Primary OH Groups Research Articles

Low-temperature catalytic oxidation of glycerol into glyceric acid over Zr@MCM-41-supported Pt catalyst

Abstract A series of mesoporous Zr@MCM-41 zeolites with different Si/Zr molar ratios were prepared using cetyltrimethylammonium bromide as a template. Pt nanoparticles were then successfully loaded onto the Zr@MCM-41 using a colloid-deposition method. These catalysts were evaluated for the catalytic oxidation of glycerol into glyceric acid at low temperature. The results showed that the molar ratios of the Si/Zr affected the oxidization of the primary OH group, in turn affecting the catalytic activity and selectivity. The Pt/Zr@MCM-41-100 catalyst showed the best catalytic activity, with the conversion of glycerol and selectivity for glyceric acid reaching 50.7% and 65.0%, respectively. The characterization result confirmed that the catalytic activity of Pt/Zr@MCM-41-100 was related to the support structure, the dispersion of the Pt nanoparticles, and the interaction between the Pt nanoparticles and the support. Meanwhile, we investigated the stability of the Pt/Zr@MCM-41-100 catalyst and analyzed the deactivation reasons using characterization.

Preparation and Characterization of Chitosan from Schistocerca Gregaria (Desert Locust)

The current study prepared and characterized chitosan from Schistocerca gregaria (Desert Locust) by a chemical method. The Schistocerca gregaria (Desert Locust) chitin and chitosan have yield of 42% and 43% with acceptable physicochemical properties such as Moisture content: 6.0±0.014 and 5.0±0.007 %, Ash content: 2.20 and 2.0%, Protein content 12.0±0.028 and 9.01 ±0.021, Crude fibre8.95±0.041 and 8.95 ± 0.014%, Lipid content, 10.0 ±0.014 and10.0 ±0.014%, however, high carbohydrate, 60.5±0.01 and 65.0±0.01% are quantified in the obtained chitin and chitosan. properties of chitosan prepared were compared with that of the other sources chitosan extracted, the study revealed that (FTIR) analysis confirmed the structure of chitosan by the presence of their characteristic IR bands such as the N–H and O–H stretching bands around 3630 cm−1, oxygen stretching (C-O-C) glycosidic linkage bands around 1125 cm−1, (C-O) in primary OH group bands around 976 cm−1, and 1634 cm−1 corresponding to the free amino group (−NH2) at C2 of glucosamine with 69% degree of deacetylation. Also, the obtained XRD peaks were around 19.01, and 20.48◦ which indicated the amophores structure. This indicates the successive formation of chitosan biopolymer. Therefore, the Schistocerca gregaria chitosan can be said to be of better quality and hence can replace commercial chitin and chitosan in terms of utilization and applications.

Improvement of Flexible Polyurethane Foam Characteristics of Palm Oil Polyols with the Addition Of Polyethylene Glycol-400

Palm oil polyol-based foams have inflexible and brittle properties. Palm oil polyols have a lower molecular weight. Polyols with a high molecular weight and a primary OH group, such as polyethylene glycol (PEG)-400, can provide the flexible properties of polyurethane foam. This study aimed to examine the optimum concentration of PEG-400, which can produce flexible polyurethane foam made from palm oil polyols with the best characteristics based on mechanical properties. This study compared palm oil polyol and PEG-400 treatment of 80%:20%, 70%:30%, 60%:40%, 50%:50%. Flexible polyurethane foam was characterized, including FTIR analysis, SEM, tensile strength, yield strength, elongation at break, and TGA. The results showed that the use of PEG-400 at an optimum concentration of 40% resulted in flexible polyurethane foam with a greater number of open cells and increased flexible properties but decreased the value of tensile strength and yield strength, as well as decreased the foam’s stability to heat. However, tensile strength and elongation at break of PEG-based palm oil polyurethane obtained from this study followed the flexible polyurethane foam requirements.

