Showing papers on "Infrared spectroscopy published in 1991"

The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules

Abstract: Alkanes. Halocompounds. Alcohols and Phenols. Ethers and Peroxides. Alkenes. Acetylenes. The -C=N and -N=C Groups. Compounds Containing the Carbonyl Group. Compounds Containing -NH2, -NHR, and -NR2 Groups. The Nitro Group. Double Bonds Containing Nitrogen Atoms. Cumulated Double Bonds. Organic Sulfur Compounds. Organosilicon Compounds. Organophosphorous Compounds. Aromatic and Heteroaromatic Rings. Selected Infrared and Raman Spectra. Appendices. Index.

Classification of Lignins from Different Botanical Origins by FT-IR Spectroscopy

Abstract: A Classification System of lignin spectra is presented, based on FT-IR Spectroscopy of more than hundred MWL's.The spectra were baseline corrected and normalized.Three main categories can be differentiated: (1) G type, (2) GS type, and (3) HGS type.The spectra within the GS category constitute a continuum, but they can be further subdivided into four groups. The Classification criteria are presented äs a table and illustrated by authentic FT-IR spectra obtained from MWL's. Peak-peaker lists of typical representatives of the three principal groups show the band intensities of normalizcd IR spectra. A triangulär coordinate System displays the correlation between the spectral features on the one hand, and the possible Normalization of FT-IR spectra composition of a lignin from its H, G, and S units on the other.

Microbiological characterizations by FT-IR spectroscopy

Abstract: Infrared signals of microorganisms are highly specific fingerprint-like patterns that can be used for probing the identity of microorganisms. The simplicity and versatility of Fourier-transform infrared spectroscopy (FT-IR) makes it a versatile technique for rapid differentiation, classification, identification and large-scale screening at the subspecies level.

Reduction of CeO2 by hydrogen. Magnetic susceptibility and Fourier-transform infrared, ultraviolet and X-ray photoelectron spectroscopy measurements

Abstract: The reduction of CeO2 by hydrogen has been studied from 300–1200 K by several complementary techniques: temperature-programmed reduction (TPR) and magnetic susceptibility measurements, Fourier-transform infrared (FTIR), UV–VIS diffuse reflectance and X-ray photoelectron (XP) spectroscopy. Two CeO2 samples were used with B.E.T. surface areas of 115 and 5 m2 g–1, respectively. The concentration of Ce3+ was determined in situ by measuring the magnetic susceptibility and the CeIII photoemission line. The reduction began at 473 K, irrespective of the initial surface area of the ceria. In the case of the low-surface-area sample, an intermediate reduction step was observed between 573 and 623 K, corresponding to the reduction of the surface. This intermediate step was less easily observed in the case of the high-surface-area ceria. In both cases, the reduction led to a stabilised state with the formal composition CeO1.83. Temperatures higher than 923 K were required to reduce the ceria further. The surface CeIII content determined by XPS was close to that determined by magnetic susceptibility measurements. The intensity of the 17 000 cm–1 band in the UV–VIS reflectance spectrum also varied with the degree of reduction. Finally, the evolution of the surface species observed by IR spectroscopy was in good agreement with the results from the other techniques. The IR results indicated large changes in the concentration and nature of both the hydroxyl and the polydentate carbonate species during the reduction process. The adsorption of oxygen on samples previously reduced to the composition CeO1.83 led to almost complete reoxidation at room temperature. The state of the initial B.E.T. surface did not influence the oxidation process. A slight excess adsorption of oxygen was evident on the surface. This was thermodesorbed at 380 K under vacuum.

