Depolarized Raman Spectra Research Articles

Polarizability relaxation in water/ethanol mixtures

Molecular dynamics simulations are used to investigate structural and dynamical properties of liquid mixtures of water and ethanol over the entire range of compositions from neat liquid water to neat liquid ethanol. Particular emphasis is given to the time relaxation of the anisotropic collective polarizability and the low-frequency part of the depolarized Raman spectra. While the dynamics was carried out using simulations with standard force fields for water and ethanol, the post-analysis was performed employing the chemical potential equalization (CPE) method. For comparison purposes, the CPE results are compared to those obtained with the traditional interaction-induced model. Both methods are able to capture the basic shape and position of the Raman bands. Polarizability response in time and frequency domains is disentangled in permanent, interaction-induced and induced-permanent (cross) terms. Our findings suggest that the polarizability relaxation of water is more sensitive to environmental fluctuations. Once water signal remains covered by the ethanol contribution even in water-rich concentrations, some water polarizability features were unveiled and correlated with H-bond dynamics. The permanent term, representative of the reorientational dynamics, captures the short-time oscillations and responds for a part of the total relaxation, while the long-range interaction-induced contribution is responsible for great part of the total polarizability relaxation.

The Raman jet spectrum of trans-formic acid and its deuterated isotopologs: Combining theory and experiment to extend the vibrational database.

Revisiting recently published Raman jet spectra of monomeric formic acid with accurate high order perturbative calculations based on two explicitly correlated coupled-cluster quality potential energy surfaces from the literature, we assign and add 11 new vibrational band centers to the trans-HCOOH database and 53 for its three deuterated isotopologs. Profiting from the synergy between accurate calculations and symmetry information from depolarized Raman spectra, we reassign eight literature IR bands up to 4000 cm-1. Experimental detection of highly excited torsional states (ν9) of trans-HCOOH, such as 4ν9 and ν6 + 2ν9, reveals substantial involvement of the C-O stretch ν6 into the O-H bend/torsion resonance ν5/2ν9, which is part of a larger resonance polyad. Depolarization and isotopic C-D substitution experiments further elucidate the nature of Raman peaks in the vicinity of the O-H stretching fundamental (ν1), which seem to be members of a large set of interacting states that can be identified and described with a polyad quantum number and that gain intensity via resonance mixing with ν1.

Non network-former cations in oxide glasses spotted by Raman scattering.

The depolarized Raman spectra can be used as a probe to reveal the presence of non-network formers in oxide glasses. Two spectral responses involving the cations are observed below 400 cm-1 in more than 30 compositions of binary and ternary aluminosilicates. One of the two bands arises solely from cations close to non-bridging oxygen providing thereby a simple test for qualifying the polymerization state of the glass. The second feature involves all cations whatever their role in the glass and is found to be twofold: one contribution arises from cations charge compensating (AlO4)- tetrahedra and the other one from network modifier cations. These results confirm the net vibrational contrast of cations depending on their structural surrounding.

Structure, the lattice dynamic, and the dielectric characteristics of Sr0.5Ba0.5Nb2O6 films

Solid solution Sr0.5Ba0.5Nb2O6 films have been synthesized on a (111)Pt/(001)Si substrate by rf deposition in an oxygen atmosphere. The depolarized Raman spectra, the structure, and the dielectric characteristics of the films have been studied over a wide temperature range. It is found that the films were singlephase, had the tetragonal tungsten bronze structure, and had a pronounced axial texture with axis 001 directed perpendicular to the substrate surface. It is shown that the film material undergoes a diffuse phase transition to the state of a relaxor ferroelectric in the temperature range 300–425 K. Possible reasons of the regularities observed are discussed.

Glass formation and Raman spectra of CaO–SiO2 glasses towards the orthosilicate limit

A series of silicate glasses formed in the binary system (1-X)CaO-XSiO2with silica mole fractions X ranging from 0.61to 0.38have been prepared using container-less aerodynamic levitation techniques and CO2-laser heating. Glasses with X<0.45were prepared for the first time but, no glass formation was possible at compositions X<0.38. Ambient temperature polarized and depolarized Raman spectra were measured for all these glasses. Qi-speciation analysis of the isotropic Raman spectra shows that near X∼0.38the predominant structures present are the SiO44− tetrahedra and the single bridged Q1species. Oxygen bridging was present at all compositions studied while at X<0.45 small amounts of free oxygen anions was present. The data are compared with the resent NMR measurements obtained with the same glass samples used in the present study. Stokes and anti-Stokes Raman spectra were measured in low frequencies revealing the Boson peak (BP) at ∼50 and ∼70cm−1 for the corresponding polarized and depolarized configurations. On the Stokes side the BP frequencies exhibit a fictional shift due to contributions from the low frequency vibrational modes of the glass.