Effective Organotin-Mediated Regioselective Functionalization of Unprotected Carbohydrates

Regioselective functionalization of unprotected carbohydrates at a secondary OH group in the presence of primary OH groups based on the commonly used organotin-mediated reaction has been improved. We found that the preactivation of the dibutylstannylene acetal intermediate with tetrabutylammonium bromide in toluene is a key to the improved condition for the efficient, high-yielding, and regioselective tosylation, benzoylation, or benzylation of unprotected carbohydrates. The counteranion of tetrabutylammonium ion with a weak coordination ability plays a crucial role in the improved regioselective reactions. A convenient access to the intermediates of synthetic value is also demonstrated in the organotin-mediated regioselective tosylation of unprotected carbohydrates, followed by the nucleophilic inversion reaction to give sulfur-containing and azide-modified carbohydrates.

ANALYSIS OF CATALYTIC BEHAVIOR OF RHENIUM PROMOTED PT/TIO2 CATALYST IN ERYTHRITOL HYDROGENOLYSIS

Rhenium modified Pt/TiO2 catalyst has been tested during erythritol (ERY) hydrogenolysis reaction in a slurry reactor. N2 physisorption, temperature programmed reduction and CO chemisorption characterizations were performed. After reduction treatment, Pt species were fully reduced with a dispersion of 26% whereas Re species remained partially oxidized. ERY may convert through four pathways: isomerization, dehydration and C-O or C-C hydrogenolysis. Pt-Re/TiO2 was active and selective to butanetriols and butanediols formation. The influence of temperature (423–498 K) and reactant concentrations (15-35 barH2 and 0.2-0.6 MERY) on catalytic activity and products distribution were evaluated; the activation energy and the reaction orders for each route were estimated. Dehydration pathway displayed the highest activation energy whereas isomerization showed the lowest value. The order to ERY and H2 were 0.23 and 0.97 respectively, for C-O hydrogenolysis route and is consistent with two different mechanisms proposed for the removal of primary or secondary OH group.

Chemical Synthesis of the Linker‐Armed Trisaccharide Repeating Unit of the O‐Antigen from Pseudomonas putida BIM B‐1100

AbstractThe manuscript reports the concise chemical synthesis of the trisaccharide repeating unit of the O‐antigen from Pseudomonas putida BIM B‐1100 in the form of its aminoethyl glycoside through a linear strategy with rational protecting group manipulations. The challenging aspects of this synthesis include the incorporation of the β‐D‐ManpNAcA and the α‐D‐GlcpNAc residues. The introduction of the desired carboxylic acid moiety has been achieved through late‐stage TEMPO‐mediated oxidation of the primary OH group on the protected trisaccharide. 1,2‐cis aminoethyl glycoside with central glucosamine is used at the reducing end to the leave the scope for further glycoconjugate formation with suitable aglycone without hampering the anomeric stereochemistry.

The Influence of the Hydroxyl Type on Crosslinking Process in Cyclodextrin Based Polyurethane Networks.

The influence of the hydroxyl groups (OH) type on the polyaddition processes of isocyanates represents a critical approach for the design of multicomponent polyurethane systems. Herein, to prove the effect of hydroxyl nature on both the isocyanate-OH polyaddition reactions and the structure/properties of the resulting networks, two structurally different cyclodextrins in terms of the primary and secondary groups’ ratio were analyzed, namely native β-cyclodextrin (CD) and its derivative esterified to the primary hydroxyl groups with oligolactide chains (CDLA). Thus, polyurethane hydrogels were prepared via the polyaddition of CD or CDLA to isophorone diisocyanate polyethylene glycol-based prepolymers (PEG-(NCO)2). The degradable character of the materials was induced by intercalating oligolactide short sequences into the polymer chains composing the polymer network. In order to establish the influence of the OH type, the synthesis of polyurethane hydrogels was analyzed by a rheological investigation of the overall system reactivity. Materials properties such as swelling behavior, thermal properties and hydrolytic degradation were influenced by the reaction feed. Specifically, the presence of primary OH groups leads to more compact networks with similar water uptake, disregarding the CD content, while the predominance of secondary OH groups together with the presence of oligolactide spacers leads to the fine tuning of the water swelling properties.

Cyclodextrin-Oligocaprolactone Derivatives-Synthesis and Advanced Structural Characterization by MALDI Mass Spectrometry.

