Showing papers on "Infrared spectroscopy published in 1992"

Fourier Transform Infrared Spectroscopy

Abstract: Typically, lignins are characterized by IR spectroscopy in the solid state. The examination of lignins in solution has long been a domain of UV and NMR spectroscopy. However, with the advent of the FTIR technique, the IR spectra of liquid samples now can be readily obtained. The need to measure IR spectra in solution exists when there is not enough time to isolate lignin from solution as, for example, is the case when spent pulping liquors have to be analyzed rapidly during a pulping process for process control. This section describes briefly the common liquid sampling accessories and applications of solution state FTIR spectroscopy to lignins. For more details on FTIR spectroscopy, the reader is referred to Chapter 4.1.

Spectroscopy of polymers

Abstract: Theory of polymer characterization vibrational spectroscopy of polymers experimental IR spectroscopy of Polymers applications of IR spectroscopy to polymers Raman spectroscopy of polymers high-resolution NMR spectroscopy of polymers in solution special editing techniques for high-resolution NMR spectroscopy of polymers high-resolution NMR spectroscopy of solid polymers applications of high-resolution solid-state NMR spectroscopy of polymers NMR relaxation spectroscopy of polymers NMR imaging of polymeric materials.

Characterization of Humic Acids, Composts, and Peat by Diffuse Reflectance Fourier-Transform Infrared Spectroscopy

Abstract: Transmission infrared (IR) spectroscopy, either dispersive or Fourier transformed (FTIR), has been used extensively in studies on humic substances. A variety of bands characteristic of molecular structures and functional groups have been identified for these substances. The development of an attachment mounted onto FTIR spectrophotometers has allowed the determination of diffuse reflectance Fourier-transformed infrared (DRIFT) spectra. The purpose of this work was to study the applicability of DRIFT to soil organic-matter research and to compare the use of this instrumentation to dispersive and Fourier-transformed transmission IR spectroscopy. The DRIFT spectra were determined for humic acids, peat samples, and composts. In addition, the possibility of using DRIFT spectra to quantitatively measure sample concentration and measuring the relative concentration of functional groups was assessed. Sample preparation for DRIFT is much simpler than for transmission IR spectroscopy, interferences due to water adsorption are reduced, and resolution is improved. The spectra obtained using DRIFT had a higher degree of resolution as compared with dispersive and Fourier-transform transmission IR spectroscopy. Bands indicative of aliphatic C-H, carboxyl and corboxylate functional groups, aromatic C=C, and C-O stretch of polysaccharides were prominent and very well resolved. The DRIFT spectra obtained can also he used to fingerprint organic matter acquired from various sources. Spectra obtained at various concentrations of humic acid indicated that DRIFT cannot be used to estimate concentrations of organic matter in a given mixture. Relative concentrations of functional groups, however, were found to be fairly constant regardless of sample concentration. Therefore, changes in the relative concentration of functional groups can he measured during the humification process. It is expected that the application of DRIFT to organic-matter research will prove especially useful for characterizing bulky heterogeneous samples such as peat and composts.

Low-temperature Fourier-transform infrared investigation of the interaction of CO with nanosized ZSM5 and silicalite

Abstract: Nanosized ZSM5 zeolites with microcrystal dimensions in the 20–120 nm range have been characterized by means of IR spectroscopy and HRTEM microscopy. The vibrational spectrum of the OH groups on the external and internal surfaces of H-ZSM5 and Na-ZSM5 samples of different crystallite dimensions has been investigated. For the sake of comparison the spectra of silicalite samples containing different concentrations of sodium and aluminium are also shown. For this purpose high-purity silicalite samples were prepared following a novel synthesis route.Carbon monoxide (a very weak Lewis base) was used to probe the acidity present on the external and internal surfaces of the zeolites through formation of 1 : 1 adducts with silanols (both internal and external), Bronsted-acid groups (both framework and extraframework), Na+ ions, and Lewis Al3+ centres (in extraframework and framework positions). The IR-active CO stretching modes of the complexes are shifted to higher wavenumber with respect to the free molecule; the positive shift can be used to estimate the acid strength. CO that was physically adsorbed in the zeolite channels has also been investigated.

