. The 95% confidence intervals were derived by a Monte-Carlo analysis. The reaction kinetics are compared to those obtained by peak fitting of low-field spectra.

Synthesis of Bi-deficient monolayered Bi2WO6 nanosheets with enhanced photocatalytic activity under visible light irradiation

The strong protonated hydroxyl groups around Bi vacancies could efficiently promote the separation of photoexcited electron–hole pairs.

Primary and secondary photochemical transformations of biologically active precursor - Nitro-Nitrosyl ruthenium complex

Photolysis of [Ru(II)NOPy2(NO2)2OH] (A) complex was studied under blue light (445 nm) irradiation. The primary photo-process results in the formation of nitric oxide (NO) and a paramagnetic RuIII compound, the latter was detected by EPR technique. The quantum yield (6–11%) of primary photolytic process was determined from the evolution of UV–vis spectra in different solvents (water, ethanol, dimethyl sulfoxide and acetonitrile). The secondary processes compete with the NO-release and lead to a variety of ruthenium forms including new nitrosyl forms. Namely, processes of hydroxyl protonation, pyridine photocleavage and nitro – nitrito bond isomerization take a place. HPLC combined with IR and 15N – NMR spectroscopies clarified the composition of the new forms and their transformations during the photolysis. Supporting EPR and IR DFT calculations confirmed the structure of photoproducts. The oxidation of NO in the secondary processes significantly influences on the total amount of nitric oxide release and reaction routes.

Mass density, sound velocity, mixing enthalpy, 1H NMR, Ab initio calculations and intermolecular interactions in binary mixtures of N-methylimidazole + water, +methanol, +ethanol, +1-propanol, +2-propanol

Over a full molar fraction range, for binary mixtures of x N-methylimidazole (hereafter abbreviated to N-mim) + (1 − x) water, +methanol, +ethanol, +1-propanol, +2-propanol, the mass density and sound velocity at (288.15, 298.15, 308.15, 318.15, 328.15) K, the mixing enthalpy at 298.15 K, and 1H NMR measurements, combined with the Ab initio calculations, have been carried out.The excess molar volume, excess average volume expansivity, excess sound velocity, excess isentropic compressibility, and the molar mixing enthalpy were calculated and fitted to a Redlich-Kister equation. The partial molar enthalpy of N-mim, the difference between the partial molar volume and the molar volume of N-mim, and the protons chemical shift changes were calculated and compared. The chemical shifts of N-mim protons, hydroxyl protons, and α-H (proton on the carbon atom next to the –OH group) of alcohols in the mixtures containing alcohols were explained by a model proposed by Gupta et al..The properties indicate that the intermolecular interactions between unlike molecules are stronger than those between like molecules. An increase in temperature has a greater influence on the mixture containing water than on pure components, and has a greater influence on pure components than on mixtures containing alcohols. The predominant interactions between unlike molecules are hydrogen-bonded complexes in the mixture containing water, and are N-mim: alcohols = 1:1 hydrogen-bonded networks in the mixtures containing alcohols, where the alkyl groups of alcohols are electron-withdrawing groups, and the methyl group of N-mim is an electron-donating group. Ab initio calculations proved that the N⋯HO hydrogen bond in the mixtures is more stable and stronger than HO⋯HO in pure water or in pure alcohols and N⋯HC in pure N-mim. The strength sequence of the intermolecular interactions between unlike molecules for the mixtures at 298.15 K is N-mim–water > N-mim–methanol > N-mim–ethanol > N-mim–1-propanol ≥ N-mim–2-propanol.

Classical and ab initio molecular dynamics simulation of p-coumaric acid in methanol-modified supercritical carbon dioxide

Classical and ab initio molecular dynamics simulation of p-coumaric acid in methanol-modified supercritical carbon dioxide has been performed. Structural properties such as radial distribution functions, coordination numbers, and average numbers of hydrogen bonds were obtained. Vibration density of states was calculated from velocity autocorrelation functions. The redshift in the frequency of the OH stretching mode for hydroxyl and carboxyl groups of the solute in fluid compared with the same values for the p-coumaric acid in gas phase is an evidence for H-bond formation. Analysis of the hydrogen bond lifetimes indicates that p-coumaric acid forms stable hydrogen bonds with methanol through hydroxyl and carboxyl protons. At the same time, the longest period of the absence of hydrogen bond is observed for carbonyl oxygen.