Cyclodextrins have previously been proven to be active in the catalysis of cyclic ester ring-opening reactions, hypothetically in a similar way to lipase-catalyzed reactions. However, the way they act remains unclear. Here, we focus on β-cyclodextrin’s involvement in the synthesis and characterization of β-cyclodextrin-oligocaprolactone (CDCL) products obtained via the organo-catalyzed ring-opening of ε-caprolactone. Previously, bulk or supercritical carbon dioxide polymerizations has led to inhomogeneous products. Our approach consists of solution polymerization (dimethyl sulfoxide and dimethylformamide) to obtain homogeneous CDCL derivatives with four monomer units on average. Oligomerization kinetics, performed by a matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) optimized method in tandem with 1H NMR, revealed that monomer conversion occurs in two stages: first, the monomer is rapidly attached to the secondary OH groups of β-cyclodextrin and, secondly, the monomer conversion is slower with attachment to the primary OH groups. MALDI MS was further employed for the measurement of the ring-opening kinetics to establish the influence of the solvents as well as the effect of organocatalysts (4-dimethylaminopyridine and (–)-sparteine). Additionally, the mass spectrometry structural evaluation was further enhanced by fragmentation studies which confirmed the attachment of oligoesters to the cyclodextrin and the cleavage of dimethylformamide amide bonds during the ring-opening process.

Scalable single-step synthesis of lignin-based liquid polyols with ethylene carbonate for polyurethane foams

A new method for the synthesis of lignin-based liquid polyols was developed. Organosolv lignin was reacted with ethylene carbonate in polyethylene glycol as solvent, leading to a full conversion of the phenolic OH into primary aliphatic OH groups. These aromatic polyols are obtained in a single step, without any purification. Upon modification of the polyethylene glycol molar mass, a wide range of hydroxyl values (IOH) can be covered. The polyols with up to 30%wt lignin have a viscosity suitable for the direct elaboration of polyurethane (PUR) foams. The method presents significant advantages over oxypropylation, the most common method for producing lignin-based polyols since it is performed at ambient pressure, without any toxic chemicals, does not require purification or post treatment, and allows to produce polyols with tunable properties. Four different aromatic polyols were then synthesized to produce rigid PUR foams, with substitution of up to 100% of a standard polyether polyol. The developed polyols showed very high reactivity, allowing to reduce the catalyst content in the PUR formulation by 75%. Rigid PUR foams prepared with 25% substitution of the standard polyol showed properties in the range of commercial PUR foams, with more than 90% closed cells and thermal conductivity of about 25 mW m−1/K, perfectly adequate for thermal insulation applications.

Polyaniline/sulfonated cassava starch: A green biocomposite for sensing of pyridoxine

Schematic representation of the electrode modification using PANi-SCS biocomposite • The synthesized PANi/SCS composite by simple chemical method towards selective determination of Pyr was evaluated for the very first time in biosensor field. • The fabricated PANi/SCS green biocomposite showed excellent sensitivity towards the oxidation of Pyr with detection limit of 39 nM. • The real sample analysis demonstrated better sensing results proposing it for food diagnostic applications. Interest in natural polymers has increased because of their unique properties, and these polymers have recently found broad applications. However, further exploration of biosensor applications is necessary. Vitamin deficiencies lead to numerous diseases worldwide; consequently, detection of the vitamin pyridoxine (Pyr) is crucial. To that end, we investigated a polyaniline (PANi)-sulfonated cassava starch (SCS)-based nanocomposite for electrochemical sensing of Pyr. Unlike other metal/metal oxide and polymer based composites, this biopolymer was prepared through a simple green synthesis route. The hydrophilic/hydrophobic ends in SCS provide greater surface area to accommodate PANi for composite formation. In addition, the SO 3 groups replace the primary OH groups through sulfonation, thus increasing binding to the OH groups of Pyr. The enhanced redox behavior of the PANi/SCS nanocomposite enabled selective sensing of Pyr over a broad range of 400 nM to 900 µM, with a low detection limit of 39 nM. Finally, Pyr detection was evaluated on banana extract, thereby demonstrating the practical utility of this method. Thus, our composite is a promising platform for food diagnostic applications.