Silicon oxycarbide glasses: Part II. Structure and properties

Abstract: Silicon oxycarbide glass is formed by the pyrolysis of silicone resins and contains only silicon, oxygen, and carbon. The glass remains amorphous in x-ray diffraction to 1400 °C and shows no features in transmission electron micrographs (TEM) after heating to this temperature. After heating at higher temperature (1500–1650 °C) silicon carbide lines develop in x-ray diffraction, and fine crystalline regions of silicon carbide and graphite are found in TEM and electron diffraction. XPS shows that silicon-oxygen bonds in the glass are similar to those in amorphous and crystalline silicates; some silicons are bonded to both oxygen and carbon. Carbon is bonded to either silicon or carbon; there are no carbon-oxygen bonds in the glass. Infrared spectra are consistent with these conclusions and show silicon-oxygen and silicon-carbon vibrations, but none from carbon-oxygen bonds. 29Si-NMR shows evidence for four different bonding groups around silicon. The silicon oxycarbide structure deduced from these results is a random network of silicon-oxygen tetrahedra, with some silicons bonded to one or two carbons substituted for oxygen; these carbons are in turn tetrahedrally bonded to other silicon atoms. There are very small regions of carbon-carbon bonds only, which are not bonded in the network. This “free” carbon colors the glass black. When the glass is heated above 1400 °C this network composite rearranges in tiny regions to graphite and silicon carbide crystals. The density, coefficient of thermal expansion, hardness, elastic modulus, index of refraction, and viscosity of the silicon oxycarbide glasses are all somewhat higher than these properties in vitreous silica, probably because the silicon-carbide bonds in the network of the oxycarbide lead to a tighter, more closely packed structure. The oxycarbide glass is highly stable to temperatures up to 1600 °C and higher, because oxygen and water diffuse slowly in it.

Hartree–Fock second derivatives and electric field properties in a solvent reaction field: Theory and application

Abstract: A compact formalism for the second and third derivatives of the Hartree–Fock energy in the presence of an Onsager solvent reaction field is presented. All three standard algorithms (MO, AO, and direct) are extended to include the reaction field in a unified way. Predictions of the infrared spectrum of formaldehyde in a variety of solvents and of solvent‐induced shifts in carbonyl stretching frequencies are presented along with the results of new measurements. As for the gas‐phase case, analytical second derivatives are far more efficient than numerical ones. The reaction field provides very good predictions of solvent effects at negligible computational cost.

Polarization Modulation FT-IR Spectroscopy of Surfaces and Ultra-thin Films: Experimental Procedure and Quantitative Analysis

Abstract: Polarization modulation of the incident electromagnetic field is used to increase the sensitivity and the in situ experiment ability of the well-known method of reflexion absorption FT-IR spectroscopy, for the characterization of surfaces and ultra-thin films. The experimental procedure and signal processing of the detected intensity are described and illustrated with the use of results obtained with Langmuir-Blodgett monolayers. The quantitative analysis of the spectra is then developed, and a linear behavior of the band intensities is found for ultra-thin films exhibiting no strong absorptions. This result is checked with the use of organic and inorganic ultra-thin films of increasing thicknesses.

Pressure-induced silicon coordination and tetrahedral structural changes in alkali oxide-silica melts up to 12 GPa: NMR, Raman, and infrared spectroscopy

Abstract: The 29 Si and 23 Na NMR, Raman, and infrared spectra of glasses quenched from liquids at pressures up to 12 GPa for three alkali silicate compositions (Na 2 Si 2 O 5 , Na 2 Si 4 O 9 , K 2 Si 4 O 9 ) and silica (SiO 2 ) reveal systematic changes in the melt structure with pressure. The most novel change is the occurrence of [5] Si and [6] Si species at high pressures identified by peaks in the 29 Si MAS NMR spectra of the alkali silicate glasses. -from Authors

Chemical etching of vicinal Si(111): Dependence of the surface structure and the hydrogen termination on the pH of the etching solutions

Abstract: Infrared spectroscopy is used to study the etching process of stepped Si(111)9° surfaces as a function of the pH of the etching HF solutions. This process results in complete H termination of the silicon surface, including terraces, steps, and defects; the surface structure can therefore be well studied using infrared (IR) spectroscopy. Polarized IR absorption spectra of the Si–H stretching vibrations (i.e., in the region 2060–2150 cm−1) vary dramatically as the pH of the etching solutions increases from 2.0 to 7.8. In general, higher pH solutions yield sharper bands and more easily assigned spectra, making it possible to identify the step and terrace species and thus to infer the surface structure and step morphology (i.e., to investigate the etching process). The data are explained by a model involving different etching rates for each individual surface species: The highest rate of removal is for isolated adatom defects located on (111) planes and the lowest is for the ideally H‐terminated (111) planes ...

