Raman band shapes and the dynamics of liquid N2O4

The Raman spectra of the totally symmetric A _g modes, v ₁, v ₂ and v ₃, of the N₂O₄ molecule have been measured in the liquid state at 262, 279 and 297 K. The vibrational and the rotational correlation functions are calculated. The long-time exponential decay of the rotational correlation functions of all the A _g modes reflects an asymptotic diffusional behaviour of molecular reorientation. The rotational relaxation rate is found to increase with increasing temperature. A marked point of inflection from the short time inertial correlation to the long time exponential decay appears at about 0·35 ps for the v ₂ mode. This is an indication of orientational rebound arising from the librational motion in a temporary solvent cage. The isotropic bandwidth increases in the order v ₁ < v ₂ < v ₃, which is also the order of decreasing vibrational frequency. The temperature dependence of the peak frequency and of the bandwidth are also found to increase in the same order. These observations are analysed qualitatively in terms of two models of vibrational dephasing which take into account the effect of vibrational anharmonicity.     

Vibrational Spectroscopic Characterization of the Sulphate-Carbonate Mineral Burkeite: Implications for Evaporites

Burkeite formation is important in saline evaporites and in pipe scales. Burkeite is an anhydrous sulphate-carbonate with an apparent variable anion ratio. Such a formula with two oxyanions lends itself to vibrational spectroscopy. Two symmetric sulphate stretching modes are observed, indicating at least at the molecular level the nonequivalence of the sulphate ions in the burkeite structure. The strong Raman band at 1065 cm^?1 is assigned to the carbonate symmetric stretching vibration. The series of Raman bands at 622, 635, 645, and 704 cm^?1 are assigned to the ν₄ sulphate bending modes. The observation of multiple bands supports the concept of a reduction in symmetry of the sulphate anion from T _d to C _3v or even C _2v.     

Investigation of hydrogen bonding in neat dimethyl sulfoxide and binary mixture (dimethyl sulfoxide + water) by concentration-dependent Raman study and ab initio calculation

In this study,our vibrational spectroscopic analysis is made on hydrogen-bonding between dimethyl sulfoxide and water comprises both experimental Raman spectra and ab initio calculations on structures of various dimethyl sulfoxide/water clusters with increasing water content.The Raman peak position of the v(S=O) stretching mode of dimethyl sulfoxide serves as a probe for monitoring the degree of hydrogen-bonding between dimethyl sulfoxide and water.In addition,the two vibrational modes,namely,the CH 3 symmetric stretching mode and the CH 3 asymmetric stretching mode have been analysed under different concentrations.We relate the computational results to the experimental vibrational wavenumber trends that are observed in our concentration-dependent Raman study.The combination of experimental Raman data with ab initio calculation leads to a better knowledge of the nature of the hydrogen bonding and the structures of the hydrogen-bonded complexes studied.     

Fine band structure of the vibrational spectra of fullerite C<Subscript>60</Subscript> and enhancement of intermolecular interaction in high-temperature phase

The fine structure of the fundamental vibrational bands and some combination tones of fullerite C₆₀ in its IR absorption and reflection spectra, as well as in Raman spectra, has been studied. This structure is due to the overlapping components of Davydov and isotopic splittings and the removal of vibrational degeneracy with symmetry lowering. It is shown that for IR F _u (i) bands (i = 1–4) and low-frequency H _g (1) and A _g (1) bands in the Raman spectrum the splittings at room temperature exceed those for the low-temperature phase. The enhancement of intermolecular interaction at elevated temperatures is explained by the nonequilibrium vibrational excitation of the medium as a result of nonlinear interaction of vibrational modes and by the change in the electronic states.     

XRD studies,vibrational spectra,and molecular structure of 1H‐imidazo [4,5‐b]pyridine based on DFT quantum chemical calculations

The molecular structures and vibrational properties of 1H‐imidazo[4,5‐b]pyridine in its monomeric and dimeric forms are analyzed and compared to the experimental results derived from the X‐ray diffraction (XRD), infrared (IR), and Raman studies. The theoretical data are discussed on the basis of density functional theory (DFT) quantum chemical calculations using Lee–Yang–Parr correlation functional (B3LYP) and 6‐31G(d,p) basis. This compound crystallizes in orthorhombic structure, space group Pna2₁(C_2v⁹) and Z = 4. The planar conformation of the skeleton and presence of the N H···N hydrogen bond was found to be characteristic for the studied system. The temperature dependence of IR and Raman modes was studied in the range 4–294 K and 8–295 K, respectively. The normal modes, which are unique for the imidazopyridine derivatives are identified. Copyright © 2009 John Wiley & Sons, Ltd.     

