scispace - formally typeset
Search or ask a question

Showing papers on "Infrared spectroscopy published in 1990"


Journal ArticleDOI
27 Sep 1990-Nature
TL;DR: In this article, a new form of pure, solid carbon has been synthesized consisting of a somewhat disordered hexagonal close packing of soccer-ball-shaped C60 molecules.
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.

6,650 citations


Journal ArticleDOI
TL;DR: In this article, the effect of varying the solution pH on the surface structure was studied by measuring the SiH stretch vibrations with infrared absorption spectroscopy, and the surface was found to be very homogeneous with low defect density (<0.5%) and narrow vibrational linewidth.
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 ).

1,250 citations


Journal ArticleDOI
TL;DR: A method of analysis of second-derivative amide I spectra whereby the frequencies of bands due to different secondary structures can be obtained is demonstrated and the band intensities obtained provide a useful method for estimating the relative amounts of different structures.
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.

1,220 citations


Journal ArticleDOI
Isao Noda1
TL;DR: In this paper, a two-dimensional infrared (2D IR) spectroscopy was proposed for elucidating various chemical interactions among functional groups, which simplifies complex spectra consisting of many overlapped peaks and enhances spectral resolution by spreading peaks over the second dimension.
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.

771 citations


Journal ArticleDOI
TL;DR: In this paper, the physical and chemical characterization of two new molecular forms of carbon, C{sub 60} and C {sub 70} were reported. But the results demonstrate a high yield of production (14%) under optimized conditions and reveal only C{ sub 60 and Csub 70 in measurable quantity, in an 85:15 ratio.
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.

727 citations


Journal ArticleDOI
TL;DR: 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.
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.

561 citations


Book
01 Sep 1990
TL;DR: In infrared spectroscopy, the state of surface cations on oxide catalysts was characterized by IR study of the absorbed probe molecules CO and NO complexation of alkenes on oxide surfaces as mentioned in this paper.
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

532 citations


Journal ArticleDOI
TL;DR: 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.
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.

500 citations


Journal ArticleDOI
TL;DR: In this article, Fourier transform infrared absorption spectroscopy has been used to characterize the structure of porous silica gel films, both deposited on c-Si substrates and free standing.
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...

410 citations


Journal ArticleDOI
TL;DR: The results show that the best agreement between the secondary structure determined by X-ray crystallography and that predicted by infrared spectroscopy is obtained when both the amide I and II bands are used to generate the calibration set.
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

384 citations


Journal ArticleDOI
TL;DR: In this paper, the resonance Raman spectra with variable-wavelength excitation were 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.
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.

Journal ArticleDOI
TL;DR: Fourier-transform infrared spectroscopy has been used together with direct electrochemistry of the protein at a modified metal electrode surface and the resulting reduced-minus-oxidized infrared difference spectra show the changes in the frequencies and intensities of molecular vibrations which arise from the redox-linked conformational change.
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.

Journal ArticleDOI
TL;DR: In this article, a detailed vibrational analysis of the infrared and Raman spectra of doped and pristine polypyrrole is presented and the theory of the effective conjugation coordinate is applied and fully justifies the observed spectra.
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.

Journal ArticleDOI
TL;DR: In this paper, the mechanism by which alcohol washing of ceramic powders produces soft agglomerates was investigated by studying the interaction of ethanol with hydrous ZrO{sub 2} powders using Fourier transform infrared (FTIR) spectroscopy.
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.

Journal ArticleDOI
TL;DR: It is concluded that all reactions in various parts of the protein are synchronized to each other and that no independent cycles exist for different parts.
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.

Journal ArticleDOI
TL;DR: In this article, the adsorption and coadsorption of NO, NO 2 and NH 3 on TiO 2 anatase has been studied by Fourier transform infrared (FT-IR) spectroscopy.

Journal ArticleDOI
TL;DR: The results suggest that two different forms of H2O lattice are produced in interstellar ices, and the implications of these data for cometary models and the understanding of cometary formation are considered.
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.

Journal ArticleDOI
TL;DR: In this paper, five samples of alumina (one amorphous and four γ-aluminas prepared from different boehmites) were compared using physical methods (X-ray diffraction, transmission electron microscopy, 27 Al NMR and Fourier transform infrared spectrometry) and the butene isomerization test.

Journal ArticleDOI
TL;DR: In this paper, the chemistry of methanol adsorbed on zeolites H-ZSM-5, Na-ZMM-5 and K-ZMSA-5 was studied.
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.

Journal ArticleDOI
TL;DR: In this paper, Kramers-Kronig analysis of the experimental near-normal infrared reflectivity spectra of a series of silica xerogels, porous Vycor and bulk vitreous silica.
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.

Journal ArticleDOI
TL;DR: In this paper, the first spectroscopic evidence that bisulfate anion adsorption is associated with the anomalous peaks seen in the cyclic voltammetry of Pt(111) in sulfuric acid was presented.

Journal ArticleDOI
TL;DR: Infrared spectra of 13 globular proteins have been obtained in the 1800–1480‐cm−1 region for H2O solutions and a method for estimating protein secondary structure from the ir spectrum has been developed.
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.

