Structure and molecular conformation of tussah silk fibroin films: Effect of methanol

Structural changes of tussah (Antheraea pernyi) silk fibroin films treated with different water-methanol solutions at 20°C were studied as a function of methanol concentration and immersion time. X-ray diffraction measurements showed that the α-helix structure, typical of untreated tussah films, did not change for short immersion times (2 min), regardless of methanol concentration. However, crystallization to β-sheet structure was observed following immersion of tussah films for 30 min in methanol solutions ranging from 20 to 60% (v/v). IR spectra of tussah films untreated and methanol treated for 2 min exhibited strong absorption bands at 1265, 892, and 622 cm^?1, typical of the α-helix conformation. The intensity of the bands assigned to the β-sheet conformation (1245, 965, and 698 cm^?1) increased for the sample treated with 40% methanol for 30 min. Raman spectra of tussah films with α-helix molecular conformation exhibited strong bands at 1657 (amide I), 1263 (amide III), 1106, 908, 530, and 376 cm^?1. Following α → β conformational transition, amide I and III bands shifted to 1668, and to 1241, 1230 cm^?1, respectively. The band at 1106 cm^?1 disappeared and new bands appeared at 1095 and 1073 cm^?1, whereas the intensity of the bands at 530 and 376 cm^?1 decreased significantly. ©1995 John Wiley & Sons, Inc.     

On the polymerization mechanism of the Chromium(III) and Chromium(II)-B Silica Gel Catalysts

FTIR spectra of the chromium(III)/silica gel catalyst after short polymerization with ethylene show weak bands at 2 685 and 1 447 cm^?1 from the stretching and the deformation vibration of the methylene group, which binds the growing polymer to the active chromium(III) site. The band at 2 685 cm^?1 is reversibly removed by CO adsorption at low temperatures (?145°C). This adsorbed CO shows a broad band at 2 184 cm^?1. From the intensity of CO IR bands on unchanged chromium(III) before and after polymerization it was calculated that 12% of the chromium(III) is catalytically active, which is in good agreement with previous measurements by an entirely different determination method (11%). The chromium(II)-B catalyst showed also a weak band at 2 685 cm^?1 and it is therefore concluded that in this case the chromium(II) is oxidised by the ligating surface silanol group (in cooperation with an ethylene molecule). The band at 2 685 cm^?1 is discussed in relation to that from normal methylene groups at 2 925 and 2 855 cm^?1 and to that from the chromium(II)-A catalyst at 2 750 cm^?1. Evidence for the existence of a mononuclear chromium(II)-A species is found. This one is, in contrast to the dinuclear chromium(II)-A species, not polymerization active.     

Thermodynamics of Carbon Dioxide Adsorption on the Protonic Zeolite H‐ZSM‐5

Adsorption of carbon dioxide on H‐ZSM‐5 zeolite (Si:Al=11.5:1) was studied by means of variable‐temperature FT‐IR spectroscopy, in the temperature range of 310–365 K. The adsorbed CO₂ molecules interact with the zeolite Brønsted‐acid OH groups bringing about a characteristic red‐shift of the O? H stretching band from 3610 cm^?1 to 3480 cm^?1. Simultaneously, the ν₃ mode of adsorbed CO₂ is observed at 2345 cm^?1. From the variation of integrated intensity of the IR absorption bands at both 3610 and 2345 cm^?1, upon changing temperature (and CO₂ equilibrium pressure), the standard adsorption enthalpy of CO₂ on H‐ZSM‐5 is ΔH⁰=?31.2(±1) kJ mol^?1 and the corresponding entropy change is ΔS⁰=?140(±10) J mol^?1 K^?1. These results are discussed in the context of available data for carbon dioxide adsorption on other protonic, and also alkali‐metal exchanged, zeolites.     