Resonance Raman investigation of the short-time dynamics of the ultraviolet photodissociation of bromoform

We report ultraviolet resonance Raman spectra of bromoform (CHBr₃) in cyclohexane solution. The resonance Raman spectra show significant intensity in the overtones of the nominal Br-C-Br symmetric bend (v ₆), the nominal H-C-Br asymmetric bend (v3), the nominal Br-C-Br symmetric stretch (v ₂) and the nominal Br-C-Br asymmetric stretch (v ₅) vibrational modes suggesting that the short-time photodissociation dynamics have noticeable multidimensional character. The lack of strong combination bands between several of the Franck-Condon active modes suggests that more than one electronic transition contribute to the resonance Raman spectra. We briefly discuss the ultraviolet short-time photodissociation dynamics of bromoform and the potential implications for the secondary photodissociation reactions of the initially formed CHBr₂ radical.     

Vibrational spectra of 2, 3 and 2, 6 dichloro anilines

The infrared and laser Raman spectra of 2, 3 dichloro aniline and 2, 6 dichloro aniline have been recorded. The vibrational spectra have been analysed assumingC _s andC _2v point groups for 2, 3 dichloro aniline and 2, 6 dichloro aniline respectively. Assignments for fundamental vibrations, combination and overtone frequencies and internal modes of vibration of amino group have been proposed.     

Tropolone monomer: Vibrational spectrum and proton tunneling

Infrared matrix-isolation spectra are reported for the tropolone monomers C₇H₅O₂H and C₇H₅O₂D. Tropolone possesses an intramolecular hydrogen bond and the possibility for proton tunneling from one oxygen atom to the other. Clearcut multiplets attributed to tunneling are observed for the OD stretching mode and for heavy atom modes of both C₇H₅O₂D and C₇H₅O₂H. The (mixed) carbonyl stretching mode particularly seems to facilitate tunneling from one conformation to the other. The tunneling phenomena suggests that tropolone monomer has nearly C_2v symmetry. As a crude estimate, the tunneling potential energy barrier is calculated to be less than 5600 cm^?1 in the ground electronic state of C₇H₅O₂D. The barrier is lower in the $\text{Π}{}^1$ electronic state than in the ground electronic state. A vibrational assignment that encompasses most of the 39 fundamental modes is proposed. The vibrations are classified using C_2v symmetry species and a parallel vibrational analysis is presented for tropone, C₇H₆O, which is a true C_2v molecule.     

Concentration effects on absolute depolarized Rayleigh ratios and effective polarizability derivatives of CS2 in liquid mixtures with CCl4

Experimental results are reported from polarized and depolarized Raman intensity measurements in combination with absolute depolarized Rayleigh ratios of liquid CS₂ in CCl₄ mixtures at 25°C. Using a calibrated Raman spectrometer it was possible to determine the concentration dependence of g ₂ · γ² _eff of CS₂. Intensity measurements of the polarized and depolarized Raman spectra of the symmetric stretching vibration of CS₂ under the same conditions allowed the determination of the effective values of α₀₁ and γ₀₁ respectively to the normal mode Σ⁺ _g in the liquid phase. In the neat liquid, α₀₁ = 0·19 ų and γ₀₁ = 0·22 ų. In mixtures with CCl₄ the values of α₀₁ decrease at lower CS₂ concentration whereas the values of γ₀₁ increase. These results indicate that the isotropic induced dipole field is increasingly cancelled as the environment becomes more isotropic while the anisotropic part is amplified in CCl₄. A report is also given of an anomalous concentration behaviour of the intensity of the combination mode v ₁ + v ₂-v ₂ on the low frequency side of the v ₁ transition which is amplified relative to the intensity of the v ₁ mode as the CCl₄ concentration increases.     

C 60 and the icosahedral group Ih: Simplification methods and group-theoretical applications

Group-theoretical methods are used to induce “global” distortions of C ₆₀ from basis function(s) of a single “local” distortion in two cases: (1) The 60 vibrational modes arising from radial displacements of carbon atoms are found in terms of basis functions of A′, the one-dimensional symmetric representation of C_s; (2) The 24 vibrational modes arising from tangential displacements of hypothetical atoms placed at pentagon centers are found in terms of basis functions of E ₁, the vector irreducible representation of C_5v . The induction process is simplified by an icon notation (v) or (vμ) which uniquely labels the operations in I_h if μ,v = 0, 1, 2, 3, 4 and under/over lining of a digit is included. These icons describe the transformations IUS^vTS^μ of Felix Kline referred to a five-fold z-axis and two-fold x-axis and serve to label and “distinguish” symmetrically equivalent points of the truncated icosahedron.     

Finite perturbation approach to nuclear spin couplings in three-membered ring systems

A progression in both the v ₂ and v ₃ vibrational modes, extending for several thousand cm^-1 has been observed in the 266 nm Raman spectrum of NOCl.     