Hydrogen-bonded liquids

Abstract: 1. Introduction.- i) Basic concepts and tools of liquid state theory.- ii) Methods for calculating intermolecular energy surfaces.- iii) Quantum effects in hydrogen-bonded liquids.- iv) Transport processes, relaxation, and glass formation in hydrogen-bonded liquids.- v) Thermodynamic properties of some H-bonded liquids in their undercooled and/or overcompressed states.- i) X-ray diffraction studies of liquids..- ii) Neutron diffraction techniques.- iii) Recent structural studies of liquid D2O by neutron diffraction..- iv) H/D substitution in neutron diffraction..- v) Structural analysis of liquid formic acid using neutron diffraction..- vi) Structural studies of water near an interface..- vii) SANS and QENS studies of Vycor containing D2O/H2O mixtures..- i) Molecular dynamics simulation studies of the hydrogen-bond network in water..- ii) A molecular model for aqueous solutions..- iii) A model for hydrogen-bonding effects at aqueous interfaces..- iv) A collective approach to the dynamics of water..- i) Properties of H-bonding in the infrared spectral range.- ii) Infrared spectra of H-bonded molecules..- iii) Vibrational and collision-induced Raman scattering from water and aqueous solutions..- iv) Acoustic S- and P-wave Character of the Intermolecular 60 and 175 cm-1 Raman Bands from Liquid Water..- v) Quasi-elastic and inelastic neutron scattering and molecular dynamics of water at supercooled temperature..- vi) The properties of hydrogen-bonded liquids studied by high pressure NMR..- vii) Low frequency light scattering in hydrogen-bonded liquids..- viii)Dielectric Measurements of water, alcohols and their mixtures..- ix) Ultrasonic properties of alcoholic aqueous solutions..- x) Multi-nuclear relaxation time studies in undercooled aqueous electrolytes..- xi) Vibrational dynamics of liquids and glassy electrolytes..- 6. The Programme.- Posters.- 7. Postscript.- Author index.- Chemical index.

Effects of formaldehyde fixation on protein secondary structure: a calorimetric and infrared spectroscopic investigation.

Abstract: We investigated the effects of formaldehyde fixation on the secondary structure of isolated proteins (bovine serum albumin, ribonuclease A, and hemoglobin) using high-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy. Whereas thermograms obtained by scanning calorimetry on unfixed purified proteins demonstrated denaturation transitions in the 70-90 degrees C temperature range, the thermograms showed no denaturation transitions in this temperature range when the proteins had been placed in formaldehyde solutions. Thus, fixation destroyed the denaturation transition of bovine serum albumin, ribonuclease A, and hemoglobin. Infrared spectra obtained on the unfixed and fixed proteins were essentially identical. This demonstrates that the "fixed" proteins retain the secondary structure present before fixation. We therefore conclude that the cross-linking of proteins that occurs in the process of formaldehyde fixation "locks in" the secondary structure of these protein molecules.

The structure of alkaline earth aluminosilicate glasses as determined by vibrational spectroscopy

Abstract: The structure of glasses in the CaOAl2O3SiO2 (CAS) and MgOAl2O3SiO2 (MAS) systems have been studied using Raman and infrared (IR) reflectance spectroscopies. Kramers-Kronig analysis was performed on the IR data. Two compositional series were chosen in each system, one which has a constant average polymerization of 1 non-bridging oxygen per tetrahedral cation, and the other in which silica content is constant at 50 mol%, but polymerization varies. The results indicate that the presence of alkaline earth network modifiers favors the formation of species with high negative charge density. In the NBO/T = 1 series this leads to the equilibrium 2Q3 = Q4 + Q2 (where the superscript is the number of bridging oxygens per tetrahedron) shifting to the right at low Al content and to the left at high Al/Si. In the fully polymerized, 50% silica Mg glass, there is evidence of clustering into Al-rich and Si-rich domains. Transformation of the IR data yields a well-resolved low-frequency band in the CAS spectra whose frequency depends on polymerization and whether Ca is acting as a network modifier or charge balancer. The analogous Mg band is apparently buried in the mid-frequency network mode.