IR Studies of Coordinatively Unsaturated Surface Compounds on Silica Gel. IV. CO Complexes of Chromium(II) and Chromium(III)

Three different chromium(II) surface compounds have been identified by their IR spectra of adsorbed CO. Especially important in the interpretation of the experiments are the bands between 2050 and 2035 cm^?1 and the relative intensities of the CO bands at 2100 and 2120 cm^?1. The bands between 2192 and 2179 cm^?1 show also a different pattern for each of the different chromium(II) surface compounds. Two chromium(III) surface compounds could be identified by the presence of either one CO band near 2200 cm^?1 or two bands near 2206 and 2196 cm^?1. Three models are proposed for the chromium(II) title compounds, all related to dinuclear chromium(II) surface complexes: Models one and two have chromium(II) ions with trans and cis configuration in different combinations, respectively, and the third model has a bridging oxygen ion from a siloxan group as a distorting ligand. The three models explain the CO absorptions sufficiently and one is nearly quantitatively in agreement with previous volumetric adsorption studies of CO on chromium(II), which showed the surprising effect of decreasing CO adsorption with decreasing temperature. Two models for the chromium(III) surface compounds are proposed with either an oxygen double bridge or only one bridging oxygen.     

Infrared molecular beam depletion spectroscopy of size-selected methanol clusters

Molecular beam depletion spectroscopy has been employed to study the dissociation of small methanol clusters in the spectral region between 1000 and 1100 cm^?1 which covers thev ₈ CO stretch (1033.5 cm^?1) and thev ₇ CH₃ rock (1074.5 cm^?1) monomer vibrations. Size selection has been achieved by dispersing the (CH₃OH) _n cluster beam by a secondary He beam. Aside from the recently published CH₃OH dimer absorption bands at 1026.5 and 1051.6 cm^?1 which are assigned to the excitation of the CO stretching vibrations in the non-equivalent subunits of the hydrogen-bonded complex, a previously unobserved band was found at 1071.3 cm^?1. This absorption band is attributed to the excitation of the CH₃ rocking vibration in the dimer. It appears that this transition which is very weak in the free methanol monomer receives substantial oscillator strength due to the intermolecular interaction in the complex. A splitting of this band could not be observed. The trimer and tetramer spectra feature single peaks for the CO stretching vibration being centered at 1042.2 cm^?1 and 1044.0 cm^?1, respectively. This observation is consistent with the cyclic structures of these species. The trimer and tetramer rocking vibrations are observed near 1060.5 cm^?1 but cannot be localized exactly, due to a gap in the CO₂ laser tuning range.     

Surface reactions of dimethyl ether on γ-Al2O3

The surface reactions of dimethyl ether (DME) on industrial alumina (γ-Al₂O₃) were studied by chromatographic analysis of the products at the outlet of the flow reactor and (independently) by diffuse reflectance IR spectroscopy. The major products of the reactions at 250°С were found to be methanol formed in the reaction of DME with hydroxyl groups (the 3720 and 3674 cm^–1 bands in the diffuse reflectance spectrum) and various methoxy groups (the 1121, 1070, 695, and 670 cm^–1 bands in the differential spectra). The presence of molecularly adsorbed methanol was confirmed by experiments with methanol fed in a high-temperature IR cell. The interaction of the resulting methanol molecule with the hydroxyl group led to the formation of a water molecule in the gas phase and a methoxy group on the oxide surface. Strong adsorption of molecular DME was revealed, which was favored by an increase in the temperature of the preliminary calcination of oxide from 250 to 450–500°С; treatment of alumina with water vapor after its preliminary contact with DME led to a recovery of the hydroxyl coating and a replacement of molecularly adsorbed DME with hydroxyl. The thermal effect recorded in a flow reactor was positive during the adsorption of DME and negative during the desorption of weakly bonded DME. Schemes of formation of methoxy groups in the interaction of DME and methanol with surface hydroxyls were suggested.     