The integrated number of vibrational states in acetylene (12C2H2, 13C2H2, 12C2D2)

A calculated exhaustive set of vibrational state energies in ¹²C₂H₂, ¹³C₂H₂ and ¹²C₂D₂ has been used to analyse the evolution of the integrated number of states with increasing vibrational energy N(E) up to 15000 cm^?1, 12000cm^?1 and 10000 cm^?1 in each isotopomer, respectively. The regular contribution to N(E) was modelled analytically and numerical parameters were fitted. The other expected contribution to N(E), which is of oscillatory nature, was quantified and is discussed using energyand time-dependent theories. Related periods of oscillation and temporal recurrences are interpreted consistently in terms of the constant of the motion N_r = 5v₂ + 3v₂ + 5v₃ + v₄ + v₅ and of an average vibrational quantum. More pragmatically, the vibrational dynamics appear to be dominated by the bending vibrations, i.e., by the slowest oscillators.     

Structural conformations and electronic interactions of the natural product,oroxylin: a vibrational spectroscopic study

The oroxylin, 5,7‐dihydroxy 6‐methoxy flavone is a potent natural product extracted from ‘Vitex peduncularis’. Density functional theory (DFT) at B3LYP/6‐311G(d,p) level has been used to compute energies of different conformers of oroxylin to find out their stability, the optimized geometry of the most stable conformer and its vibrational spectrum. The conformer ORLN‐1 with torsion angles 0, 180, 180 and 0 degrees, respectively, for H₁₃ O₁₂ C₆ C₅, H₁₄ O₁₀ C₄ C₅, H₁₃ O₁₂ C₆ C₅ and H₁₄ O₁₀ C₄ C₅ is found to be most stable. The optimized geometry reveals that the dihedral angle φ between phenyl ring B and the chrome part of the molecule in − 19.21° is due to the repulsive force due to steric interaction between the ortho‐hydrogen atom H₂₉ of the B ring and H₁₈ of the ring C (H₂₉·H₁₈ = 2.198 Å). A vibrational analysis based on the near‐infrared Fourier transform(NIR‐FT) Raman, Fourier transform‐infrared (FT‐IR) and the computed spectrum reveals that the methoxy group is influenced by the oxygen lone pair‐aryl p_z orbital by back donation. Hence the stretching and bending vibrational modes of the methoxy group possess the lowest wavenumber from the normal values of methyl group. The carbonyl stretching vibrations have been lowered due to conjugation and hydrogen bonding in the molecules. The intramolecular H‐bonding and nonbonded intramolecular interactions shift the band position of O₁₀ H₁₄ and O₁₂ H₁₃ stretching modes, which is justified by DFT results. Copyright © 2008 John Wiley & Sons, Ltd.     

Lattice vibrational study of the phase transition in the plastic crystal adamantane (C10H16)

Using the Raman technique we have studied the lattice vibrations of the prototype plastic crystal adamantane (C₁₀H₁₆). The measurements, as a function of temperature through the phase transition (208K), show splittings in certain lines. At room temperature using hydrostatic pressure, we also induced a phase transition. The transition is detected by Raman measurements, and we observe splittings of the very same vibrational lines. This indicates that the pressure induced phase transition yields the same crystal structure as the low temperature structure. Some analysis of the pressure and temperature derivatives of the vibrational modes is presented.     

Raman study of temperature and pressure effects on vibrational relaxation in liquid CHCl3 and CDCl3

The Raman line shapes of the ν₁(A ₁)C-H and C-D stretching fundamentals in liquid CHCl₃ and CDCl₃ have been measured as a function of pressure from 1 bar to 4·5 kbar within the temperature range 30°C to 90°C. Densities have also been determined under the same experimental conditions. The vibrational relaxation rates are obtained from the isotropic component of the Raman band and the experimental results can be summarized as follows: (i) as T increases at constant density the vibrational relaxation rate increases; (ii) at constant T, the increase in density produces an increase in the relaxation rate; (iii) an increase in temperature at constant pressure results in an increased relaxation rate. The above three cases hold for the CDCl₃ liquid, whereas only (ii) may be stated for the CHCl₃ liquid.

The experimental vibrational data are interpreted in terms of the Kubo stochastic line-shape theory and the collinear-isolated-binary-collision model proposed by Fischer and Laubereau. Application of the Kubo formalism shows that vibrational dephasing is the dominant relaxation mechanism and that the modulation is fast both in liquid CHCl₃ and CDCl₃.

Interpretation in terms of the binary collision dephasing model leads to the following results: (i) the pure dephasing mechanism seems to be the dominant broadening mechanism for the isotropic Raman line shapes studied; (ii) the calculated dephasing rates as a function of density and temperature show agreement with the experimental data. In these calculations the elastic collision times are obtained from the modified Enskog theory.     