Vibrational Properties of Strontium Barium Niobate Relaxor Single Crystals Studied by Raman Spectroscopy

Vibrational properties of relaxor ferroelectric strontium barium niobate (SrxBa1−xNb2O6, SBN-x) with x = 0.61 were investigated by high-resolution Raman scattering. The depolarized Raman spectra with the a(c,b)ā configuration, where a, b and c denote the crystallographic axes of the tetragonal system, were measured in a wide temperature range from 600°C to room temperature. Most optic modes were insensitive to or showed a change in the slope at the ferroelectric phase transition (Tc ∼73°C), while the two optic modes located at ∼200 and ∼280 cm−1 exhibited a noticeable softening in a specific temperature range from Tc to ∼150°C. This temperature range was the same to that where substantial acoustic anomalies were observed, which could be attributed to the formation of quasi-static precursor polar regions near Tc in the paraelectric phase.

Terahertz underdamped vibrational motion governs protein-ligand binding in solution.

Low-frequency collective vibrational modes in proteins have been proposed as being responsible for efficiently directing biochemical reactions and biological energy transport. However, evidence of the existence of delocalized vibrational modes is scarce and proof of their involvement in biological function absent. Here we apply extremely sensitive femtosecond optical Kerr-effect spectroscopy to study the depolarized Raman spectra of lysozyme and its complex with the inhibitor triacetylchitotriose in solution. Underdamped delocalized vibrational modes in the terahertz frequency domain are identified and shown to blue-shift and strengthen upon inhibitor binding. This demonstrates that the ligand-binding coordinate in proteins is underdamped and not simply solvent-controlled as previously assumed. The presence of such underdamped delocalized modes in proteins may have significant implications for the understanding of the efficiency of ligand binding and protein-molecule interactions, and has wider implications for biochemical reactivity and biological function.

Y3Al5O12–SiO2 Glasses: Structure and Polyamorphism

Aerodynamic levitation and CO2 laser melting have been used to synthesize the yttrium aluminosilicate glasses zY2O3–yAl2O3–xSiO2 with z/y = 3/5 corresponding to the YAG (Y3Al5O12) composition and x between ~5 and ~45 mol%. The low‐ and high‐density (LDA inclusion and HDA matrix) polyamorphic phases in glasses with less than ~14 mol% SiO2 were identified with backscattering electron imaging. Polarized and depolarized Raman spectra show the formation of various Qn SiO4 species whose relative populations change smoothly as the SiO2 content is altered. The AlOs (s = 4–6) and YOz (z = 6–9) polyhedra formed in the YAG glass are preserved upon silica additions while the terminal oxygens of the Q2AlO4 tetrahedra are gradually bridged to the Qn‐SiO4 species. The low‐frequency Boson Peak overlaps with the vibrational spectrum and its maximum is redshifted with increasing silica content. Micro‐Raman spectra measured for the LDA and HDA amorphous phases are found to be similar to the spectra of the bulk glass indicating common structural characteristics. The stability of the LDA phase against crystallization appears to be lower than that of the HDA phase. The crystallinity on certain inclusions consisted of YAG microcrystals and a new unidentified microcrystalline phase within Y4Al2(1−x)Si2xO(9+x) solid solution.

Volume properties and spectroscopy: A terahertz Raman investigation of hen egg white lysozyme

The low frequency depolarized Raman spectra of 100 mg/ml aqueous solutions of hen egg white lysozyme (HEWL) have been collected in the 25-85 °C range. Short and long exposures to high temperatures have been used to modulate the competition between the thermally induced reversible and irreversible denaturation processes. A peculiar temperature evolution of spectra is evidenced under prolonged exposure of the protein solution at temperatures higher than 65 °C. This result is connected to the self-assembling of polypeptide chains and testifies the sensitivity of the technique to the properties of both protein molecule and its surrounding. Solvent free spectra have been obtained after subtraction of elastic and solvent components and assigned to a genuine vibrational contribution of hydrated HEWL. A straight similarity is observed between the solvent-free THz Raman feature and the vibrational density of states as obtained by molecular dynamics simulations; according to this, we verify the relation between this spectroscopic observable and the effective protein volume, and distinguish the properties of this latter respect to those of the hydration shell in the pre-melting region.