Quantitative determination of molecular structure in multilayered thin films of biaxial and lower symmetry from photon spectroscopies. I. Reflection infrared vibrational spectroscopy

Abstract: A semitheoretical formalism based on classical electromagnetic wave theory has been developed for application to the quantitative treatment of reflection spectra from multilayered anisotropic films on both metallic and nonmetallic substrates. Both internal and external reflection experiments as well as transmission can be handled. The theory is valid for all wavelengths and is appropriate, therefore, for such experiments as x‐ray reflectivity, uv–visible spectroscopic ellipsometry, and infrared reflection spectroscopy. Further, the theory is applicable to multilayered film structures of variable number of layers, each with any degree of anisotropy up to and including full biaxial symmetry. The reflectivities (and transmissivities) are obtained at each frequency by solving the wave propagation equations using a rigorous 4×4 transfer matrix method developed by Yeh in which the optical functions of each medium are described in the form of second rank (3×3) tensors. In order to obtain optical tensors for materials not readily available in single crystal form, a method has been developed to evaluate tensor elements from the complex scalar optical functions (n) obtained from the isotropic material with the limitations that the molecular excitations are well characterized and obey photon–dipole selection rules.This method is intended primarily for infrared vibrational spectroscopy and involves quantitative decomposition of the isotropic imaginary optical function (k) spectrum into a sum of contributions from fundamental modes, the assignment of a direction in molecular coordinates to the transition dipole matrix elements for each mode, the appropriate scaling of each k vector component in surface coordinates according to a selected surface orientation of the molecule to give a diagonal im(n) tensor, and the calculation of the real(n) spectrum tensor elements by the Kramers–Kronig transformation. Tensors for other surface orientations are generated by an appropriate rotation matrix operation. To test the viability of this approach, three sets of experimentally derived infrared spectra of oriented monolayer assemblies on quite distinctively different substrates were chosen for simulation: (1) n‐alkanethiols self‐ assembled onto gold, (2) n‐alkanoic acid salt Langmuir–Blodgett (LB) monolayers on carbon, and (3) n‐alkanoic acid salt LB monolayers on silica glass. The formalism developed was used to simulate the spectral response and to derive structural features of the monolayers. Good agreement was found where comparisons with independent studies could be made and, in general, the method appears quite useful for structural studies of highly organized thin films.

Infrared spectroscopy of DNA.

Abstract: Publisher Summary This chapter presents a general survey of the Fourier-transform infrared (FTIR) spectrum of a nucleic acid, describes the characterization of the different classic families of DNA conformations, and then discusses the way FTIR spectroscopy can be used in the study of DNA recognition, particularly in the case of triple-stranded structures and DNA–drug interactions. Compared to other techniques some of the advantages of infrared spectroscopy in the study of nucleic acids are (1) the possibility of obtaining data from samples under an extremely wide variety of physical states, (2) no limitation introduced by the size of the investigated molecule—that is, it can work well with short oligonucleotides as well as long polynucleotides, DNA fragments obtained by enzymatic or chemical cleavage, or high molecular weight native DNAs, (3) infrared spectroscopy is a nondestructive technique, which requires only small amounts of samples, and (4) it gives vibrational information characteristic of the helical conformations of the nucleic acids.

Acidic Properties of Alumina-Supported Metal Oxide Catalysts: An Infrared Spectroscopy Study

Abstract: The infrared spectra of the hydroxyl region and that of surface chemisorbed CO 2 species for Re 2 O 7 /Al 2 O 3 , CrO 3 /Al 2 O 3 , MoO 3 /Al 2 O 3 , V 2 O 5 /Al 2 O 3 , TiO 2 /Al 2 O 3 , and Nb 2 O 5 /Al 2 O 3 catalytic systems have been investigated. A sequential consumption of the alumina OH groups upon deposition of the supported metal oxide has been found for all the investigated catalytic systems. A possible relationship between BrOnsted acidity and a new low-frequency band in the hydroxyl region observed at high loadings of the supported metal oxide systems is postulated

Silicalite characterization. 2. IR spectroscopy of the interaction of carbon monoxide with internal and external hydroxyl groups

