Showing papers on "Infrared spectroscopy published in 1990"

Solid C60: a new form of carbon

Abstract: A new form of pure, solid carbon has been synthesized consisting of a somewhat disordered hexagonal close packing of soccer-ball-shaped C60 molecules. Infrared spectra and X-ray diffraction studies of the molecular packing confirm that the molecules have the anticipated 'fullerene' structure. Mass spectroscopy shows that the C70 molecule is present at levels of a few per cent. The solid-state and molecular properties of C60 and its possible role in interstellar space can now be studied in detail.

Ideal hydrogen termination of the Si (111) surface

Abstract: Aqueous HF etching of silicon surfaces results in the removal of the surface oxide and leaves behind silicon surfaces terminated by atomic hydrogen. The effect of varying the solution pH on the surface structure is studied by measuring the SiH stretch vibrations with infrared absorption spectroscopy. Basic solutions ( pH=9–10) produce ideally terminated Si(111) surfaces with silicon monohydride ( 3/4 SiH) oriented normal to the surface. The surface is found to be very homogeneous with low defect density (<0.5%) and narrow vibrational linewidth (0.95 cm−1 ).

Protein secondary structures in water from second-derivative amide I infrared spectra.

Abstract: Infrared spectra have been obtained for 12 globular proteins in aqueous solution at 20 degrees C. The proteins studied, which vary widely in the relative amounts of different secondary structures present, include myoglobin, hemoglobin, immunoglobulin G, concanavalin A, lysozyme, cytochrome c, alpha-chymotrypsin, trypsin, ribonuclease A, alcohol dehydrogenase, beta 2-microglobulin, and human class I major histocompatibility complex antigen A2. Criteria for evaluating how successfully the spectra due to liquid and gaseous water are subtracted from the observed spectrum in the amide I region were developed. Comparisons of second-derivative amide I spectra with available crystal structure data provide both qualitative and quantitative support for assignments of infrared bands to secondary structures. Band frequency assignments assigned to alpha-helix, beta-sheet, unordered, and turn structures are highly consistent among all proteins and agree closely with predictions from theory. alpha-Helix and unordered structures can each be assigned to only one band whereas multiple bands are associated with beta-sheets and turns. These findings demonstrate a method of analysis of second-derivative amide I spectra whereby the frequencies of bands due to different secondary structures can be obtained. Furthermore, the band intensities obtained provide a useful method for estimating the relative amounts of different structures.

Two-Dimensional Infrared (2D IR) Spectroscopy: Theory and Applications

Abstract: A novel concept in vibrational spectroscopy called two-dimensional infrared (2D IR) spectroscopy is described. In 2D IR, a spectrum defined by two independent wavenumbers is generated by a cross-correlation analysis of dynamic fluctuations of IR signals induced by an external perturbation. 2D IR spectra are especially suited for elucidating various chemical interactions among functional groups. Notable features of the 2D IR approach are: simplification of complex spectra consisting of many overlapped peaks; enhancement of spectral resolution by spreading peaks over the second dimension; and establishment of unambiguous assignments through correlation analysis of bands selectively coupled by various interaction mechanisms. The procedure for generating 2D IR correlation spectra and the properties of the 2D spectra are discussed in detail. Examples of 2D IR spectra are presented for atactic polystyrene and the proteinacious component of human stratum corneum to demonstrate the utility of this technique.

Characterization of the soluble all-carbon molecules C60 and C70

Abstract: The authors report on the further physical and chemical characterization of the new forms of molecular carbon, C{sub 60} and C{sub 70}. Results demonstrate a high yield of production (14%) under optimized conditions and reveal only C{sub 60} and C{sub 70} in measurable quantity, in an 85:15 ratio. These two new molecular forms of carbon can be completely separated in analytical amounts by column chromatography on alumina. Comparison among mass spectra obtained by the electron impact, laser desorption, and fast atom bombardment (FAB) methods allows a clear assessment of the composition of the mixed and pure samples, and of the fragmentation and double ionization patterns of the molecules. In addition, spectroscopic analyses are reported for the crude mixture by {sup 13}C NMR and by IR spectroscopy in KBr pellet, and for pure C{sub 60} and C{sub 70} in solution by UV-vis spectroscopy.