Unraveling the sequence of the electronic flow along the water-assisted ring-opening reaction in mutagen MX

The ring-opening reaction of 3-chloro-4-(dichloromethyl)-5-hydroxy-5H-furan-2-one (mutagen X or MX) remains an intriguing subject of both theoretical and experimental interest. This relies on the fact of uncertainty concerning which structure acts as the real mutagen. Challenging the current accepted idea of a direct tautomeric process transforming the furanone ring to the oxobutenoic acid chain structure, a water-assisted process via a six-membered transition structure (TS) is revealed to proceed with activation energy of 24.5 kcal/mol (i.e., 26.8 kcal/mol lower than the conventional tautomeric process). An analysis based on the application of catastrophe theory to the evolution of the electron localization function topology along the intrinsic reaction coordinate reveals that the process can be explained, using a Lewis-like chemical language, as a result of the electronic activation of the furanone specie by the water molecule: on the activation pathway (i.e., before TS is reached), the electronic perturbation introduced by the water reagent is first observed on the valence shell of the ring oxygen and then on that of the hydroxylic one. Thereafter, the release of the hydroxylic hydrogen and the breaking of the C–O bond in the furanone ring follow the electronic rearrangement. On the de-activation pathway, the water molecule first captures the hydroxylic proton. Then, a hydronium-like structure transfers a proton to the oxygen of the furanone moiety contributing to stabilize the open ring structure. The above description of such a favorable ring-opening process become thus associated with nine topological structural stability domains, featuring the following sequence of catastrophes: C5H3O3Cl3 + H2O: 10-C†F†[FU]UC[C†C†]CC-0: C5H3O3Cl3 + H2O.

Assessing the pKa‐Dependent Activity of Hydroxyl Hydrogen Bond Donors in the Organocatalyzed Cycloaddition of Carbon Dioxide to Epoxides: Experimental and Theoretical Study

AbstractThe development of hydrogen bond donors (HBDs) as catalytic moieties in the cycloaddition of carbon dioxide to epoxides is an active field of research to access efficient, inexpensive and sustainable metal‐free systems for the conversion of carbon dioxide to useful chemicals. Thus far, no systematic attempt to correlate the activity of a diverse selection of HBDs to their physico‐chemical properties has been undertaken. In this work, we investigate factors influencing the catalytic activity of hydroxyl HBDs from different chemical families under ambient conditions by considering the HBDs Brønsted acidity (expressed as pKa), the number of hydroxyls and structural aspects. As an effect, this study highlights the crucial role of the hydroxyl protons’ Brønsted acidity in determining the catalytic activity of the HBDs, identifies an ideal range for the hydroxyl HBDs proton acidity (9 <pKa <11) and leads to a revaluation of phenol and to the discovery of a simple ascorbic acid derivative as efficient HBDs for the title cycloaddition reaction. Density functional theory (DFT) calculations show mild reactions barriers for the reaction catalysed by phenol and suggest the occurrence of aggregation between molecules of ascorbic acid as a further factor affecting catalytic activity.magnified image

Anthocyanidins structural study using positive electrospray ionization triple quadrupole mass spectrometry and H/D exchange.

We report herein a detailed structural study by collision-induced dissociation (CID) of nonglycosylated anthocyanins (anthocyanidins) using electrospray ionization triple quadrupole mass spectrometry (ESI-QqQ) and isotope labeling experiments to understand the fragmentation process often used in mass spectrometry analysis of this class of compounds. Tandem mass spectrometric product ion spectra for three anthocyanidins (cyanidin, delphynidin, and pelargonin) were evaluated to propose fragmentation mechanisms to this natural colorant class of organic compounds. The proposed rearrangements, retro Diels-Alder reaction, water loss, CO losses, and stable acylium ion formation, were evaluated based on tandem mass spectrometric experiments of normal and labeled precursor ions together to computational thermochemistry. B3LYP/6-311+G** ab initio calculations studies were carried out to obtain energy diagrams to show the viability of the proposed mechanisms. The CO losses fragmentation channels have lower energies when compared with water losses and the other proposed fragmentations. The isotope labeling experiments indicate the H/D exchange of the hydroxyl protons and corroborate the proposed general fragmentation mechanism for anthocyanidins.

Experimental Evidence of Synergistic Interactions in Pyrrole-Phenol Complexes at Low Temperatures under Isolated Conditions.