Abstract: In paper 1 we have shown that the structural properties of silicalites depend upon the preparation procedure. In particular Na- and Al-free sample (S) prepared following a specifically designed method is characterized by the presence of internal atomic-dimension defects (nanodefects and microcavities). On the basis of spectroscopic data and computer graphic simulations, it is shown that the microcavities derive from one or more missing [SiO 4 ] units and can contain up to four OH groups per missing tetrahedron

Spectroscopy of the Earth's Atmosphere and Interstellar Medium

Abstract: Far infrared and microwave spectroscopy of the earth's atmosphere, B. Carli and M. Carlotti remote sensing of the atmosphere by high resolution infrared absorption spectroscopy, L.R. Brown, et al intensities and collision broadening parameters from infrared spectra - an update, M.A.H. Smith, et al collisional line mixing, A. Levy, et al spherical top spectra, J.P. Champion, et al spectroscopy among the stars, G. Winnewisser, et al.

Surface vibrational modes of silanol groups on silica

Abstract: Infrared spectroscopy has been used to study the vibrational modes associated with free (isolated) silanol groups on an aerosil silica and a precipitated silica which have been activated in vacuum in the temperature range from 450 to 800 o C. Both silicas exhibit two Si-O-H angle deformation modes at 760 and 840 cm -1 , indicative of two types of isolated silanol species, called type I and II, respectively

FTIR difference spectroscopy of bacteriorhodopsin: toward a molecular model.

Abstract: Bacteriorhodopsin (bR) is a light-driven proton pump whose function includes two key membrane-based processes, active transport and energy transduction. Despite extensive research on bR and other membrane proteins, these processes are not fully understood on the molecular level. In the past ten years, the introduction of Fourier transform infrared (FTIR) difference spectroscopy along with related techniques including time-resolved FTIR difference spectroscopy, polarized FTIR, and attenuated total reflection FTIR has provided a new approach for studying these processes. A key step has been the utilization of site-directed mutagenesis to assign bands in the FTIR difference spectrum to the vibrations of individual amino acid residues. On this basis, detailed information has been obtained about structural changes involving the retinylidene chromophore and protein during the bR photocycle. This includes a determination of the protonation state of the four membrane-embedded Asp residues, identification of specific structurally active amino acid residues, and the detection of protein secondary structural changes. This information is being used to develop an increasingly detailed picture of the bR proton pump mechanism.

Luminescence and structural study of porous silicon films

Abstract: The luminescence properties of 3 μm thick, strongly emitting, and highly porous silicon films were studied using a combination of photoluminescence, transmission electron microscopy, and Fourier transform infrared spectroscopy. Transmission electron micrographs indicate that these samples have structures of predominantly 6–7 nm size clusters (instead of the postulated columns). In the as‐prepared films, there is a significant concentration of Si—H bonds which is gradually replaced by Si—O bonds during prolonged aging in air. Upon optical excitation these films exhibit strong visible emission peaking at ≊690 nm. The excitation edge is shown to be emission wavelength dependent, revealing the inhomogeneous nature of both the initially photoexcited and luminescing species. The photoluminescence decay profiles observed are highly nonexponential and decrease with increasing emission energy. The 1/e times observed typically range from 1 to 50 μs. The correlation of the spectral and structural information suggest...

Fourier transform Raman and infrared vibrational study of human skin: Assignment of spectral bands

Abstract: Fourier transform (FT) Raman and infrared spectra of the outermost layer of human skin, the stratum corneum, were recorded. Assignments consistent with the FT Raman vibrations were made for the first time and compared with assignments from the FTIR spectrum. The results demonstrate that FT Raman spectroscopy holds several advantages over FTIR in studies of human skin. The molecular and conformational nature of human skin, and modifications induced by drug or chemical treatments, may be assessed by FT Raman spectroscopy.

