Showing papers on "Raman spectroscopy published in 1975"

Structure and properties of oriented polymers

Abstract: Introduction Physicochemical Approaches to the Measurement of Molecular Anisotropy Structure and Morphology of Oriented Polymers Infrared Dichroism, Polarised Fluorescence and Raman Nuclear Magnetic Resonance Mechanical Anisotropy at Small Strains Anisotropic Creep Behaviour Anisotropic Yield Behavior Film Formation Liquid Crystalline Polymers Index

Interpretation of the doublet at 850 and 830 cm-1 in the Raman spectra of tyrosyl residues in proteins and certain model compounds.

Abstract: The doublet at 850 and 830 cm-1 in the Raman spectra of proteins containing tyrosyl residues has been examined as to its origin and the relation of its components to the environment of the phenyl ring, the state of the phenolic hydroxyl group, and the conformation of the amino acid backbone. Raman spectral studies on numerous model molecules related to tyrosine, including certain deuterium derivatives, show that the doublet is due to Fermi resonance between the ring-breathing vibration and the overtone of an out-of-plane ring-bending vibration of the para-substituted benzenes. Further examination of the effects of pH and solvents on the Fermi doublet and of the crystallographic data demonstrates that the intensity ratio of the two components depends on changes in the relative frequencies of the two vibrations. These in turn are found to be sensitive to the nature of the hydrogen bonding of the phenolic hydroxyl group of its ionization, but much less so to the environment of the phenyl ring and the conformation of the amino acid backbone. By use of the relative intensities of the doublet in model systems where the phenolic hydroxyl group is strongly hydrogen-bonded, weakly hydrogen-bonded, free or ionized, the reported Raman intensities of the doublets observed in the Raman spectra of several proteins have been interpreted. The results are compared with those obtained by other techniques.

Raman spectroscopic investigation of the structure of silicate glasses. I. The binary alkali silicates

Abstract: The Raman spectra of some binary alkali silicate glasses and crystals have been measured and the results interpreted. The glasses are of composition M2O⋅xSiO2, 1⩽x⩽4 and M = Li, Na, K. There is a strong resemblance between glassy and crystalline spectra which is consistent with a considerable amount of structural disorder of the glass. The reason is that the disorder acts as a perturbation on the spectrum of an elementary vibrating unit from which the glass is largely constructed. As a result, the general nature of the glass structure may be determined. Furthermore, glasses of the same x but with different alkali have different amounts of short‐range order. This is demonstrated by showing that the widths of certain Raman peaks increase with increasing disorder of the Si–O network of the glass. A strong argument is made against the microcrystallite theory of glass structure. For any composition x, glasses increase in disorder in the direction K → Na → Li. The crystallization rates increase in the same dire...

Raman microprobe and microscope with laser excitation

Abstract: New devices, have been developed to generate maps or images of heterogeneous samples using a characteristic Raman frequency line and to obtain the surface distribution of a given compound. Samples are illuminated by a laser beam causing Raman lines of the different substances to be emitted. Different techniques using a triple monochromator or a holographic grating system are described. Several examples of Raman images of heterogeneous samples are presented.

Raman scattering of collagen, gelatin, and elastin.

Abstract: The Raman spectra of collagen, gelatin, and elastin are presented. The Raman lines in the latter two spectra are assigned by deuterating the amide N-H groups in gelatin and by studying the superposition spectra of the constituent amino acids. Two lines appear at 1271 and 1248 cm−1 in the spectra of collagen and gelatin that can be assigned to the amide III mode. Possibly, the appearance of two amide III lines is related to the biphasic nature of the tropocollagen molecule, i.e., proline-rich (nonpolar) and proline-poor (polar) regions distributed along the chain. The melting, or collagen-to-gelatin transition, in water-soluble calf skin collagen is studied and the 1248-cm−1 amide III line is assigned to the 31 helical regions of the tropocollagen molecule. Elastin is thought to be mostly random and the Raman spectrum confirms this assertion. Strong amide I and III lines appear at 1668 and 1254 cm−1, respectively, and only weak scattering is observed at 938 cm−1. These features have been shown to be characteristic of the disordered conformation in proteins.

The structure of borosilicate glasses studied by Raman scattering

Abstract: Raman spectra of alkali and alkaline earth borosilicate glasses are reported. These spectra are used to discuss the molecular structure of the glasses. The influence of Al 2 O 3 additions on the structure of borosilicate glass is also discussed. It is shown that the same type of groups are present in borosilicate glasses as in borate and silicate glasses. The presence of large borate groups such as tetraborate and metaborate groups is strongly suggested by the Raman spectra. It appears that boron ions are hardly taken up in the silicon-oxygen network. Our results suggest that the region of phase separation is larger than the region presently acknowledged.