Spectroscopic detection of free “lone-pair” groups or terminal methanol molecules in methanol solutions

When basic aprotic solvents are added to methanol they become hydrogen bonded, and there is a consequent growth in non-bonded lone-pairs, (LP)_free. Although corresponding non-bonded OH groups, (OH)_free, have been detected for alcohols and for water, using overtone infrared spectroscopy, no different spectroscopic evidence for (LP)_free groups has previously been reported. We have found that unique OH stretching bands develop when strongly basic solvents such as dimethylsulphoxide are added to methanol. Band maxima assigned to (LP)_free groups occur at 3440 cm^?1 in the fundamental and 6790 cm^?1 in the overtone region. These are at considerably higher frequencies than those for bulk methanol (3340 cm^?1 and 6600 cm^?1) showing that the hydrogen bond is weakened in this unit, as expected. Proton resonance shifts for the OH protons of methanol on adding basic aprotic cosolvents are reported, and explained in terms of these results.     

FT-IR study of hydrocarbon conversion on dealuminated HY zeolites in working conditions

The acidity of two dealuminated HY zeolites, obtained either by isomorphous substitution or by hydrothermal treatment at 550°C followed by acid leaching, was characterized by IR spectroscopy in dynamic or static modes. The probe molecules used were 2,6-dimethylpyridine, pyridine and deuterated acetonitrile. They showed that the steamed sample presented very strong acidic Brønsted sites, characterized by OH groups giving rise to a ν (OH) band at 3600 cm⁻¹, and Lewis acidity. The steamed sample was found active for n-hexane cracking at 400°C. Use of an IR cell working as a flow reactor and specific poisoning of hydroxyl groups by 2,6-dimethylpyridine evidenced the catalytic role played by the 3600 cm⁻¹ OH groups. On the other hand, cyclohexene conversion, a much less demanding reaction, occurred on both types of zeolite. All the accessible acidic hydroxyls appeared to be active and results were discussed according to the various reactions observed: cyclohexene isomerization, hydrogen transfer and cracking. The intensity of a band at 1586 cm⁻¹, assigned to carbonaceous deposits, was correlated to catalyst deactivation. The results showed that the 3600 cm⁻¹ OH groups were first consumed under cyclohexene flow, due to a rapid poisoning by coke. Adsorption of probe molecules on deactivated samples evidenced that the Lewis acid sites were almost unaffected whatever the reaction, suggesting that they do not play a significant role. This was confirmed by comparison of activity measurements on both types of zeolite towards cyclohexene conversion.     

Sulfophthalide cycle cleavage and formation of fluorenyl structures upon poly(diphenylene sulfophthalide) thermolysis

Thermolysis of poly(diphenylene sulfophthalide) (PDSP) in the temperature range from 100 to 500 °C was studied by IR and UV-Vis spectroscopy and thermogravimetric analysis. A series of absorption bands in the IR spectrum of PDSP were assigned on the basis of the theoretical calculations of the IR spectrum of diphenyl sulfophthalide used as a model compound, in particular, ν_as(S=O) = 1352 cm^?1, ν_s(S=O) = 1196 cm^?1, ν(C-O) ～ 920 cm^?1, ν(S-O) = 824 cm^?1, and δ(SO₂) = 576 cm^?1. The sulfophthalide cycle (SPC) in PDSP decomposes at the thermolysis temperatures in a range of 260–400 °C. An analysis of the IR spectra of the thermolyzate and the quantum chemical calculations of the IR spectra of the model compounds confirmed the predominant formation of fluorenyl structures in the thermolyzed polymer. The changes in the UV-Vis spectra observed upon the thermolysis of thin films of PDSP (the hypsochromic shift of the long-wavelength absorption band from 271 to 263 nm and the appearance a shoulder at ～310 nm) and the results of TD-DFT calculations of the UV-Vis spectra of the model compounds are consistent with the hypothesis about the formation of fluorenyl structures. The general scheme of PDSP thermolysis at 260–400 °C was proposed in which the major process is the formation of fluorenyl fragments in macromolecules of the polymer due to the intramolecular ring closure in biradicals formed by the SPC cleavage.     