Reaching the ionic limit in the (1 − X)[Ca0.5–Mg0.5]O–XSiO2 pseudo binary glass system with 0.5 < X < 0.27: Glass formation and structure

Container-less levitation techniques and CO2-laser heating were used to prepare a series of silicate glasses along the compositional join (1−X)(0.5Ca–0.5Mg)O–XSiO2 with X ranging between 0.50 and 0.27; thus reaching compositions beyond the orthosilicate limit (X=0.33). Polarized and depolarized Raman spectra measured at ambient conditions for all these glasses show systematic and smooth band intensity changes with composition. Qi-species analysis of the isotropic Raman spectra shows the presence a) of bridging oxygens in all glasses studied with silica mole fractions down to X=0.33 and b) of free oxygen at all compositions. Beyond the X=0.33 limit, the connectivity between the SiO4 tetrahedra decreases drastically and the corresponding glass structures are characterized by isolated negatively charged “tetrahedral” SiO44− and O2− anions with M2+ (M=Mg, Ca) as counter cations, held together by pure Coulombic (ionic) interactions. The Boson peak appears in the low-frequency range 70–90cm−1 and its polarization characteristics and frequency shifts with glass composition are pointed out and discussed.

Simultaneous acquisition of all four forms of circular polarization Raman optical activity: results for α‐pinene and lysozyme

AbstractA new circularly polarized (CP) Raman spectrometer is described that demonstrates simultaneous acquisition of all four forms of circular polarization Raman optical activity (ROA). The instrument is a design extension of a commercially available back scattering circular polarization (SCP) ROA spectrometer. Circular polarization of the incident beam is introduced with a quarter‐wave plate, and a half‐wave plate alternately positioned in and out of the beam controls the modulation between right circular polarization (RCP) or left circular polarization (LCP) states. Combining this modulation with the simultaneous detection of RCP and LCP scattered Raman radiation allows the measurement of incident circular polarization (ICP), SCP, in‐phase dual circular polarization(DCPI) and out‐of‐phase DCPII‐ROA. In addition, three different forms of backscattered Raman spectra, namely unpolarized, highly polarized, and depolarized Raman spectra, as well as a degree of circularity spectrum are obtained. The performance of the new all‐CP ROA spectrometer is evaluated with neat α‐pinene and aqueous hen lysozyme solution. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Solubility of CO2in 1-Butyl-3-methyl-imidazolium-trifluoro Acetate Ionic Liquid Studied by Raman Spectroscopy and DFT Investigations

The polarized and depolarized Raman spectra of 1-butyl-3-methyl-imidazolium-trifluoro acetate (Bmim TFA) ionic liquid and of the dense phase obtained after introduction of supercritical carbon dioxide (313K) under pressure (from 0.1 MPa up to 9 MPa) in the ionic liquid have been recorded. The spectrum of the pure ionic liquid has been assigned by comparison with the spectra of ionic liquids sharing the same cation and using literature data concerning the vibrational modes of the TFA anion. It was found that the spectra of the ionic liquid is almost unaffected by the CO(2) dilution. The only noticeable perturbation concerns a weak enhancement of the mode assigned here to the symmetric stretch vibration of the COO group of the TFA anion. The band shape analysis of the ν(CC) band in pure Bmim TFA shows that the carboxylate groups probe a variety of environments which are almost not affected by the dilution in carbon dioxide. The analysis of the Fermi dyad of carbon dioxide shows that this molecule is perturbed upon dilution in the ionic liquid. The spectra suggest the presence of carbon dioxide in two different environments. In the first one, carbon dioxide molecules interact with themselves, whereas in the second environment, this molecule interacts with the COO group of the TFA anion. This is supported by B3LYP-DFT calculations aimed at assessing the interaction between an ion pair dimer and a carbon dioxide molecule. It is shown that dissolved CO(2) molecules preferentially interact with the TFA anion through a weak charge transfer interaction taking place between the carbon atom of CO(2) (acting as a Lewis acid) and a oxygen atom of the COO group of TFA (as a Lewis base). The results show that Bmim TFA is able to accommodate a large amount of carbon dioxide without having its short-range local structure significantly perturbed. Most CO(2) is hosted in the voids existing among the ion pairs, while some also weakly interact with the anion. It is finally argued that the evolution of the local organization of the IL upon carbon dioxide dilution presents similarities with the microsegregation phenomena reported for IL upon increasing the alkyl chains lengths.