Vibrational Analysis of Coherent Anti-Stokes Resonance Raman Spectra from Bacteriorhodopsin Containing Isotopically Substituted Retinal Chromophores

Vibrational spectra recorded by coherent anti-Stokes resonance Raman scattering (CARS) from bacteriorhodopsin (BR) samples containing isotopically substituted (²H and ¹³C) retinal chromophores were measured using high repetition rate, low-power, picosecond pulsed excitation (λ₁=580 nm and λ_s=640±3 nm). These picosecond resonance CARS (PR/CARS) data were analyzed via third-order susceptibility relationships [χ ^{( 3 )} ] to obtain band origins, bandwidths, relative intensities, and electronic phase factors assignable to all significant vibrational Raman features in the 1490–1700 cm⁻¹ wavenumber region (the ethylenic stretching and C = N–H rocking or Schiff base modes). Isotopic substitution selectively places ²H at C₁₅, ¹³C singly at the C₁₀ position and at the C₁₄ position, and ¹³C simultaneously in positions of C₁₄ and C₁₅. Each isotopic BR sample was examined not only in H₂O, but also in D₂O, which places a ²H at the Schiff base nitrogen of the retinal. In addition, PR/CARS data were recorded from each isotopic BR sample following either light adaptation [i.e. the BR sample contained a single retinal isomer (all- trans , 15- anti or BR-570)] or dark adaptation [i.e. the BR sample contained a mixture of comparable amounts of retinal isomers (BR-570 and 13- cis , 15- syn or BR-548)]. Excellent agreement was found between the vibrational features observed by PR/CARS and those obtained from spontaneous resonance Raman measurements from the same isotopically substituted BR pigments. Several new vibrational features were also found from the PR/CARS data. Vibrational Raman data from three of the isotopic BR samples in D₂O are reported for the first time.     

Raman diagnostics of CVD systems: Determination of local temperatures

The suitability of local temperature measurements by cw Raman spectroscopy for the CH₄/H₂ CVD system has been established. The temperature profiles in a model reactor were derived from H₂ pure rotational lines and from hot bands of thev ₁ vibrational band of CH₄. Experimental results are presented for substrate temperatures of 773 K and of 1473 K. High accuracy of measurement and excellent agreement with theoretical solutions for the temperature field within the reactor were found.     

Density functional study on the structural conformations and intramolecular charge transfer from the vibrational spectra of the anticancer drug combretastatin‐A2

Combretastatin‐A2 (CA2), a potential anticancer drug in advanced preclinical development, is extracted from the medicinal plant C ombretum caffrum. The NIR‐FT Raman and FT‐IR spectral studies of the molecule were carried out and a b initio calculations performed at the B3LYP/6‐31G(d) level to derive the equilibrium geometry as well as the vibrational wavenumbers and intensities of the spectral bands. The vibrational analysis showed that the molecule has a similar geometry as that of c is‐stilbene, and has undergone steric repulsion resulting in twisting of the phenyl ring with respect to the ethylenic plane. Vibrational analysis was used to investigate the lowering of the stretching modes, and enhancement of infrared band intensities of the C–H stretching modes of Me2 may be attributed to the electronic effects caused by back‐donation and induction from the oxygen atom. Analysis of phenyl ring modes shows that the CA2 stretching mode 8 and the aromatic C–H in‐plane bending mode are equally active as strong bands in both IR and Raman spectra, which can be interpreted as the evidence of intramolecular charge transfer (ICT) between the OH and OCH₃groups via conjugated ring path and is responsible for bioactivity of the molecule. Copyright © 2008 John Wiley & Sons, Ltd.     

The adsorption of cyclic hydrocarbons on Ru(001): II. Cyclohexane

The adsorption of cyclohexane on Ru(001) at 90 K has been investigated by thermal desorption mass spectrometry, EELS, UV photoemission and LEED. Thermal desorption indicates the adsorption of the undissociated molecule first in a chemisorbed monolayer (T_d = 200 K) with subsequent formation of multilayers (T_d = 165 K) at higher exposures. The vibrational spectrum obtained by EELS is characterized by a frequency shift of the C-H stretching mode from 2920 cm^?1 (multilayer) to 2560 cm^?1 for the chemisorbed monolayer. Off-specular EELS data indicate two different electron scattering mechanisms for the C-H stretching mode. Whereas for the C-H stretching mode of the multilayer, large angle electron impact scattering is observed, the C-H soft-mode of the monolayer is largely due to small angle dipolar scattering. The He I photoelectron spectra of cyclohexane multilayers are characteristic of the undissociated molecule. A new assignment of C(2s) and the lowest C(2p) level, based on a comparison with benzene, shows that the chemisorbed monolayer is characterized by the absence of emission or broadening of the 2a_1u level. This is attributed to C_3v symmetry of the chemisorbed layer and to a possible interaction of the 2a_Iu orbital with the metal surface.