Correlation between silicon hydride species and the photoluminescence intensity of porous silicon

Abstract: The role of silicon hydride species in the photoluminescence intensity behavior of porous Si has been studied. The surfaces of luminescent porous Si samples were converted to a predominate SiH termination using a remote H plasma. The as‐passivated samples were then immersed in various concentrations of hydrofluouric solutions to regulate the recovery of SiH2 termination on the surface. Photoluminescence measurements and transmission Fourier‐transform infrared spectroscopy have shown that predominant silicon monohydride (SiH) termination results in weak photoluminescence. In contrast, it has been observed that the appearance of silicon dihydride (SiH2) coincides with an increase in the photoluminescence intensity.

Spectroscopic study of the interaction of carbon monoxide with cationic and metallic palladium in palladium–alumina catalysts

Abstract: Palladium–alumina catalysts prepared from various precursors and by different preparation methods have been studied after calcination by O2 and reduction by H2, using IR spectroscopy of adsorbed CO, EPR and diffuse reflectance measurements. In addition to Pd2+ ions, the unreduced samples contain Pd3+ and Pd+ species. Pd2+ and Pd– entities are still present on catalysts reduced at 573 K. The influence of the precursor on the metal fraction exposed (MFE) and on the nature of adsorbed CO species is clearly shown. Bridged entities are more sensitive to the MFE value than the linear ones. At 100 Torr [1 Torr ≈(101 325/760) Pa], the bridged entities are compressed, which leads to a CO-(surface Pd) ratio of near unity. The MFE increases when the precursors are taken in the following order nitrate < chloride (impregnated)≈ acetylacetonate < chloride (exchanged). The sintering brought about by increasing the reduction temperature seems to affect the largest metal particles rather than the small ones which strongly interact with the carrier.

FTIR spectroscopic study of carbon dioxide adsorption/desorption on magnesia/calcium oxide catalysts

Abstract: The nature of adsorption states of CO{sub 2} on mixed Mg/Ca oxide catalysts was investigated by FTIR spectroscopy. At room temperature, the unidentate is predominantly formed besides bicarbonate. On CaO-rich catalysts additional structures were detected and attributed to bridging carbonate and linearly adsorbed CO{sub 2}. Desorption behavior of these species is discussed in dependence on temperature. Surface basicity was quantitatively measured by the wavenumber difference of symmetric and asymmetric carbonate absorption bands and related to catalytic activity for oxidative coupling of methane. 22 refs., 6 figs., 2 tabs.

An infrared study of monomeric and oligomeric (n=2, 3, and 4) hydrogen chloride in liquified noble gases

Abstract: Infrared spectra (3100–2600 cm−1) of HCl dissolved in liquid argon (94–124 K), liquid krypton (117–167 K), and liquid xenon (161–221 K) at concentrations varying from 0.8×10−3 to 2.8×10−2 M are reported. At low concentrations in all three solvents, only the spectrum due to monomeric species is observed. For solutions in liquid argon, the observed rotational fine structure was assigned. For all solvents, the monomeric stretching frequency shows a linear relation to the relative density of the solvent, extrapolating to the dilute vapor phase frequency. At higher concentrations in liquid argon and liquid krypton, bands due to oligomeric species are found. Factor analysis shows at least three oligomeric species are present. The band profile analysis of the oligomer absorptions allows the assignment of observed bands to dimer, trimer, and tetramer. From the temperature dependence of the oligomer band intensities, the enthalpy difference for dimerization is found to be 3.78±0.33 kJ mol−1 in liquid argon and 5.0...

Porous silica gels and TiO2/SiO2 mixed oxides prepared via the sol-gel process: characterization by spectroscopic techniques