Molecular dynamics simulation of liquid methanol. II. Unified assignment of infrared, raman, and sum frequency generation vibrational spectra in methyl C–H stretching region

Vibrational spectra of methyl C-H stretching region are notoriously complicated, and thus a theoretical method of systematic assignment is strongly called for in condensed phase. Here we develop a unified analysis method of the vibrational spectra, such as infrared (IR), polarized and depolarized Raman, and ssp polarized sum frequency generation (SFG), by flexible and polarizable molecular dynamics simulation. The molecular model for methanol has been developed by charge response kernel model to allow for analyzing the methyl C-H stretching vibrations. The complicated spectral structure by the Fermi resonance has been unraveled by empirically shifting potential parameters, which provides clear information on the coupling mechanism. The analysis confirmed that for the IR, polarized Raman, and SFG spectra, two-band structure at about 2830 and 2950 cm(-1) results from the Fermi resonance splitting of the methyl C-H symmetric stretching and bending overtones. In the IR spectrum, the latter, higher-frequency band is overlapped with prominent asymmetric C-H stretching bands. In the depolarized Raman spectrum, the high frequency band at about 2980 cm(-1) is assigned to the asymmetric C-H stretching mode. In the SFG spectrum, the two bands of the splitted symmetric C-H stretching mode have negative amplitudes of imaginary nonlinear susceptibility χ(2), while the higher-frequency band is partly cancelled by positive imaginary components of asymmetric C-H stretching modes.

Far Wing Asymmetry of Rotational Raman Lines in Hydrogen

Depolarized Raman spectra of compressed hydrogen gas have been computed rigorously previously for 36 K and 50 K (Gustafsson et al. (2009)). The far wings of the rotational lines show asymmetry that goes beyond that expected from the theory for intracollisional interference and Fano line shapes. Here we analyze the (0) line for pure hydrogen at 36 K in detail. The added asymmetry stems partly from a shape resonance which adds significant intensity to the higher frequency side of the line profile. The influence of the threshold energy for the rotational transition accounts for the remainder.

Glass formation and structure in the MgSiO3–Mg2SiO4 pseudobinary system: From degraded networks to ioniclike glasses

A series of glasses xMgO-(1-x)SiO(2) with compositions from enstatite MgSiO(3) (x=0.5) to forsterite Mg(2)SiO(4) (x=0.667) in mole fraction intervals of x approximately 0.02 have been prepared by containerless levitation techniques and CO(2) laser heating. Polarized and depolarized Raman spectra measured at ambient conditions for all these glasses show systematic and smooth band intensity changes with composition. Analysis of the Raman band contours in terms of vibrations due to different oxygen bridged SiO(4) tetrahedra (Q(i), species analysis) undoubtedly shows that bridging oxygens are present in all glasses studied even in the limit of the forsterite composition where bridged Si(2)O(7) (6-) ionic dimers are formed. Furthermore the relative amounts of the Q(i) species change smoothly with composition while at high MgO content "free" oxygens are present presumably forming Mg-O-Mg bridges, which contribute to the glass stability at these compositions. Raman spectra measurements at different temperature below T(g) show small alterations in the Q(i) species in the MgSiO(3) region while no changes were observed in the Mg(2)SiO(4) region. The Boson peak frequency is practically invariant on both composition and temperature and this is in contrast to the systematics followed by most silicate glasses. It is suggested that at compositions near the forsterite ioniclike glasses are formed arising from a very fragile liquid.

Levitation / CO2 Laser Melting and Glass Formation in MgO Rich Mixtures with SiO2: Raman Spectra and Structure

Container-less levitation techniques and CO2 laser heating were used to melt SiO2-MgO mixtures and prepare a series of glasses with compositions from enstatite MgSiO3 to forsterite Mg2SiO4 in intervals of ~0.02 mol% SiO2. Polarized and depolarized Raman spectra measured at ambient conditions for all these glasses show systematic and smooth band intensity changes with composition. Analysis of the Raman band contours in terms of vibrations due to different oxygen bridged SiO4 tetrahedral (Qi, species analysis), undoubtedly shows that bridging oxygens are present in all glass studied even in the limit of the forsterite composition. The Boson peak frequency is invariant and the vibrational modes of the Qi species do not reflect any abrupt structural changes with composition. It is suggested that the forsterite forms an ionic-like glass arising from a very fragile liquid.

Roto-translational Raman spectra of pairs of hydrogen molecules from first principles

We calculate the collision-induced, roto-translational, polarized, and depolarized Raman spectra of pairs of H(2) molecules. The Schrodinger equation of H(2)-H(2) scattering in the presence of a weak radiation field is integrated in the close-coupled scheme. This permits the accounting for the anisotropy of the intermolecular potential energy surface and thereby it includes mixing of polarizability components. The static polarizability invariants, trace and anisotropy, of two interacting H(2) molecules were obtained elsewhere [Li et al., J. Chem. Phys. 126, 214302 (2007)] from first principles. Here we report the associated spherical tensor components which, along with the potential surface, are input in the calculation of the supramolecular Raman spectra. Special attention is paid to the interferences in the wings of the rotational S(0)(0) and S(0)(1) lines of the H(2) molecule. The calculated Raman pair spectra show reasonable consistency with existing measurements of the polarized and depolarized Raman spectra of pairs of H(2) molecules.