Quantitative IR spectrophotometry of peptide compounds in water (H2O) solutions. I. Spectral parameters of amino acid residue absorption bands.

Abstract: Infrared spectra of the amino acid residues in H2O solution have been obtained in the 1800-1400-cm-1 region. It has been established that amino acid residues of arginine, asparagine, glutamine, aspartic and glutamic acids, lysine, tyrosine, histidine, and phenylalanine have intensive absorption in this spectral region. Infrared spectra for a set of model compounds have been measured. On the basis of these data, spectral parameters of amino acid residue absorption bands have been determined.

Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides

Abstract: Nature of oxide surface centres characterization of the state of surface cations on oxide catalysts by IR study of the absorbed probe molecules CO and NO complexation of alkenes on oxide surfaces application of infrared spectroscopy to mechanistic studies of heterogeneous catalytic reactions

Secondary structure and dosage of soluble and membrane proteins by attenuated total reflection Fourier-transform infrared spectroscopy on hydrated films.

Abstract: Attenuated total reflection Fourier-transform infrared spectroscopy of thin hydrated films of soluble and membrane protein included in a phospholipid bilayer is shown to provide useful information as to the secondary structure of the protein. The analysis of the amide I band of deuterated samples by Fourier self-deconvolution followed by a curve fitting was performed by a new procedure in which all the input parameters are generated by the computer rather than by the investigator. The results of this analysis provide a correct estimation of the alpha-helix and beta-sheet structure content with a standard deviation of 8.6% when X-ray structures are taken as a reference. We also show that the orientation of the different secondary structures resolved by the Fourier self-deconvolution/curve-fitting procedure and of the phospholipid acyl chains can be simultaneously evaluated for membrane proteins reconstituted in a lipid bilayer. Of special interest for reconstitution of membrane proteins, the lipid/protein ratio can be accurately and quickly determined from the infrared spectrum.

Structural investigation of silica gel films by infrared spectroscopy

Abstract: Fourier transform infrared absorption spectroscopy has been utilized to characterize the structure of porous silica gel films, both deposited on c‐Si substrates and free standing. The films were either dried at room temperature or subjected to partial densification at 400–450 °C. The spectra of the gel films are compared to those of thermal SiO2 grown on c‐Si and to Kramers–Kronig analysis of the reflection spectra of bulk SiO2 gels and v‐SiO2. The gel films show small frequency shifts compared to the latter spectra and they also exhibit new bands due to the presence of OH groups, although very little molecular water or residual organic species were found. The results are interpreted in terms of the gel structure. Compared to the thermal oxide, the sharp peak near 1070 cm−1 is narrower for the gels and the spread in intertetrahedral angles is estimated at 24° and 27° for room temperature dried and partially densified gels, respectively, compared to 33° for the thermal oxide. This is in agreement with a st...

Determination of the secondary structure content of proteins in aqueous solutions from their amide I and amide II infrared bands. Comparison between classical and partial least-squares methods.

Abstract: A method for estimating protein secondary structure from infrared spectra has been developed. The infrared spectra of H{sub 2}O solutions of 13 proteins of known crystal structure have been recorded and corrected for the spectral contribution of water in the amide I and II region by using the algorithm of Dousseau et al. This calibration set of proteins has been analyzed by using either a classical least-squares (CLS) method or the partial least-squares (PLS) method. The pure-structure spectra calculated by the classical least-squares method are in good agreement with spectra of poly(L-lysine) in the {alpha}-helix, {beta}-sheet, and undefined conformations. The results show that the best agreement between the secondary structure determined by X-ray crystal-lography and that predicted by infrared spectroscopy is obtained when both the amide I and II bands are used to generate the calibration set, when the PLS method is used, and when it is assumed that the secondary structure of proteins is composed of only four types of structure: ordered and disordered {alpha}-helices, {beta}-sheet, and undefined conformation. Attempts to include turns in the secondary structure estimation have led to a loss of accuracy. The spectra of the calibration proteins were also recorded in {sup 2}H{sub 2}O solution.more » After correction for the contribution of the combination band of {sup 2}H{sub 2}O in the amide I{prime} band region, the spectra were analyzed with PLS, but the results were not as good as for the spectra obtained in H{sub 2}O, especially for the {alpha}-helical conformation.« less

Studies of the temperature-dependent phase behavior of long chain n-alkyl thiol monolayers on gold

Abstract: The influences of temperature on the infrared spectra of a docosylthiol (C22) monolayer on a gold substrate are reported. The data suggest that this adsorbate exhibits a complex phase state at 300 K in which gauche conformations concentrated at the chain termina figure prominently. The detailed temperature dependence of the spectral band intensities is found to be very similar in many regards to that of bulk hydrocarbon crystals. At low temperature, band splittings are observed which suggest the lowest energy structure for this adsorbate is one containing two chains per unit cell. A simple lattice argument, consistent with steric models and low‐energy electron diffraction data, is presented to explain these observations.