The simultaneous possession of π-electron clouds and acidic hydrogen atoms in pyrrole (C4H5N) and phenol (C6H5OH) framework opens the potentiality in exploring the synergistic interactions in their weakly bonded complexes. In this work, the synergistic hydrogen bonding in C4H5N-C6H5OH complexes is therefore investigated using FTIR spectroscopy under isolated conditions at low temperatures. Computations performed at DFT, DFT-GD3, M06, and MP2 level of theories employing aug-cc-pVDZ basis set yielded three minima on the potential energy surface for the 1:1 complex of C4H5N-C6H5OH. All three optimized structures showed synergistic interactions, where both C6H5OH and C4H5N simultaneously act as a proton donor and acceptor at MP2/aug-cc-pVDZ level of theory. In the global minimum complex A, the hydroxyl proton and the C-H group of C6H5OH interact with the π-cloud of C4H5N. The first local minimum corresponds to complex B, where the N-H and π-electrons of C4H5N interact with π-electrons of C6H5OH. In complex C, the N-H and C-H groups of C4H5N interact with O-H and π-cloud of C6H5OH, respectively. Complex A was the lowest energy structure at all levels of theory, whereas the stabilization energies of complexes B and C varied depending upon the levels of theory used. Interestingly, the stabilization energies as predicted by the DFT method are in accordance with Etter's and Legon-Millen rules; however, a deviation in the Legon-Millen rule was discerned with empirical (DFT-GD3, M06) and dispersion corrected (MP2) methods. On comparing the experimental vibrational wavenumber shifts in the N-H stretching and bending modes of C4H5N and O-H stretching mode of C6H5OH submolecules with the computed shifts, all three complexes were identified in the N2 matrix. Natural Bond Orbital and Energy Decomposition analyses were performed to characterize the nature of the synergistic interaction in these complexes.

High-efficiency adsorption for acid dyes over CeO2·xH2O synthesized by a facile method

In this study, CeO2·xH2O with high-efficiency adsorption for acid dyes was synthesized via a facile method. Effect of temperature, pH and aging time in synthesis on the removal of acid red 14 (AR14) was investigated. It was found that the CeO2·xH2O prepared at 25 OC, pH of 4.5, and aged for 6 h is suitable for adsorption of AR14 and acid orange 7 (AO7) because of a large number of hydroxyl groups, NH4+ and oxygen vacancies in the resulting sample. The unary adsorption isotherm of AR14 on the optimized CeO2·xH2O adsorbent is well described by the Langmuir model, with an estimated adsorption capacity of 540 mg/g, which is a much higher value than those reported in literature. 100% of AR14 or 33% of AO7 in 40 ml of 250 mg/l aqueous solution can be removed by 0.02 g of the optimized CeO2·xH2O adsorbent. The CeO2·xH2O adsorbent also shows a good stability through several adsorption-regeneration cycles. Based on these excellent properties, the application in the removal of acid dyes from wastewater is anticipated. Furthermore, a novel adsorption mechanism was proposed based on the interaction between sulfonic groups (−SO3−) in acid dyes and protonated hydroxyl groups (−OH2+), ammonium radicals (NH4+), and oxygen vacancies (VO••) on the surface of the optimized CeO2·xH2O sample.

An equilibrium isotope effect due to a strong hydrogen bond

The mono deuterium equilibrium isotope effect (EIE) for the exchange of hydroxyl deuterons and protons on 2-hydroxyl and 4-hydroxyl of 2, 4-dihydroxybenzaldehyade were calculated theoretically and also measured with NMR. The equilibrium constant (Keq = 1.108) was predicted using the geometry and vibrational force constants derived from electronic structure theory followed by the program THERMISTP, which is highly consistent with the NMR results (experimental Keq = 1.081 and Keq = 1.066), indicating the formation of the strong hydrogen bond between 2-hydroxyl and the aldehyde group.

Distinguishing Intra- and Intermolecular Interactions in Liquid 1,2-Ethanediol by 1H NMR and Ab Initio Molecular Dynamics.

The central OCCO backbone of the 1,2-ethanediol molecule adopts the gauche conformer in the gaseous and crystalline states but exists in conformational equilibrium between gauche and trans in the liquid; an observation that has been attributed to the competition between intra- and intermolecular interactions. Here, we show that the nuclear Overhauser effect (NOE) has the ability to distinguish inter- from intramolecular interactions in liquid 1,2-ethanediol. We do so by exploiting the secondary isotope effect to distinguish the hydroxyl protons of HOCH2CH2OH and the deuterated HOCD2CD2OH in the 1H NMR spectra of mixtures of the two and, in conjunction with ab initio MD simulations, show how the interplay between inter- and intramolecular interactions gives rise to the conformational isomers in the liquid state of 1,2-ethanediol.