Musa Paradaisica and <i>Vitis vinifera</i> Functionalised Ag-NPs: Electrochemical and Optical Detection of <i>Escherichia coli</i> in Seawater

Herein, we demonstrate a simple and inexpensive one-pot green synthesis of silver nanoparticles (Ag-NPs) functionalised with a combination of banana peel (Musa paradisiaca) and grape (Vitis vinifera) fruit extracts. The reaction mixture of aqueous silver nitrate, banana peel and grapefruit extracts revealed a dark brown colour after a reaction time of 18 minutes, which indicates the presence and the successful synthesis of silver nanoparticles. The optical and structural properties of the green synthesised nanoparticles were analysed using UV-Visible spectroscopy (UV-Vis) which confirmed an absorption band at 440 nm. The polydispersity nature and the AgNPs sizes of 30 nm were revealed using small angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (HR-TEM) techniques. Fourier transform infrared spectroscopy (FT-IR) studies revealed the structure of these nanoparticles which included carbonyl groups, primary amine groups, OH groups and other stabilizing functional groups characteristic of the properties of combined extracts. A simple, quick, less time-consuming surface plasmon resonance (SPR) and electrochemical method in the form of optical and electrochemical sensors have been developed for the detection of Escherichia coli 0157:H7. The obtained limit of detection (LOD) values for SPR and GBPE-Ag-NPs/GCE-based sensor systems were found to be 1 × 102 CFU/mL and 3.5 × 101 CFU/mL, respectively. The obtained values fall within the range for E. coli 0157:H7 in seawater.

Immobilized Fe3O4-Polydopamine-Thermomyces lanuginosus Lipase-Catalyzed Acylation of Flavonoid Glycosides and Their Analogs: An Improved Insight Into Enzymic Substrate Recognition.

The conversion of flavonoid glycosides and their analogs to their lipophilic ester derivatives was developed by nanobiocatalysts from immobilizing Thermomyces lanuginosus lipase (TLL) on polydopamine-functionalized magnetic Fe3O4 nanoparticles (Fe3O4-PDA-TLL). The behavior investigation revealed that Fe3O4-PDA-TLL exhibits a preference for long chain length fatty acids (i.e., C10 to C14) with higher reaction rates of 12.6–13.9 mM/h. Regarding the substrate specificity, Fe3O4-PDA-TLL showed good substrate spectrum and favorably functionalized the primary OH groups, suggesting that the steric hindrances impeded the secondary or phenolic hydroxyl groups of substrates into the bonding site of the active region of TLL to afford the product.

Proton Conductive, Low Methanol Crossover Cellulose-Based Membranes.

This work describes the development of sulfated cellulose (SC) polymer and explores its potential as an electrolyte-membrane for direct methanol fuel cells (DMFC). The fabrication of our membranes was initiated by the preparation of the novel sulfated cellulose solution via controlled acid hydrolysis of microcrystalline cellulose (MCC). Ion-conductive crosslinked SC membranes were prepared following a chemical crosslinking reaction. SC solution was chemically crosslinked with glutaraldehyde (GA) and cured at 30 °C to produce the aforementioned membranes. Effects of GA concentration on methanol permeability, proton conductivity, water uptake and thermal stabilities were investigated. The crosslinking reaction is confirmed by FTIR technique where a bond between the primary OH groups of cellulose and the GA aldehyde groups was achieved, leading to the increased hydrophobic backbone domains in the membrane. The results show that the time of crosslinking reaction highly affects the proton conduction and methanol permeability. The proton conductivity and methanol crossover (3M) of our GA crosslinked SC membranes are 3.7 × 10−2 mS cm−1 and 8.2 × 10−9 cm2 s−1, respectively. Crosslinked sulfated cellulose films have lower ion conductivity than the state-of-the-art Nafion (10.2 mS cm−1); however, the methanol crossover is three orders of magnitude lower than Nafion membranes (1.0 × 10−5 cm2 s−1 at 1 M). Such biofilms with high methanol resistivity address the major hurdle that prevents the widespread applications of direct alcohol fuel cells.