Formation of rod-like zinc oxide microcrystals in homogeneous solutions

Abstract: The formation of zinc oxide microcrystals obtained from hydrolysis of zinc nitrate and zinc chloride in the presence of hexamethylenetetramine has been studied by X-ray diffraction, electron microscopy and IR spectroscopy. Prisms, needles and spherulitic aggregates are formed depending on several factors such as reactant concentrations, pH and temperature. It is shown that the formation of prisms and needles is mainly determined by kinetic conditions with prisms favoured at lower temperatures (< 80 °C), where faces can be developed from the solution complexes. The formation mechanism of the monodispersed prisms and needles occurs by coupling of microcrystalline spheres through their c axes forming embryonic rod particles which later grow. On the other hand, the formation of spherulitic aggregates is more the consequence of solution heterogeneity, i.e. high reactant concentrations.

Consistent porphyrin force field. 2. Nickel octaethylporphyrin skeletal and substituent mode assignments from nitrogen-15, meso-d4, and methylene-d16 Raman and infrared isotope shifts

Abstract: Resonance Raman spectra with variable-wavelength excitation are reported for nickel octaethylporphyrin and its isotopomers containing {sup 15}N, and {sup 2}H at the methine (meso-d{sub 4}) and methylene (methylene-d{sub 16}) carbon atoms. The {sup 15}N, meso-d{sub 4} double isotopomer is also examined. The infrared spectrum of the methylene-d{sub 16} isotopomer is reported, and the frequencies are combined with recently published infrared results for the other isotopomers. Essentially all of the porphyrin skeletal modes have been assigned and have been allocated to local coordinates which recognize the pyrrole rings as cooperative vibrational units. The assignments are supported by a normal-coordinate analysis with a valence force field involving standard ethyl force constants and porphyrin in-plane force constants which are transferred nearly intact from Ni porphine and Ni tetraphenylporphine. Many vibrational modes of the NiOEP ethyl substituents have also been located in the spectra and assigned. Bands assignable to ethyl C-C stretching and C-H bending modes are surprisingly strong in the resonance Raman spectra and suggest appreciable involvement of the ethyl groups in the porphyrin {pi}-{pi}{sup *} excited states. The conformations of the ethyl substituents have a marked influence on the low-frequency vibrational spectra.

Vibrational and Electronic Energy Levels of Polyatomic Transient Molecules. Supplement B

Abstract: A summary is presented of recently published, critically evaluated experimental vibrational and electronic energy level data for approximately 1700 neutral and ionic transient molecules and high temperature species possessing from three to sixteen atoms. Although the emphasis is on species with lifetimes too short for study using conventional sampling techniques, there has been selective extension of the compilation to include data for isolated molecules of inorganic species such as the heavy-metal oxides, which are important in a wide variety of industrial chemical systems. Radiative lifetimes and the principal rotational constants are included. Observations in the gas phase, in molecular beams, and in rare-gas and diatomic molecule matrices are evaluated, and several thousand references are cited. The types of measurement surveyed include conventional and laser-based absorption and emission techniques, laser absorption with mass analysis, and photoelectron spectroscopy.

Human colorectal cancers display abnormal Fourier-transform infrared spectra.