In situ surface‐enhanced Raman spectroscopic monitoring electrochemical and surface plasmon resonance synergetic catalysis on dehydroxylation of PHTP at Ag electrodes

AbstractSurface plasmon resonance (SPR)‐driven heterogeneous catalytic reactions have attracted considerable interests. However, the application of SPR was restricted in few types of surface reaction; the extension of SPR‐catalyzed reaction still remains significant challenge. The introduction of additional external fields is expected to generate the synergetic effect for improving the performance of SPR catalytic reaction. Herein, the roughened Ag electrode played as a substrate for introducing the second external field, that is, electrochemical control. A probe molecule, p‐hydroxythiophenol (PHTP), was preanchored onto Ag electrodes and surface‐enhanced Raman spectroscopy was employed to monitor the surface processes. It demonstrated that the PHTP underwent the dehydroxylation reaction to produce the thiophenol (TP) by the appropriate excitation line and potentials. It disappeared on Ag roughened electrodes without electrochemical control or on a smooth charged surface attached with shell‐isolated nanoparticles. It suggested that the dehydroxylation reaction was contributed by the synergetic effects of localized SPR and applied potential. The results revealed that the efficiency of dehydroxylation was critically depended on wavelength and power of laser, potential, as well as solution pH. The hydrogen sources were essential for dehydroxylation and protonated hydroxyl group promoted the process for producing TP, thus, the dehydroxylation of PHTP was more favorable in acidic solution than that in a neutral case, while it was inhibited completely in alkaline environment. With the appropriate potential, the size of the nanostructures on Ag electrode surface and the corresponding surface plasmon band resulted in the higher efficiency with matched laser wavelength at about 532 nm. The synergetic effects of SPR and potential allowed the extension of the SPR catalysis to a new catalogue of surface reaction, that is, dehydroxylation. It was beneficial to develop the SPR catalysis as a promising approach in the surface chemistry.

3-O-Methyl-D-glucose mutarotation and proton exchange rates assessed by 13C, 1H NMR and by chemical exchange saturation transfer and spin lock measurements.

3-O-Methyl-D-glucose (3OMG) was recently suggested as an agent to image tumors using chemical exchange saturation transfer (CEST) MRI. To characterize the properties of 3OMG in solution, the anomeric equilibrium and the mutarotation rates of 3OMG were studied by 1H and 13C NMR. This information is essential in designing the in vivo CEST experiments. At room temperature, the ratio of α and β 3OMG anomers at equilibrium was 1:1.4, and the time to reach 95% equilibrium was 6h. The chemical exchange rates between the hydroxyl protons of 3OMG and water, measured by CEST and spin lock at pH 6.14 and a temperature of 4°C, were in the range of 360-670s-1.

GlucoCEST imaging with on-resonance variable delay multiple pulse (onVDMP) MRI.

To examine the detection sensitivity for the rapidly exchanging hydroxyl protons of D-glucose using the recently developed on-resonance variable delay multi-pulse (onVDMP) chemical exchange saturation transfer (CEST) technique. The onVDMP method was applied for the detection of water signal changes upon venous D-glucose infusion in mice with 9L glioma xenografts. The effect size of onVDMP MRI during infusion was compared with that of conventional continuous wave (CW) CEST MRI. Both methods highlighted the tumor and the blood vessels on D-glucose infusion. In interleaved studies, the mean signal changes detected by onVDMP were found to be 1.8 times higher than those by CW-CEST, attributed to its high labeling efficiency for fast exchanging protons and the labeling of the OH protons over a larger frequency range. The onVDMP method is a more sensitive technique for the detection of exogenous CEST agents with fast-exchanging protons compared to CW-CEST MRI.