A molecular dynamics study of the effect of functional groups and side chain on adsorption of alcoholic surfactant and interfacial thermal transport

In the present study, we investigated the effect of the number and position of functional groups, and the length of the main chain and side chain in organic surfactant on adsorption behavior and interfacial heat transfer between silica surface and alkane solvent by non-equilibrium molecular dynamics simulation, where the surfactants were primary/secondary alcohol, monohydric/dihydric alcohol, and linear/branched alcohol. The results showed a similar adsorption behavior for all the surfactant types, where hydroxyl functional (OH) groups adsorbed onto the silica surface and alkyl chain was in contact with the solvent, which produced a heat path from silica via surfactant to solvent. The number of adsorbed OH groups did not directly translate to significantly decreased thermal boundary resistance due to the adsorption structure. Coulomb interaction enabled the closer distance between primary terminal OH groups in surfactant and silica surface, which enhanced the solid-surfactant intermolecular heat transfer. However, Coulomb interaction contributed less to shorten the molecular distances between secondary OH groups in surfactant and silica surface, which was connected to less efficient heat transfer from silica to surfactant and thereby did not enhance the interfacial heat transfer as much as surfactants with terminal OH. The increase in terminal OH groups in the surfactant molecules could not significantly reduce thermal boundary resistance, although the adsorption amount of OH was distinctly greater than that of surfactants with single OH. The side chain in surfactant enabled the efficient surfactant-solvent intermolecular heat transfer but related to the desorption of surfactant when decreasing the temperature. Thus branched-chain dihydric alcohol performed better than other surfactants on reducing thermal boundary resistance when the interfacial temperature was high enough to maintain the sufficient adsorption amount. We considered such reverse temperature-sensitive surfactant has a great potential application to fulfill multiple needs for heat dissipation of electronic devices, especially the high temperature operation. The new insights obtained in the present study were a step towards a molecular structure design of surfactant enhancing solid–liquid interfacial heat transfer.

Kinetically Equivalent Functionality and Reactivity of Commonly Used Biocompatible Polyurethane Crosslinking Agents

In this paper, the kinetics of the reaction of phenyl isocyanate with crosslinking agents such as sucrose, sorbitol, and glycerol are reported. Crosslinking agents were used in high molar excess to isocyanate to obtain pseudo-first-order rate dependencies, and the reaction products were separated by high-performance liquid chromatography and detected by UV spectroscopy and mass spectrometry. It was found that the glycerol’s primary hydroxyl groups were approximately four times reactive than the secondary ones. However, in the case of sorbitol, the two primary OH groups were found to be the most reactive, and the reactivity of hydroxyl groups decreased in the order of kOH(6)(8.43) > kOH(1)(6.91) > kOH(5)(1.19) > kOH(2)(0.98) > kOH(3)(0.93) > kOH(4)(0.64), where the numbers in the subscript and in the brackets denote the position of OH groups and the pseudo-first-order rate constants, respectively. The Atomic Polar Tenzor (APT) charges of OH groups and dipole moments of monosubstituted sorbitol derivatives calculated by density functional theory (DFT) also confirmed the experimental results. On the other hand, the reactions of phenyl isocyanate with crosslinking agents were also performed using high excess isocyanate in order to determine the number of OH-groups participating effectively in the crosslinking process. However, due to the huge number of derivatives likely formed in these latter reactions, a simplified reaction scheme was introduced to describe the resulting product versus reaction time distributions detected by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS). Based on the results, the kinetically equivalent functionality (fk) of each crosslinking agent was determined and found to be 2.26, 2.6, and 2.96 for glycerol, sorbitol, and sucrose, respectively.

Interfacial adhesion of a grafted polymer on a cellulose surface: a first-principles study

This paper presents the first-principles investigation on interfacial adhesive strength between grafted polymer and cellulose. Focusing on maleic anhydride-grafted polypropylene, which is expected to be used in various applications including automobiles, a simple interfacial model was constructed under a local tensile loading condition. Then the stress–strain relation, deformation of molecular structure and breaking process were discussed. From comparison of mechanical responses among three chemically possible bonding configurations, it was clarified that the monoester bond exhibited higher interfacial strength than the diester bond. We also found that controlling the bonding configuration can improve the interface strength by about 50% compared to when primary OH groups react to form ester bonds. It was proved that this first-principles calculation model was effective for the definitive design and development of chemical modification of nanocellulose.

Isolation of Low Dispersity Fractions of Acetone Organosolv Lignins to Understand their Reactivity: Towards Aromatic Building Blocks for Polymers Synthesis.