Infrared spectra of water. I. Effect of temperature and of H/D isotopic dilution

Abstract: The implementation of a cell operating with the technique of Attenuated Total Reflection (ATR) enabled us to obtain good quality IR spectra of water for the whole region above 750 cm−1 (which is the limit of the effect of the cutoff wave number of the crystal) and up to 5000 cm−1. ATR spectra have been recorded as a function of temperature ranging from −5 °C to 80 °C and of H/D molar concentration c(0

Polarization modulation Fourier transform infrared reflectance measurements of thin films and monolayers at metal surfaces utilizing real-time sampling electronics

Abstract: The technique of polarization modulation Fourier transform infrared (PM-FTIR) spectroscopy is applied to the reflectance spectra of thin polymer films and spontaneously organized monolayers adsorbed onto gold, sliver, and chromium surfaces. The differential PM-FTIR reflectance spectra are obtalned by the photoelastic modulation of the FTIR beam polarization and a novel real-time sampling methodology that generates the average and differential FTIR interferograms from measurements of the Infrared signal during each modulation cycle. In comparison with conventional electronics that utilize a lock-in amplifier, the real-time electronics permit the operation of the FTIR spectrometer at normal mirror velocities. The use of polarization-independent optics after the metal surface ensures that the true surface Infrared differential reflectance spectrum is obtained. The theoretical wavelength dependence of the PM-FTIR spectrum is rederlved for the case of the real-time sampling measurement and compared to the experimental data. Spectra of a 15-nm film of polyimide on chromium, a spontaneously Organized monolayer of octadecanethioi on gold, and a spontaneously organized monolayer of arachidic acid on silver are shown to demonstrate the applicability of the method to different metals, samples, and spectral regions.

Infrared spectra of nitric and hydrochloric acid hydrate thin films

Abstract: The mono-, di-, and trihydrates of nitric acid have been prepared as thin crystalline films and characterized by their distinctive infrared spectra. The infrared spectra of the mono- and trihydrates have been found to be significantly different than previously reported, to the extent that the published spectra are of questionable use. The spectrum of the dihydrate of HNO{sub 3}, which has not been reported previously, differs sharply from that of the other hydrates primarily because of effects indicative of a much more severely distorted nitrate ion. Infrared spectra are also reported for the tetra- and hexahydrate of HCl. The spectrum of the tetrahydrate largely duplicates existing data but is included for comparison with the previously unpublished spectrum of the hexahydrate. Sharp O-D stretching bands of dilute HOD in the hexahydrate are indicative of proton ordering within the network of the waters of hydrate and affirm the view based on x-ray data that, unlike the highest crystalline hydrate of HClO{sub 4}, the structure does not mimic that of the classical clathrate hydrates.

In Situ Fourier‐Transform Electromodulated Infrared Study of Porous Silicon Formation: Evidence for Solvent Effects on the Vibrational Linewidths