Physical properties and phase transitions in WO3

Abstract: A study of the phase transitions in WO3 crystals by Raman and absorption spectroscopy reveals that WO3 possesses two stable (or metastable) modifications at room temperature, differing slightly but definitely in their physical properties. Measurements of electrical conductivity show that both are insulators at low temperatures and semiconductors at higher temperatures. One is monoclinic and the other triclinic. Both exhibit at lower temperatures discontinuities in their physical properties without accompanying changes in cell parameters.

Infrared and Raman spectroscopy of carbohydrates. Part VI: Normal coordinate analysis of V-amylose

Abstract: A normal coordinate analysis of V-amylose has been performed for an isolated 61 helical chain. Negligible splitting from interactions of vibrations of successive residues is expected between A and E vibrational species due to the large size of the monomer unit. As a result, calculation of only the totally symmetric A modes represents an adequate approximation to the vibrational spectrum of helical polysaccharides. Using this method together with a valence force field we have obtained good agreement between the observed and calculated frequencies. In addition, the computed potential energy distribution and Cartesian displacement coordinates match previous experimental assignments, based on deuterium exchange. The analysis also supports the proposed mechanism for conversion of V-amylose to the more extended B-form. This conversion results in an observed frequency shift for the Raman line at 946 cm−1 which is predicted by the calculations.

Melting and premelting phenomenon in DNA by laser Raman scattering.

Abstract: Raman spectra of DNA from calf thymus DNA have been taken over a wide range of temperatures (25°–95°) in both D2O and H2O. A study of the temperature dependence of the Raman spectra shows that the temperature profiles of the intensities and frequencies of the various bands fall into four different categories: (1) base bands that show a reversible increase in intensity prior to the melting region, i.e., a definite premelting phenomenon; (2) base bands that show little or no temperature dependence; (3) deoxyribose-phosphate backbone vibrations that show no temperature dependence up to the melting region, at which point large decreases in intensity occur; and (4) slow frequency changes in certain in-plane vibrations of guanine and adenine due to deuteration of the C-8 hydrogen of these purines in D2O. Certain Raman bands arising from each of the four bases, adenine, thymine, guanine, and cytosine have been found to undergo a gradual increase in intensity prior to the melting region at which point large, abrupt increases in intensity occur. The carbonyl stretching band of thymine, involved in the interbase hydrogen bonding actually undergoes both a gradual shift to a lower frequency as well as an increase in intensity. These changes provide evidence that some change in the geometry of the bases relative to each other begins to occur around 50°C, well below the melting region of 70°–85°C. From the spectra taken at various temperatures, the DNA appears to remain in the B conformation until the melting point is reached, at which time the DNA progresses into a disordered random-coil form. No A-form conformation is found either in the premelting or the melting region.

Resonant Raman scattering in silicon

Abstract: The resonance of the Raman scattering from the zone-center optical phonon in silicon has been measured over a wide energy range covering the 3.4-eV direct band gap. The experimental results are compared with both a theory deriving the Raman cross section from the optical constants and an ab initio calculation; good agreement is found in the region where the respective theories are expected to be reliable. The second-order Raman spectrum of silicon has also been measured at laser frequencies between 1.65 and 3.72 eV. Above the 3.4-eV gap, we observe a strong peak in the second-order spectrum corresponding to scattering from two optical phonons near the $\ensuremath{\Gamma}$ point. From the resonance behavior of the second-order scattering, several electron-two-phonon deformation potentials are determined.

Infrared and Raman spectroscopy of carbohydrates. Paper V. Normal coordinate analysis of cellulose I

Abstract: A normal coordinate analysis of cellulose I has been performed for an isolated chain using a valence force field and atomic coordinates from an x−ray refinement analysis. The calculated frequencies show good agreement with the observed infrared and Raman data, while the computed potential energy distribution is compatible with previous assignments, based on deuteration etc. The atomic displacements show that all the predicted modes in the region below 1500 cm−1 result from complex motions of a number of atoms or groups, rather than from single group vibrations. Most of the modes above 1200 cm−1 are localized within the residue and are very modes above 1200 cm−1 are localized within the residue and are very modes above 1200 cm−1 are localized within the residue and are very similar to those predicted for the monomer, β−D−glucose. Below 1200 cm−1 inter−residue coupling becomes more appreciable. Negligible splitting is predicted between equivalent A and B modes which suggests that the observed spectral lines...