Abstract: Fourier-transform infrared spectroscopy (FT-IR) was applied to the study of tissue sections of human colorectal cancer. Pairs of tissue samples from colorectal cancer and histologically normal mucosa 5-10 cm away from the tumor were obtained from 11 patients who underwent partial colectomy. All cancer specimens displayed abnormal spectra compared with the corresponding normal tissues. These changes involved the phosphate and C-O stretching bands, the CH stretch region, and the pressure dependence of the CH2 bending and C = O stretching modes. Our findings indicate that in colonic malignant tissue, there are changes in the degree of hydrogen-bonding of (i) oxygen atoms of the backbone of nucleic acids (increased); (ii) OH groups of serine, tyrosine, and threonine residues (any or all of them) of cell proteins (decreased); and (iii) the C = O groups of the acyl chains of membrane lipids (increased). In addition, they indicate changes in the structure of proteins and membrane lipids (as judged by the changes in their ratio of methyl to methylene groups) and in the packing and the conformational structure of the methylene chains of membrane lipids. The cell(s) of the malignant colon tissues responsible for these spectral abnormalities is unknown. Cultured colon adenocarcinoma cell lines displayed similarly abnormal FT-IR spectra. The diagnostic potential of the observed changes is discussed.

Redox-linked conformational changes in proteins detected by a combination of infrared spectroscopy and protein electrochemistry. Evaluation of the technique with cytochrome c.

Abstract: We have developed a new technique for the study of redox-linked conformational changes in proteins, by the combination of two established techniques. Fourier-transform infrared spectroscopy has been used together with direct electrochemistry of the protein at a modified metal electrode surface. The technique has been evaluated with cytochrome c, because of its well-characterized electrochemistry and because the availability of X-ray crystallographic and NMR studies of both redox states of the protein provides a reference against which our data can be compared. In electrochemical control experiments, it was confirmed that the spectroelectrochemical cell design allows fast, accurate and reproducible control of the redox poise of the protein. The resulting reducedminus-oxidized infrared difference spectra show the changes in the frequencies and intensities of molecular vibrations which arise from the redox-linked conformational change. In contrast to the absolute infrared spectra of proteins, such difference spectra can be sufficiently straightforward to allow interpretation at the level of individual bonds. A complete interpretation of the spectra is beyond the scope of the present paper: however, on the basis of the data presented, we are able to suggest assignments for all except one of the major bands between 1500 cm−1 and 1800 cm−1.

Lattice dynamics and vibrational spectra of pristine and doped polypyrrole: effective conjugation coordinate

Abstract: A detailed vibrational analysis of the infrared and Raman spectra of doped and pristine polypyrrole is presented. The theory of the effective conjugation coordinate is applied and fully justifies the observed spectra. The information on the structure and properties of this material derived from the proposed analysis are presented and discussed.

Metal crystallinity effects in electrocatalysis as probed by real-time FTIR spectroscopy: electrooxidation of formic acid, methanol, and ethanol on ordered low-index platinum surfaces

Abstract: The electrooxidation kinetics of 0.05-0.25 M formic acid, methanol, and ethanol in 0.1 M HClO{sub 4} on ordered Pt(111), Pt(100), and Pt(110) surfaces were examined by means of FTIR spectra obtained during slow (2-10 mV s{sup {minus}1}) thin-layer potential sweeps. This procedure enables the potential-dependent CO coverage, {theta}{sub CO}, formed by dissociative reactant chemisorption to be followed in real time in relation to the appearance of CO{sub 2} and partial electrooxidation products as assayed quantitatively from their characteristic infrared bands. Terminally bonded (i.e., on-top) CO is the major form of this adsorbate detected under most conditions from the characteristic C-O stretching frequencies at ca. 2,030-2,060 cm{sup {minus}1}, although bridging CO was also observed in some cases. For formic acid electrooxidation, high CO coverages ({theta}{sub CO} {approx lt} 0.7) are formed on Pt(100) and Pt(110) that inhibit severely the reaction at low overpotentials during the positive-going sweep.

Alcohol Interaction with Zirconia Powders

Abstract: The mechanism by which alcohol washing of ceramic powders produces soft agglomerates has been investigated by studying the interaction of ethanol with hydrous ZrO{sub 2} powders using Fourier transform infrared (FTIR) spectroscopy. Unambiguous evidence of ethoxide formation has been found, which apparently prevents bond formation between adjacent particles and thus the formation of hard agglomerates.