Two organosolv lignins extracted during pilot runs of the Fabiola process were analyzed, fractionated and chemically modified with ethylene carbonate (EC) to produce building blocks suitable for polymer synthesis. Isolation of low dispersity fractions relied on the partial solubility of the lignins in organic solvents. Lignins solubility was first evaluated and analyzed with Hansen and Kamlet‐Taft solubility parameters, showing a good correlation with the solvents dipolarity/polarizability parameter π*. The results were then used to select a sequence of solvents able to fractionate the lignins into low dispersity fractions of increasing molar masses, which were analyzed by 31P NMR, SEC and DSC. The lignins were then reacted with EC, to convert the phenolic OH groups into primary aliphatic OH groups. The reactivity of the organosolv lignins was high, and milder reaction conditions than previously reported were sufficient to fully convert the phenolic OH groups. A gradual reduction in reactivity with increasing molar mass was evidenced and attributed to reduced solubility of high molar mass fragments in EC. Undesirable crosslinking side reactions were evidenced by SEC, but were efficiently limited thanks to a fine control of the reaction conditions, helping to maximize the benefits of the developed lignin modification with EC.

Spatially dependent H-bond dynamics at interfaces of water/biomimetic self-assembled lattice materials

Understanding hydrogen-bond interactions in self-assembled lattice materials is crucial for preparing such materials, but the role of hydrogen bonds (H bonds) remains unclear. To gain insight into H-bond interactions at the materials' intrinsic spatial scale, we investigated ultrafast H-bond dynamics between water and biomimetic self-assembled lattice materials (composed of sodium dodecyl sulfate and β-cyclodextrin) in a spatially resolved manner. To accomplish this, we developed an infrared pump, vibrational sum-frequency generation (VSFG) probe hyperspectral microscope. With this hyperspectral imaging method, we were able to observe that the primary and secondary OH groups of β-cyclodextrin exhibit markedly different dynamics, suggesting distinct H-bond environments, despite being separated by only a few angstroms. We also observed another ultrafast dynamic reflecting a weakening and restoring of H bonds between bound water and the secondary OH of β-cyclodextrin, which exhibited spatial uniformity within self-assembled domains, but heterogeneity between domains. The restoration dynamics further suggest heterogeneous hydration among the self-assembly domains. The ultrafast nature and meso- and microscopic ordering of H-bond dynamics could contribute to the flexibility and crystallinity of the material--two critically important factors for crystalline lattice self-assemblies--shedding light on engineering intermolecular interactions for self-assembled lattice materials.

A Mild Methylation of Whole Leaf Producing Per-O-methyl Cellulo Di-phospho Amino Acid (Dipeptide)

A mild methylation method is detailed in this work. It uses 1N NaOH to deprotonate primary and secondary hydroxyl groups for short time. Excess 1N NH4OH (1.00 ml, pH 11.4) is added, followed by NaHCO3 to make the carbonate esters by nucleophilic substitution of the deprotonated primary and secondary OH groups. The reaction conditions include; ambient temperature and for an appropriate time. Then NaBH4 is added and the reaction conditions are; standing at ambient temperature for at least 4 hours. This converts the carbonate esters to the methyl ethers. Work-up includes transferring the methylated switchgrass leaf section to a vial containing 1N HCl (1.00 ml). The leaf needs to be submerged in the acid. The reaction conditions include; heating the reaction vessel at 72°C, for 4 days. The contents of the vial are reduced in volume to syrup with a stream of air. The mixture dissolves completely in methanol. Mass spectrometry is done on a methanol solution in the negative ion ESI mode. The mass spectral interpretation reveals the presence of methylated glucan amimo acids and glucan dipeptides. The molecules identified were per-O-methylated glucans linked to either serine or asparagine through a di-(hydrido) di-phospho di-hydratephe moeity. It is possible that one ion is derived from a phosphorylated tyrosine linked to either the serine or asparagine linked to an originally di-phospho cellulo-glucan.

Mechanism of HF condensive complex plasma modification of polyurethane membrane coatings in the air environment

The mechanism of the high frequency condensive complex plasma modification of polyurethane membrane coatings in air is investigated. The study revealed that after plasma modification in air, oxidation and hydrolysis of polyurethane membrane coatings occurs, which is expressed in an increase in the intensity of absorption bands corresponding to primary OH groups in the range of wave numbers 1005–1070 cm−1, 1310 cm−1 and 1420 cm−1, as well as in partial destruction of polyurethane chains with the formation of urea groups, which corresponds to the vibrations of CO-groups in the range of wave numbers 1590–1690 cm−1.