The negative ion mass spectra of methanol and deuterated methanol

The negative ion mass spectra of methanol, methanol-d and trideuteromethanol have been measured at 70 eV. Use of deuterated methanols has enabled the ion at m/e 31 to be identified as CH₃O^? and not CH₂OH^?. Isotope effects have been determined for H^? and D^? formation, and for OH^? and OD^? formation. It has been shown that OH^? formation occurs as a rearrangement reaction as well as by simple C? O bond fission.     

Temperature‐dependent polymorphic crystalline structure and melting behavior of poly(butylene adipate) investigated by time‐resolved FTIR spectroscopy

The polymorphic crystalline structure and melting behavior of biodegradable poly(butylene adipate) (PBA) samples melt‐crystallized at different crystallization temperatures were studied by differential scanning calorimetry (DSC) and fourier transform infrared (FTIR) spectroscopy. The crystalline structure and melting behavior of PBA were found to be greatly dependent on the crystallization temperature. By comparison of the FTIR spectra and the corresponding second derivatives between the α‐ and β‐crystal of PBA, the spectral differences were identified for the IR bands appeared at 1485, 1271, 1183, and 930 cm^?1 and the possible reasons were presented. Especially, the 930 cm^?1 band was found to be a characteristic band for the β‐crystal. Combining the DSC data with the analysis of normalized intensity changes of several main IR bands during the melting process, the melting behaviors of the α‐ and β‐crystal were clarified in detail. It is demonstrated by the in situ IR measurement that the β‐crystalline phase would transform into the α‐crystalline phase during the melting process, and the solid–solid phase transition from the β‐ to α‐crystal was well elucidated by comparing the intensity changes of the 1170 and 930 cm^?1 bands. The dependence of the β‐ to α‐crystal phase transition on the heating rate was revealed by monitoring the intensity ratio of the 909 and 930 cm^?1 band. It was suggested that at the heating rate of 0.5 or 1 °C/min, the percent amount of the transformed α‐crystal from the β‐crystal was much higher than that at the higher heating rate. The β‐crystal transforms into the α‐crystal incompletely at the higher heating rate because of the less time available for the phase transition. In addition, the β‐ to α‐crystal phase transition was further confirmed by the IR band shifts during the melting process. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1997–2007, 2009     

Pulse radiolysis study on the initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene

Initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene have been studied by means of microsecond pulse radiolysis. For styrene, absorption bands caused by the monomer cation radical St⁺? appear at 630 and 350 nm in a mixture of isopentane and n-butyl chloride at about ?165°C. In parallel with the decay of St⁺?, three absorption bands appear in the near-infrared (IR) region, and at 600 and 450 nm. The IR and 600 nm bands are assigned to the associated dimer cation radical St₂⁺?, and the 450 nm band to the bonded dimer cation radical St-St⁺?. The kinetic behavior of these species shows that reaction of St⁺? with styrene monomer forms both St₂⁺? and St-St⁺?. With the decay of St-St⁺?, another absorption band appears at 340 nm, and the lifetime of this band is relatively long. The 340 nm band may be due to carbonium ions of the growing polystyrene. For α-methylstyrene, the monomer cation radical (at 690 and 350 nm), the associated dimer cation radical (in the near-IR region and at 620 nm) and the bonded dimer cation radical (at 480 nm) behave in a manner similar to that of the corresponding styrene species. The absorption band caused by carbonium ions of growing poly(α-methylstyrene) appears at 340 nm.     