Evidence for a strong polarization of n-hexane in zeolite H-ZSM-5 by FT-IR and solid-state NMR spectroscopy

Abstract The adsorption and interaction of n-hexane in acidic zeolites were investigated via the adsorbate-induced shifts of the FT-IR bands (Δ ν ˜ ) and 1H MAS NMR signals (Δδ) of acidic bridging OH groups (Si(OH)Al) in H-mordenite, H-Beta, 10H,Na-Y, 86H,Na-Y, H-ZSM-5, and US-Y. The plot of the experimentally determined Δ ν ˜ values as a function of Δδ shows a linear correlation. In a subsequent step, the influences of the average electronegativity Sav of the individual zeolites under study and the deprotonation energy ΔpA of their acidic hydroxyl protons on Δ ν ˜ and Δδ were investigated. The individual Sav values were calculated via the geometric average of all element electronegativities of the zeolite framework, while the ΔpA values were obtained via an evaluation of the CO-induced shifts of the FT-IR bands of Si(OH)Al groups compared with that of SiOH groups and using an empirical relationship. In the plots of the Δ ν ˜ /Sav and Δ ν ˜ /ΔpA ratios as a function of Δδ/Sav and Δδ/ΔpA, respectively, the data sets of all zeolites under study are closely arranged, excluding that of H-ZSM-5. This finding indicates that n-hexane adsorbed on zeolite ZSM-5 is not exclusively involved in the interaction with acidic Si(OH)Al groups, but is also influenced by the zeolite framework. The 13C solid-state NMR shifts of n-hexane adsorbed on the zeolites hint at a polarization of this alkane especially in the 10-ring pores of zeolite H-ZSM-5. This polarization of n-hexane in H-ZSM-5 is important for the reactivity of this alkane in heterogeneously catalyzed reactions.

Additive-free carbon nanotube dispersions, pastes, gels, and doughs in cresols

Cresols are a group of naturally occurring and massively produced methylphenols with broad use in the chemical industry. Here, we report that m-cresol and its liquid mixtures with other isomers are surprisingly good solvents for processing carbon nanotubes. They can disperse carbon nanotubes of various types at unprecedentedly high concentrations of tens of weight percent, without the need for any dispersing agent or additive. Cresols interact with carbon nanotubes by charge transfer through the phenolic hydroxyl proton and can be removed after processing by evaporation or washing, without altering the surface of carbon nanotubes. Cresol solvents render carbon nanotubes polymer-like rheological and viscoelastic properties and processability. As the concentration of nanotubes increases, a continuous transition of four states can be observed, including dilute dispersion, thick paste, free-standing gel, and eventually a kneadable, playdough-like material. As demonstrated with a few proofs of concept, cresols make powders of agglomerated carbon nanotubes immediately usable by a broad array of material-processing techniques to create desirable structures and form factors and make their polymer composites.

Exploiting the Proton Exchange as an Additional Route to Enhance the Relaxivity of Paramagnetic MRI Contrast Agents.

The relaxivity of Gd(HP-DO3A) was studied as a function of pH and buffer composition in order to identify the main factors of the observed relaxation enhancement due to the exchange of the coordinated hydroxyl proton. It was established that the paramagnetic relaxation time, T1M, of the coordinated hydroxyl proton is about 50% shorter than that of the protons in the coordinated water molecule. The control of the p K of the coordinated alcoholic -OH moiety in the ligand is fundamental to utilize the proton exchange enhanced relaxivity under physio/pathologic conditions. A new derivative of Gd(HP-DO3A) was synthesized by replacing the -CH3 group with a -CF3 moiety. In this complex, the -OH group becomes more acidic. Consequently, the maximum contribution of the proton exchange to the relaxivity is shifted to a lower pH region with the fluorinated ligand.

Multiscale modelling strategies and experimental insights for the solvation of cellulose and hemicellulose in ionic liquids

ABSTRACTThe present study investigates the dissolution behaviour of cellulose and hemicellulose in potential ionic liquids (ILs) using both the quantum chemical and experimental validation. For converging upon the recommended IL, 1428 ILs consisting of 42 cations and 34 anions were studied with the conductor like screening model for real solvents (COSMO-RS) model. Based on the infinite dilution activity coefficient of the components in IL, the selected anions and cations were visualised by observing their interactions with cellulose and hemicellulose using interaction energies, natural bonding orbital analysis and molecular dynamics simulations. The dissolution order of cellulose and hemicellulose in ILs was primarily determined by the evaluation of hydrogen bonds between the oxygen atom of anion and hydroxyl proton of cellulose/hemicellulose. From this discernible fact, the anion of the IL was observed to play a leading role in the solvation process as compared to the cation. Eventually, acetate [OAc]– anion and 1-ethyl-3-methylimidazolium [EMIM]+ cation were found to be good candidates for the dissolution of cellulose and hemicellulose. This was further confirmed by the measurement of solid-liquid equilibria with cellulose and hemicellulose. The regenerated cellulose powder was then characterised by Fourier transform spectroscopy(FTIR), X-ray diffraction (XRD) and Thermal gravimetric analysis (TGA).