Raman spectroscopy and ab initio investigations of transient complex formation in CO2-benzene mixtures

The polarized and depolarized Raman spectra of CO(2) have been measured as a function of CO(2) concentration (0.02-0.7 molar fractions) in the dense phase of the binary mixtures obtained by introducing under pressure (from 0.2 up to 6.0 MPa) supercritical carbon dioxide (at 313 K) in liquid benzene. Four main experimental features are observed. A new weak polarized band centered at approximately 660 cm(-1) has been detected in the region of the Raman inactive nu(2) bending mode of carbon dioxide. The analysis of the polarized band shapes of the Fermi dyad shows that CO(2) molecules probe two environments. In one of them carbon dioxide interacts "specifically" with benzene molecules, whereas in the other it interacts "nonspecifically" with its neighbors. The analysis of the depolarized Fermi dyad profiles shows that the rotational dynamics of CO(2) specifically interacting with benzene is strongly hindered. Finally, a new weak polarized band has been detected between the two components of the dyad. These observations rationalized at the light of ab initio calculations show that CO(2)-benzene transient complexes are formed. It is argued that ab initio predictions, limited here to a pair of molecules, are still valid in dense phase because the elementary act of formation of the transient complex can be probed on the observation time and spatial range of vibrational Raman spectroscopy.

Transient Complex Formation in CO2−Hexafluorobenzene Mixtures: a Combined Raman and ab Initio Investigation

The polarized and depolarized Raman spectra of CO2 in binary mixtures with hexafluorobenzene have been measured in the dense phase along the isotherm 313 K as a function of concentration (0.03-0.7 molar fraction in CO2) by varying the pressure (0.5-6.2 MPa). Experimental observations in the nu2 bending region, in the nu1-2nu2 Fermi resonance dyad, and in the spectral domain between the Fermi dyad peaks on CO2 are reported. These results are discussed in comparison with those obtained in previous studies on CO2-C6H6 and CO2-acetone mixtures. We conclude in agreement with previous investigations that CO2 molecules can probe two environments. In one of them carbon dioxide interacts "specifically" with hexafluorobenzene molecules to form a transient heterodimer, whereas in the other environment CO2 interacts "nonspecifically" with its neighbors. New ab initio calculations reported here allow rationalizing most of the experimental results. However, the observation of weak spectral features (bending mode and Fermi dyad regions) shows that a slight departure from the predicted structure (C6v symmetry) should exist in the dense phase. Finally, the greater solubility of CO2 in perfluorinated benzene versus perhydrogenated benzene has been discussed on the basis of this study in connection with thermodynamic measurements interpreted in the scaled particle theory framework.

Raman Investigation of the CO2 Complex Formation in CO2−Acetone Mixtures

Polarized and depolarized Raman spectra of CO2-acetone mixtures have been measured along the isotherm 313 K as a function of CO2 concentration (0.1-0.9 molar fractions in CO2) by varying the pressure from 0.2 up to 8 MPa. Upon CO2 addition, a new band appears at about 655 cm(-1) and is assigned to the lower frequency nu 2(1) component of the bending mode after degeneracy removal due to the formation of a 1:1 electron donor acceptor (EDA) CO2 complex. The equilibrium constant associated with the complex formation was estimated and found close to those of contact charge transfer complexes. The main modifications of the Fermi dyad of CO2 in the mixtures compared with that of pure CO2 at equivalent density have been assessed. The band-shape analysis revealed that each dyad component is described by two Lorentzian profiles, showing that a tagged CO2 molecule probes two kinds of environment in its first shell of neighbors. The first one involves nonspecific interactions of CO2 with surrounding acetone whereas the second is assigned to the signature of 'transient' CO2 complexes formed with acetone. An upper bound life time of the complex has been estimated to be 8 ps. In addition, a broad band has been detected between the Fermi dyad peaks at about 1320 cm(-1) and its origin interpreted as a further evidence of the CO2-acetone heterodimer formation. Finally, the values of the equilibrium concentration of the heterodimer versus the total concentration of CO2 deduced from the analysis of the nu 2(1) band and from the Fermi dyad have been compared, and the difference is interpreted as due to a lack of theoretical approach of Fermi resonance transitions associated with species existing in different environments.