Simultaneous monitoring of light-induced changes in protein side-group protonation, chromophore isomerization, and backbone motion of bacteriorhodopsin by time-resolved Fourier-transform infrared spectroscopy

Abstract: Absorbance changes in the infrared and visible spectral range were measured in parallel during the photocycle of light-adapted bacteriorhodopsin, which is accompanied by a vectorial proton transfer. A global fit analysis yielded the same rate constants for the chromophore reactions, for protonation changes of protein side groups, and for the backbone motion. From this result we conclude that all reactions in various parts of the protein are synchronized to each other and that no independent cycles exist for different parts. The carbonyl vibration of Asp-85, indicating its protonation, appears with the same rate constant as the Schiff base deprotonation. The carbonyl vibration of Asp-96 disappears, indicating most likely its deprotonation, with the same rate constant as for the Schiff base reprotonation. This result supports the proposed mechanism in which the protonated Schiff base, a deprotonated aspartic acid (Asp-85) on the proton-release pathway, and a protonated aspartic acid (Asp-96) on the proton-uptake pathway act as internal catalytic proton-binding sites.

The physical and infrared spectral properties of CO2 in astrophysical ice analogs.

Abstract: Results of measurements of the infrared spectroscopic and condensation-vaporization properties of CO2 in pure and mixed ices are presented. Detailed examination of five infrared CO2 bands, 2.20, 2.78, 4.27, 15.2, and 4.39 microns, shows that the peak position, FWHM, and profile of the bands provide important information about the composition, formation, and subsequent thermal history of the ices. Absorption coefficients and their temperature dependence for all five CO2 bands are determined. The temperature dependence variation is found to be less than 15 percent from 10 to 150 K, i.e., the temperature at which H2O ice sublimes. The number of parameters associated with the physical behavior of CO2 in CO2- and H2O-rich ices, including surface binding energies, and condensation and sublimation temperatures, are determined under experimental conditions. The implications of the data obtained for cometary models are considered.

Adsorption complexes of methanol on zeolite ZSM-5

Abstract: Infrared spectroscopy, 1H MAS NMR and thermogravimetric analysis have been used to study the chemistry of methanol adsorbed on zeolites H-ZSM-5, Na-ZSM-5 and K-ZSM-5. On H-ZSM-5, methanol forms the CH3OH+2 methoxonium ion at low coverages. At higher coverages, large protonated clusters of strongly hydrogen-bonded molecules associated with the Bronsted-acid sites are present. In zeolites Na-ZSM-5 and K-ZSM-5 methanol is coordinatively adsorbed on the cation and forms weakly hydrogen-bonded clusters at higher equilibrium pressures.

Characterization of silica gels by infrared reflection spectroscopy

Abstract: The FTIR specular reflection spectra of porous silica gels, either dried at room temperature or subjected to partial densification at 400°C, are significantly different from the spectrum of bulk vitreous silica. Namely, the high frequency SiOSi stretching vibrational peak near 1100 cm −1 exhibits shifts in the gels of up to 40 cm −1 towards lower frequencies, whereas the relative intensity of the companion high frequency shoulder simultaneously increases. The origin and possible structural meaning of the observed changes were investigated by Kramers-Kronig analysis of the experimental near-normal infrared reflectivity spectra of a series of silica xerogels, porous Vycor and bulk vitreous silica. Major differences were found for the energy loss function Im(−1/ϵ ∗ ), the peaks of which have been associated with the LO components of the dominant vibrational modes, and the largest red shifts corresponded to the highest frequency mode. Simple calculations show that the observed behavior of the infrared spectra could in part be due to a certain degree of strain in the SiOSi bridging bonds at the surface of the gel pores.

Quantitative IR spectrophotometry of peptide compounds in water (H2O) solutions. III. Estimation of the protein secondary structure.

Abstract: Infrared spectra of 13 globular proteins have been obtained in the 1800-1480-cm-1 region for H2O solutions. A method for estimating protein secondary structure from the ir spectrum has been developed. The method can also be used for estimating polypeptide and fibrous protein conformation. For the globular and fibrous proteins and polypeptides analyzed, the correlation coefficients between the ir and x-ray estimates of ordered helix, disordered helix, ordered beta-structure, disordered beta-structure, turns, and remainder were 0.98, 0.80, 0.99, 0.87, 0.90, and 0.92 respectively.

Comparative study of the surface hydroxyl groups of fumed and precipitated silicas. 2. Characterization by infrared spectroscopy of the interactions with water

Abstract: Les spectres infrarouges d'une silice pyrogenee et de silices preparees en phase liquide (aqueuse ou alcoolique) sont compares