Molecular geometry in an excited electronic state and a preresonance Raman effect.

Abstract: Observations of Raman spectra of various molecules at different exciting laser wavelengths lead to an empirical rule. If a Raman line becomes stronger when the exciting frequency is brought closer to the frequency of an electronic band, this means that the equilibrium conformation of the molecule is distorted along the normal coordinate for the Raman line in the transition from the ground to the excited electronic state.

The Analysis of Discrete Fine Particles by Raman Spectroscopy

Abstract: A conventional laser Raman spectrometer has been modified and used to obtain useful Raman spectra from discrete solid particles as small as 0.7 μm in linear dimensions. Spectra obtained from single, micrometer-sized particles of several inorganic and organic compounds are reported. Simplified calculations are discussed which provide an estimate of detectability levels and other problems associated with these measurements. Certain parameters that must be considered in the design of an instrument especially intended for use in the chemical characterization of single fine particles are reviewed in the light of this work.

Dynamics of molecular reorientational motion and vibrational relaxation in liquids. Chloroform

Abstract: Vibrational and rotational (dipole and second−order tensor) correlation functions were obtained by Fourier inversion of infrared and Raman vibrational band contours of the three ∥ and one ⊥ fundamental of liquid CHCl3, CDCl3, and isotopically pure CH35Cl3. All correlation functions are nonexponential at short times and approximately exponential for long times. The symmetry axis of the molecule reorients by ’’free’’ jumps of about 1/3 rad, turning through a root−mean−square angle of 1 radian within 2psec by about 13 orientational jumps. Computer simulations show that J diffusion is too fast beyond 1 psec and that M diffusion fits the data up to 4 psec (τJ = 0.12 psec); thereafter, M diffusion is too slow. The Raman rotational correlation time is approximately equal to the NMR quadrupolar correlation time; the infrared rotational correlation time is only 0.75 of a corresponding dielectric relaxation time. Vibrational relaxation in the symmetric near−infrared carbon−hydrogen stretch is of the same order of i...