Journal ArticleDOI
01 Aug 1990-Langmuir
TL;DR: Les spectres infrarouges d'une silice pyrogenee et de silices preparees en phase liquide (aqueuse ou alcoolique) sont compares as discussed by the authors.
Abstract: Les spectres infrarouges d'une silice pyrogenee et de silices preparees en phase liquide (aqueuse ou alcoolique) sont compares

Journal ArticleDOI
TL;DR: Fourier transform infrared (FTIR) spectroscopic studies are carried out on different, potentiometric well defined oxidation states of polyaniline in aqueous acidic and organic electrolytes.
Abstract: Fourier transform infrared (FTIR) spectroscopic studies are carried out on different, potentiometric well defined oxidation states of polyaniline in aqueous acidic and organic electrolytes. During the oxidation process ring structures transform from benzenoid into quinoid states. The fully reduced state of polyaniline shows differences in the anion contents in acidic and organic electrolytes. The 400 mV vs saturated calomel electrode (SCE) oxidized state has the maximum number of the intercalated anions in aqueous acidic media in accordance with supporting potentiometric titration experiments. This conducting form of polyaniline shows similar FTIR spectra in organic as well as in acidic media. For the oxidized state at 800 mV vs SCE, a deintercalation of anions in aqueous acidic, or further intercalation in organic electrolyte is observed. Beyond 800 mV vs SCE, polyaniline shows degradation processes in aqueous acidic media which are found to proceed via formation of benzoquinone‐like structures and finally result in a complete dissolution of the polymer.

Journal ArticleDOI
TL;DR: In this article, the types of complexes that salicylate (2-hydroxy-benzoate) forms with the surface of goethite (α-FeOOH) in aqueous medium were studied in situ by using cylindrical internal reflection (CIR) Fourier transform infrared (FTIR) spectroscopy.
Abstract: The types of complexes that salicylate (2-hydroxy-benzoate) forms with the surface of goethite ({alpha}-FeOOH) in aqueous medium were studied in situ by using cylindrical internal reflection (CIR) Fourier transform infrared (FTIR) spectroscopy. Results obtained from CIR-FTIR studies were compared with adsorption isotherm experiments in order to relate the level of salicylate coverage to the nature of the surface complex. At lower surface coverages all the interfacial salicylate has a chelate structure in which one carboxylic oxygen and the ortho phenolic oxygen bind one Fe atom of the goethite surface. At higher surface coverages this chelate complex coexists with salicylate ions, which are weakly bound in the double layer.

Journal ArticleDOI
TL;DR: Raman spectra were acquired on ox femur samples treated with hydrazine to remove the organic components of bone and showed a large increase in the signal-noise ratio of the mineral spectrum, the first time that this high resolution technique has been applied to biological minerals.

Journal ArticleDOI
01 May 1990-Langmuir
TL;DR: La spectrometrie IR a transformation de Fourier is utilisee in situ for etudier la structure des complexes superficiels formes apres adsorption de benzoate et de phenolate sur des particules de α-FeOOH en suspension dans l'eau as discussed by the authors.
Abstract: La spectrometrie IR a transformation de Fourier est utilisee in situ pour etudier la structure des complexes superficiels formes apres adsorption de benzoate et de phenolate sur des particules de α-FeOOH en suspension dans l'eau

Journal ArticleDOI
TL;DR: The role of vanadium as minority carrier lifetime killer in SiC-based optoelectronic devices is suggested from these data as discussed by the authors, which suggests that vanadium acts as a deep acceptor, V4+Si(3d1)/V3+Si (3d2)−A0/A−, and possibly also as a donor.
Abstract: Trace impurities of vanadium in Lely‐grown silicon carbide single crystals have been detected by their strong, polytype‐specific photoluminescence in the 1.3–1.5 μm near‐infrared spectral range, as well as by infrared absorption. The spectra arise from the intra‐3d‐shell transitions 2E(3d1)→2T2(3d1) of V4+Si(3d1). Electron spin resonance reveals that VSi in SiC acts as a deep acceptor, V4+Si(3d1)/V3+Si(3d2)−A0/A−, and possibly also as a deep donor. The role of vanadium as minority‐carrier lifetime killer in SiC‐based optoelectronic devices is suggested from these data.

Journal ArticleDOI
TL;DR: In this article, the formation of coordinatively unsaturated (cus) Zr{sup 3+} centers at the surface of monoclinic ZrO{sub 2}.
Abstract: CO adsorption at room temperature, followed by means of in situ FTIR spectroscopy, and EPR spectroscopy have been used to demonstrate the formation, by vacuum thermal activation, of coordinatively unsaturated (cus) Zr{sup 3+} centers at the surface of monoclinic ZrO{sub 2}. The same techniques have also been adopted for a preliminary characterization of the reactivity of (cus) Zr{sup 3+} centers: they turn out to be highly resistant to oxidation by molecular oxygen, whereas they interact with water and with carbon dioxide, yielding more easily coordinatively saturated configurations, still reduced, rather than undergoing oxidation to the 4+ state.

Book
01 Jan 1990
TL;DR: The laboratory use of computers laboratory balances organic elemental analysis continuous flow analysis atomic emission spectroscopy atomic absorption and flame emission spectrometry ultraviolet, visible and near-infrared spectrophotometries molecular fluorescence and phosphorescence rotational and vibrational spectroscopic x-ray methods photoacoustic and acoustic X-ray techniques of chiroptical Spectroscopy nuclear magnetic resonance electon paramagnetic resonance xray as mentioned in this paper.
Abstract: The laboratory use of computers laboratory balances organic elemental analysis continuous-flow analysis atomic emission spectroscopy atomic absorption and flame emission spectrometry ultraviolet, visible and near-infrared spectrophotometries molecular fluorescence and phosphorescence rotational and vibrational spectroscopy x-ray methods photoacoustic spectroscopy techniques of chiroptical spectroscopy nuclear magnetic resonance electon paramagnetic resonance x-ray