Abstract: We report on an experimental investigation of porous silicon layer (PSL) formation and silicon/hydrofluoric acid interfaces, using internal‐reflection Fourier‐transform infrared spectroscopy and Fourier‐transform electrochemically modulated infrared spectroscopy. Low‐doping Si samples (, resistivity ≈4 Ω cm) have been used for PSL growing. The in situ electromodulated spectra during the formation of PSL and the transmission spectra of the PSL in contact with the electrolyte contain only a broad band at around 2100 cm−1, whereas the transmission spectra of the dried PSL give rise to three sharp peaks at around 2085, 2115, and 2140 cm−1. All these vibrational peaks are ascribed to chemical species. The broadening of the spectrum in the former cases has been found to be due to interaction of species with the solvent. Under cathodic conditions—either for n‐Si in the dark or for p‐Si with illumination—the spectrum of the Si/HF interface exhibits three sharp peaks, characteristic of a surface in contact with a gas phase. This is attributed to the presence of hydrogen gas bubbles on the Si surface. It is observed that the Si surface is covered with Si‒H bonds in solution at all the applied potentials and current densities. An attempt has been made to detect intermediate species such as in the process of anodic dissolution of Si in , and it is concluded that if these species exist at all, their lifetime is shorter than 0.3 ms.

Molecular and Supermolecular Origins of Enhanced Electronic Conductivity in Template-Synthesized Polyheterocyclic Fibrils. Part 1. Supermolecular Effects

Abstract: : The pores in a nanoporous membrane can be used as templates for the synthesis of nanostructures. We have recently shown that conductive polymer fibrils, obtained via this template synthetic method, can show dramatically higher electronic conductivities than conventional versions of the analogous polymers. In this and a succeeding paper we explore the molecular and supermolecular origins of this enhanced electronic conductivity. This paper focuses on supermolecular effects. We have used DC and optical measurements of conductivity, X-ray diffraction, and polarized infrared absorption spectroscopy to show that the polymer chains in the narrowest template-synthesized fibrils are preferentially oriented parallel to the axes of these fibrils. This preferential polymer chain orientation is partially responsible for the observed conductivity enhancements. We also show that template-synthesis can yield poly(3-methylthiophene) fibrils with conductivities as high as 6600 S cm-1. This is the highest conductivity ever reported for a heterocyclic polymer.

A molecular dynamics study of the far infrared spectrum of liquid water

Abstract: The far infrared spectrum of liquid water at room temperature is calculated by molecular dynamics simulation over the spectral range 0.5–1000 cm−1. It is shown that the experimental absorption intensity can be reproduced satisfactorily provided that the dipole induced dipole mechanism is conveniently implemented in the calculation and the classical profile corrected for quantum effects. The contribution due to exchange overlap dipoles between O and H atoms is also investigated but its role in the genesis of the far infrared (FIR) spectrum is negligible. Although the dipole induced dipole (DID) mechanism is found to be responsible for the peculiar band shape near 200 cm−1 by revealing the intermolecular oscillations of the hydrogen bond network, no other translational band is detected in the region 10–60 cm−1, a result in contradistinction with data put forward recently. Moreover, it is shown that the absorption spectrum is the seat of various cancellation effects between permanent and induced dipoles, eff...

Estimating cerebral blood flow in newborn infants: comparison of near infrared spectroscopy and 133Xe clearance.

Abstract: A new method of measuring cerebral blood flow (CBF) in newborn infants by means of near infrared spectroscopy (CBFnirs) was compared with the i.v. 133Xe clearance technique (CBFxe). Forty CBFnirs measurements were obtained during 19 133Xe measurements in 16 infants; 79 other CBFnirs data sets were discarded because the assumptions for their use were not fulfilled. The test-retest variation or repeated near infrared-measurements during each 133Xe clearance was 17.5%. CBFnirs was closely related to CBFxe (r2 = 0.84, p < 0.0001), with a slope of 0.75 (SEM = 0.064) and a intercept of 1.58 mL/100 g/min (SEM = 0.51). The difference between the measurements obtained by the two methods (CBFnirs – CBFxe) was negative in the high range of CBF, whereas the difference was close to zero in the low range. We conclude that CBF measured with near infrared spectroscopy was in good agreement with the CBF measured with the 133Xe method. The near infrared spectroscopy method has the advantage of being noninvasive, and it does not involve ionizing radiation. Because of methodologic constraints, however, it may underestimate CBF in the high range of flow, and it may have limitations of application in clinical research. (Pediatr Res 30: 570–573, 1991)