Resonance Raman spectroelectrochemistry. I. Tetracyanoethylene anion radical

Abstract: A new spectroelectrochemical technique based on the resonance Raman effect is described whereby high resolution vibrational spectra can be obtained for electrogenerated intermediates and products. Both controlled potential coulometric electrolysis in bulk solution and cyclic potential step electrolysis under semiinfinite diffusion mass transport conditions have been tested as electrogeneration modes. Resonance Raman spectra of the tetracyanoethylene anion radical (TCNE.-), which exhibits perfectly reversible electrogeneration behavior, have been obtained in both of these electrogeneration modes with 4880, 4765, and 4579 A Ar+ laser excitation. In addition resonance Raman intensity (RRI) vs. time transients produced by single-shot, double-potential steps at a microelectrode have been obtained for the 2194 cm-I q(Ag) CEN and 1421 cm-I u2(Ag) C=C stretching modes. The Raman transients correlated exactly in time with the corresponding chronoamperometric and transmission chronoabsorptometric signals indicating that the Raman transients and spectra obtained under cyclic potential step conditions result from TCNEmin the electrochemical diffusion layer. The elucidation of electrochemical reaction pathways invariably requires the detection, identification, and kinetic monitoring of intermediates and/or products generated either on the electrode surface or in the adjacent solution. It has been demonstrated that spectroscopic monitoring of electrogenerated species (spectroelectrochemistry) is extremely useful for purposes of identification and structural characterization as well as providing superior molecular specificity for detection and kinetic monitoring relative to the usual electrochemical parameters current, potential, time, etc.' Restricting discussion to electrogenerated species in solution, the spectroscopic techniques which have made the greatest impact on electrochemistry to date are electron spin resonance2 (ESR) and electronic absorption spectroscopy' (uv-visible). ESR is routinely used for identification of paramagnetic species in bulk solution produced by controlled potential electrolysis and more recently has been applied to quantitative kinetic studies of electrogenerated radical ions in the diffusion layer following current step elect r ~ l y s i s . ~ ~ Electronic absorption spectroscopy has proven extremely versatile in that i t can be interfaced to electrochemistry in transmission and internal reflection modes at optically transparent electrodes (OTE's) as well as in the specular reflection mode at bulk metal electrodes. In addition uv-visible spectroscopy is readily applied to the most common mass transport conditions including bulk electrolysis,6 finite diffusion (thin layer electrochemi~try),~ semiinfinite diffusion,' and hydrodynamics.8 The utility of electronic absorption spectroscopy for purposes of species identification is unfortunately limited due to the relatively low information content of uv-visible spectra in fluid solution. However, one finds that because of the high sensitivity of uv-visible spectrophotometric measurements and the large values of electronic absorption extinction coefficients, electronic absorption spectroscopy is most useful for following the kinetics of electrochemical transformations over a wide dynamic range of time from hours to microseconds. Several attempts have been made to couple vibrational spectroscopy to e l ec t r~chemis t ry .~ -~ '~ The motivation for this is the greater degree of molecular specificity and sensitivity to subtle environmental factors inherent in vibrational spectroscopy relative to electronic absorption spectroscopy. The attempts made to use infrared spectroscopy (ir) to study electrogenerated species in solution have met with only limited success because of the following experimental limitations: (1) small free spectral window caused by competitive absorption from the solvent/supporting electrolyte system as well as the electroactive substrate; (2) relatively low spectrophotometric sensitivity due to the small values of infrared extinction coefficients; and (3) rapid time response is limited as a result of the high electrical resistance of infrared transparent electrode materials. Recently it has been suggested that Raman spectroscopy might be more readily applied to electrochemical systems than ir from the viewpoint of free spectral window, ease of cell construction since borosilicate glass will work as a window material, and ease of application to aqueous solutions since water is a poor Raman scatterer.' 5*16 The problem with this approach is the lack of sensitivity inherent in the normal spontaneous Raman scattering process. Typically samples must be 0.1 1.0 M in order to get Raman spectra with acceptable signal-to-noise (SN) ratios even for strong Raman scatterers. This lack of sensitivity would appear to preclude the use of ordinary Raman spectroscopy to observe spectra or kinetic transients in solution at the concentration levels of electroactive substrate normally used in electrochemical experiments. Fleischmann and coworkersI7 have, nevertheless, demonstrated that Raman spectroscopy will be very useful for observing strong Raman scatterers adsorbed on high surface area electrodes as a function of applied electrode potential. In this paper we wish to demonstrate that vibrational spectra of electrogenerated species in bulk solution or in the electrochemical diffusion layer can be readily obtained from solutions originally ca, 1 X 10-3 hf in electroactive substrate by utilizing the resonance Raman effect.Is The primary characteristic of resonance Raman spectroscopy that we capitalize on for application to electrochemistry is that laser excitation within an electronic absorption band results in enhancements of the Raman scattering cross section by factors as large as 105-106 for certain vibrational modes of the electrogenerated species. In addition we will illustrate various advantageous features of resonance Raman spectroelectrochemistry (RRSE) such as: ( I ) use of low resistance bulk metal electrodes for in situ studies in the electrochemical diffusion layer; (2) cell design criteria are relatively unrestrictive from both the Raman spectroscopic and electrochemical points of view; (3) there is essentially no spectral interference from the solvent, supporting electrolyte, or electroactive substrate (unless it is colored); (4) the only requirements for observation of an electrogenerated species by RRSE are that it have a lifetime compatible with its mode of electrogeneration and have an extinction coefficient greater than ca. 1000 M-I cm-' at one (or Van Duyne et al. / Resonance Raman Spectroelectrochemistry

Characterization of the A ⇄ B transition of DNA in fibers and gels by laser Raman spectroscopy

Abstract: Both Raman spectra and X-ray diffraction patterns have been obtained from oriented fibers of sodium deoxyribonucleic acid (Na-DNA) as a function of salt content and relative humidity. We have confirmed the previously reported X-ray results that, for oriented fibers, the A-form always exists between 75 and 92% relative humidity and that the conformation will change to the B-form at 92% relative humidity only if an excess (3–5%) of added salt is present. Oriented fibers containing low amounts of added salt remain in the A-type conformation at 92% relative humidity and higher. An exact correlation has been found between the familiar A- and B-type X-ray diffraction patterns of DNA fibers and the Raman spectra previously reported without X-ray verification from this laboratory for the A- and B-forms. In particular, a band at 807 cm−1 was always present when a fiber showed the A-type diffraction pattern, and this band shifts to 790 cm−1 in the B-form. Using the Raman spectrum to determine the specific conformation of DNA in samples less amenable to X-ray analysis, we have studied the A ⇄ Btransformation in unoriented fibrous masses of DNA and in concentrated, oriented gels. We find that in unoriented fibrous masses, the A ⇄ B transition always occurs at 92% relative humidity even at very low salt concentration (0–4%). However, in oriented DNA gels at low salt, the A-form can persist as a metastable state to concentration as low as 20% DNA. The origin of the bands at 807 and 790 cm−1 and the possible biological implications of these findings are discussed.