Abstract: Titania/silica mixed oxides prepared by sol-gel techniques are characterized by diffuse reflectance FTIR, Raman and 29Si solid state CP/MAS-NMR spectroscopy. The degree of cross-linking of the silica network is inferred from the frequency of the v(Si O Si) skeletal vibration. Gels prepared by complete hydrolysis of the alkoxide precursors (tetra-ethoxy-orthosilicate, TEOS, and tetra-isopropoxy-orthotitanate, TIOT, respectively) prior to mixing exhibit TiO2 crystallites embedded in an amorphous silica matrix. Raman spectroscopy is used to study the dependence of TiO2 crystallite size on hydrolysis pH. In an alternative two-stage hydrolysis procedure, 2 mol of water are first added per mole of TEOS, followed by mixing with the appropriate amount of TIOT, and completion of the hydrolysis. No crystalline domains are detected in the resulting oxides. The formation of Si O Ti bridges, as monitored by the intensity of a characteristic 945–960 cm−1 vibration, is particularly prominent if both steps of the two-stage hydrolysis procedure are performed in an acidic medium. A redispersion treatment in base, designed to induce a pore ripening process, if found to be only effective when applied to the wet gel, i.e., prior to drying. 29Si NMR experiments are used to investigate the processes occurring upon drying at 873 K. The extent of condensation reactions, by which Si O Si bridges are formed with concomitant desorption of water, is inferred from the fraction of completely cross-linked Si( O Si)4 sites. 1H-29Si cross-polarization dynamics are found to depend strongly on the degree of cross-linking, as well as on the number of available surface silanol groups.

Amide and Ester Surface Attachment Reactions for Alkanethiol Monolayers at Gold Surfaces As Studied by Polarization Modulation Fourier Transform Infrared Spectroscopy

Abstract: The modification of alkanethiol monolayers via amide and ester formation reactions is demonstrated as a methodology for attaching monolayer and submonolayer coveragea of specific chemical species onto metal electrodes. A monolayer of 11-mercaptoundecanolc acid (HSC 10 H 20 COOH) is used as a surface bifunctional linking agent on polycrystalllne gold surfaces. The reaction of these adsorbed molecules with gaseous thionyl chloride converts the carboxylic acid group to an acid chloride which can then be reacted further with an amine or alcohol to form an amide or ester linkage

Matrix isolation IR spectroscopy of tautomeric systems and its theoretical interpretation: 2-hydroxypyridine/2(1H)-pyridinone

Abstract: Prototropic tautomerism of a model system of 2-hydroxypyridine/2(1H)-pyridinone has been investigated in low-temperature inert matrices and in the gas phase. The ratio of tautomer concentrations, [hydroxy]:[oxo], for the 2-pyridinone sample deposited in Ar or N 2 matrices was estimated as 2.80:1 and 2.99:1, respectively. A similar ratio of tautomers was observed in the gas phase. The matrix and gas-phase infrared spectra of2-hydroxypyridine and 2(1H)-pyridinone are reported

Vibrational dependence of the anisotropic intermolecular potential of Ar-HCl

Abstract: A new intermolecular potential for Ar-HCl is obtained by fitting to results from high-resolution microwave, far-infrared, and infrared spectroscopy. The new potential is substantially more accurate than any previous Ar-HCl potential and is the first to include the dependence of the potential on the HCl monomer vibration: it is a function of the diatom mass-reduced vibrational quantum number η=(+1/2)/μ HX 1/2 as well as the intermolecular distance R and angle θ

Redox-dependent changes in .beta.-extended chain and turn structures of cytochrome c in water solution determined by second derivative amide I infrared spectra

Abstract: The redox-dependent changes in secondary structure of cytochromes c from horse, cow, and dog hearts in water at 20 OC have been determined by amide I infrared spectroscopy. Second derivative amide I spectra were obtained by use of a procedure that includes a convenient method for the effective subtraction of the spectrum of water vapor in the system. The band at 1657 cm-' representing the helix structure was unaffected by a change in redox state whereas changes in bands due to turns at 1680, 1672, and 1666 cm-', unordered structure at 1650 cm-', and @-structures at 1632 and 1627 cm-' occurred. About one-fourth of the @-extended chain spectral region and one-fifth of the @-turn region (involving a total of approximately 9-13 residues) were sensitive to the oxidation state of heme iron. No significant changes in the secondary structure of either the reduced or oxidized protein due to changes in ionic strength were detected. The localized structural rearrangements triggered by the changes in oxidation state of heme iron are consistent with differences in the binding of heme iron to a histidine imidazole nitrogen and a methionine sulfur atom from the ,&extended chain. The demonstrated ability to obtain highly reproducible second derivative amide I infrared spectra confirms the unique utility of such spectral measurements for localization of subtle changes in secondary structure within a protein, especially for changes among the multiple turns and 6-structures.