Coordination of extraframework Li+ cation in the MCM-22 and MCM-36 zeolite: FTIR study of CO adsorbed

Nature and population of Li⁺ cationic sites in MCM-22 zeolite and its pillared form (MCM-36) were investigated by means of adsorption of CO as a probe molecule. CO stretching frequency and adsorption heat were measured by FTIR spectroscopy and adsorption microcalorimetry. Intrazeolitic carbonyl complexes on Li⁺ cations in MCM-22 and MCM-36 are characterized by two main vibrational bands at 2,195 and 2,188 cm^?1. Band at higher wavenumbers is ascribed to carbonyls on Li⁺ ions coordinated only to two oxygen atoms at the intersection of 10-ring channels and interacting with CO molecule by energy around 45 kJ mol^?1. Band at 2,188 cm^?1 was assigned to the carbonyls on Li⁺ cations located on top of 5 or 6-rings on the channel walls and coordinated to three or four oxygen atoms, interacting with CO molecule by energy 33–36 kJ mol^?1. Effect of pillaring and layered form of zeolite on nature and population of Li⁺ cationic sites is also discussed, as well as the formation of dicarbonyl complexes.     

Some 1-substituted quaternary imidazolium compounds and related polymers: Qualitative and quantitative infrared analysis

It was found that 1-substituted quaternary imidazolium compounds show some characteristic infrared (IR) activity. On quarternization of 1-substituted imidazoles strong absorption bands appeared at about 1150 and 1550 cm^?1 in the IR spectra of these compounds. The band at 1150 cm^?1 was assigned to the position 2 C?H bending mode and the 1550 cm^?1 band was attributed to a ring vibration mode of the quaternary imidazolium compounds. The concentration of the quaternary imidazolium units in a polymer can be determined by measuring the intensity of the absorption bands at 1150 or 1550 cm^?1 in relation to another suitable absorption band of the spectrum.     

Confirmation of the presence of imine bonds in thermally cured polyimides

Model compounds for imines formed during the thermal curing of short chain polyimides have been synthesized and characterized. These compounds have imine bonds (C?N) formed by the nucleophilic attack of primary amines on imide carbonyls. The C?N stretching mode appears at 1649–1664 cm^?1 in the Raman and infrared spectra of these compounds and the band assigned to the carbonyl mode in an imide ring with an imine bond appears near 1740 cm^?1. These compounds have been prepared and characterized to verify the conclusions of a previously reported study in which bands observed in thermally cured short chain polyimides at 1656 and 1742 cm^?1 were assigned as the C?N and associated C?O modes, respectively. It has also been confirmed that the C?N stretching mode in the imide model compounds is inherently IR weak and can only be seen if the concentration of imine species is high. © 1993 John Wiley & Sons, Inc.     

Structural analysis of reactive dye species retained by the basic alumina surface

The alumina-dye composites were prepared by treating the basic alumina with the water solutions of Reactive Red 120 (RR 120) and Reactive Blue 15 (RB 15) dyes. The bands of low intensities in the 1400–1600 cm⁻¹ region and at 783 cm⁻¹ in the IR spectra of these composites point out that the dye species is bound weakly to the surface. In the case of mechanochemical adsorption of dye molecules, the asymmetric and symmetric S(=O)₂ and the S-O-C stretching bands together with the vibrations of aromatic ring revealed that dye types under dry conditions interacted effectively with alumina surface. After the heating of the alumina dye complexes in the temperature range 150–350°C, the intensities of the IR and XRD peaks for adsorbed types decreased. The endothermic peaks over 200°C and the bigger total mass losses for the alumina-dye composites can be ascribed to the decomposition of dye species retained by the alumina surface. The mass losses on TG curves of the alumina-dye complexes up to ∼800°C exhibit the removal of black residues occurred by decomposition of first adsorbed products. The thermal analysis data also point out that the water molecules bonded strongly to the alumina surface and dye types compete to accommodate at the surface active sites.     

The influence of thermal treatment on the nitrate band intensities in infra-red spectra of nitrate inclusion complexes of the zeolite 4A

The changes occurring in the included component in the course of thermal treatment of inclusion complexes of zeolite 4A with alkali nitrates were followed by IR spectroscopy and the results obtained were correlated with the data of TG analysis. An increase in temperature caused changes in the breadth and intensity of the nitrate bands (at about 1450 and 1380 cm^–1) and the appearance of a new band at about 1350 cm^–1. All the mentioned effects relate to the temperature interval starting from room temperature up to 670 K and are attributed to the increase in the thermal motion of included species in zeolite cages. The activation energy of the coordination change of the nitrate groups in the cage was calculated from the ratio of intensities of the nitrate bands at 1450 and 1350 cm^–1 and the intensity of the zeolite band at 460 cm^–1, originating from aT–O bending vibration as a function of temperature. At temperatures exceeding 720 K decomposition of included nitrate was noticed. Having this in mind, it was concluded that the changes of the band intensities were closely connected with this process.     

Surface modified Pt/C as a methanol tolerant oxygen reduction catalyst for direct methanol fuel cells

A new procedure has been successfully developed by which PtN_x/C is synthesized to enhance methanol tolerance while maintaining a high catalytic activity for the oxygen-reduction reaction (ORR). The nitrogen-modified Pt surface, which is prepared using a chelating agent followed by heat treatment, exhibits considerable selectivity toward the ORR in the presence of methanol. The high methanol tolerance could be attributed to the suppression of methanol adsorption resulting from the modification of the Pt surface with nitrogen. A direct methanol fuel-cell (DMFC) test showed that a power density of up to 120 m W cm⁻² was generated when PtN_x/C was used as the cathode catalyst (1 mg cm⁻²) in 6 M methanol and oxygen at 70 °C.     

IR studies of polyimides. I. Effects of chemical and physical changes during cure

A detailed study has been made of the thermal cure of two polyimides using infrared analysis. It is shown that two of the IR bands commonly used to determine the amount of imidization are affected by both interference from other species and by polymer morphology. These bands, located near 1780 cm^?1 and 730 cm^?1, overlap bands attributed to anhydride, which is formed when the polymer is heated. A correction for this effect is described. However, the measured absorbances of these bands are also subject to a dichroic effect. Any anisotropy in the samples can therefore affect the results. A third band, that located near 1370 cm^?1, does not overlap anhydride peaks and did not, using the approach described here, display any effect due to dichroism. Thus it appears to offer a reliable way of following imidization by IR.     

Infrared Spectroscopic Studies on the Surface Chemistry of High‐Surface‐Area Gallia Polymorphs

Polymorphs α, β, and γ of Ga₂O₃ having hexagonal (corundum‐type), monoclinic and cubic (spinel‐type) structure, respectively, were prepared in a high‐surface‐area form, and characterized by powder X‐ray diffraction. Nitrogen adsorption at 77 K showed these gallia samples to have specific surface areas of 77 (α‐Ga₂O₃), 40 (β‐Ga₂O₃) and 120 m² g^?1 (γ‐Ga₂O₃). Fourier transform infrared spectroscopy of adsorbed carbon monoxide (at 77 K) and pyridine (at room temperature) showed that the three gallia polymorphs have a very similar surface Lewis acidity, regardless of their different crystal structures. This Lewis acidity was assigned, mainly, to coordinatively unsaturated tetrahedral Ga³⁺ ions situated on the surface of the small crystallites which constitute the different metal oxide varieties. Ga³⁺···CO adducts formed after CO adsorption gave (in all cases) a characteristic C–O stretching band at 2195–2200 cm^?1, while Lewis‐type adducts formed with adsorbed pyridine were characterized by IR absorption bands at 1610–1612 and 1446–1450 cm^?1. The three (partially hydroxylated) gallia polymorphs showed also a very weak Brønsted acidity, which they manifested by forming hydrogen‐bonded adducts with both CO and pyridine; however no protonation of adsorbed pyridine occurred.