The Structure of Aminopropylsiloxane Polymerized in DC Electric Field

Summary : The Raman spectra of aminopropylsiloxane (APS) polymerized on PVC substrate in DC electric field were measured to study the structure of deposited films. The modelling of low wavenumber Raman spectra was achieved using contribution of acoustic phonons, intramolecular vibrations as well as optical phonons related to librational motions of ladders. Results suggest that electric field influences the orientations of ladders increasing thus the degree of order in microdomains.     

Electrical anharmonicity and vibronic couplings contributions to optical nonlinearity of N-benzyl-2-methyl-4-nitroaniline crystal studied by FT-IR, polarized FT-NIR, resonance Raman and UV-vis spectroscopy

FT-IR and Raman spectra of the orthorhombic and monoclinic N-benzyl-2-methyl-4-nitroaniline (BNA) single crystals, powders, and BNA solutions were measured in the 15-4000 cm(-1) range, and complete assignments of bands to normal vibrations are proposed. The assignments have been reinforced by density functional theory (DFT) calculations. Polarized FT-NIR spectra of the orthorhombic (010) BNA plate were measured for the overtones and combinations analysis and for the mechanical and electrical anharmonicity estimation. Resonance Raman spectra of the orthorhombic BNA polymorph together with the resonance excitation profiles of fundamental and lattice vibrations, compared with the band maxima in the measured UV-vis-NIR spectra of BNA, have revealed vibronic couplings with intramolecular and intermolecular charge transfers. The role of anharmonicity, vibronic couplings and intermolecular hydrogen bonds as well as of the N-benzyl substituent are discussed and the origin of second harmonic generation in the orthorhombic BNA crystal is proposed.     

Simulated Raman spectra of four tetraphenylbutadiene polymorphs

We report a combined experimental and theoretical investigation on the Raman spectra of the polymorphs α, β, γ, and δ of 1,1,4,4‐tetraphenyl‐1,3‐butadiene (TPB), in the region of the intramolecular modes. The interpretation of the polarized spectra is supported by ab‐initio calculations for the isolated molecules and by lattice dynamics calculations for the crystals. The calculations reproduce the peculiar, and surprisingly large, differences among the spectra of the various polymorphs. The phenyl groups of 1,1,4,4‐tetraphenyl‐1,3‐butadiene may arrange themselves around the butadiene skeleton in 2 stable conformers, which have either inversion (C_i) or 2‐fold (C₂) symmetry and therefore exhibit intramolecular vibrations with quite different Raman selection rules and spectra. The compound forms 4 crystalline polymorphs (α, β, γ, and δ) with different combinations of C_i and C₂ conformers, and correspondingly different intramolecular spectra. The theoretical calculations provide a quantitative analysis of the various spectra.     

Vibrational spectra of copper sulfate hydrates investigated with low-temperature Raman spectroscopy and terahertz time domain spectroscopy

In this paper, the vibrational spectra of copper sulfate hydrates (CuSO(4)·xH(2)O, x = 5, 3, 1, 0) have been investigated with low-temperature Raman spectroscopy and terahertz time domain spectroscopy (THz-TDS). It is found that the four groups of Raman bands between 90 and 4000 cm(-1) can be assigned to lattice vibration as well as intramolecular vibrations of a copper complex, sulfate group, and water molecules. The variation of vibrational spectra during the dehydrated process are discussed in detail considering the transformation of the crystal structure, especially the bands between 3000 and 3500 cm(-1), which are attributed to the ν(1) and ν(3) modes of water molecules. In addition, as a complement of Raman spectra, the THz spectra at 0.1-3 THz indicate the absorption due to the low-frequency lattice vibration and hydrogen bond.     

The Raman spectrum of hexamethylbenzene single crystal

A set of polarized Raman spectra obtained from suitable hexamethylbenzene single crystals at room temperature is presented. All the observed frequencies are attributed to intramolecular vibrations or lattice modes on the basis of clear polarization data.     

Vibration-Spectroscopic Studies on Solid Compounds

Considerably improved possibilities are now available for the investigation of the optical vibrations in lattice compounds by Raman and IR spectroscopy. These spectra can be interpreted on the basis of the models and findings of molecular vibration spectroscopy. For simple lattice types simple frequency equations are found for the optical lattice vibrations with in-phase vibration motions in all unit cells (wave vector k = 0). As an approximation, the force constants that occur in these equations may be interpreted as stretching force constants. For typical ionic lattices, the influence of Coulomb forces due to the lattice polarization that occurs during the vibrational motion must be taken into account. As is shown by the example of some spinels, the assignment of lattice vibrations can be facilitated by model calculations.     

Vibrational assignments for 7-methyl-4-bromomethylcoumarin, as aided by RHF and B3LYP/6-31G* calculations

Infrared (4000-400 cm(-1)) and Raman (3500-50 cm(-1)) spectral measurements have been made for the solid sample of 7-methyl-4-bromomethylcoumarin. Electronic structure calculations at RHF/6-31G* and B3LYP/6-31G* levels of theory have been performed, giving equilibrium geometries, harmonic vibrational spectra and normal modes. Different orientations of bromomethyl group have yielded only two conformers, of which the most stable one lying lower from the other conformer by approximately 7.99 kJ/mol, is non-planar with no symmetry. A complete assignment of the vibrational modes, aided by the calculations, has been proposed. Coupled vibrations are manifest in many modes. Some spectral features, compared to 6-methyl-4-bromomethylcoumarin, show changes across both IR and Raman spectra, involving mainly skeletal vibrations, and to a lesser degree, methyl and bromomethyl vibrations. Low-frequency vibrations below 150 cm(-1) are assigned to lattice modes.     

Vibrational spectroscopy of solid state molecular dimers

The IR and Raman spectra of α- and β-perylene crystal vibrations are investigated, β-perylene is monomeric and α-perylene has face-to-face dimers. Frequencies and forms of the lattice vibrations as well as vibron relaxation and localization are discussed. Polarization ratios in α crystals are perturbed by coupling to dimer vibrations.     

Low-frequency Raman-active vibrations of triclinic n-paraffins

The low-frequency Raman spectra of triclinic n-paraffins, n-C₈H₁₈ through n-C₂₄H₅₀, were observed. The normal-coordinate treatments of crystal vibrations of n-C₈H₁₈ through n-C₁₈H₃₈ were carried out. Six characteristic series of the observed Raman lines were assigned to rotatory lattice vibrations and intramolecular skeletal vibrations.     

Relationship between the Dynamic Properties of 1h Ice and Xenon Hydrate and the Interplay of Intramolecular and Intermolecular Vibrations

A new approach was used to simulate vibration spectra of 1h ice and xenon hydrate in the range 0–4000 cm^-1, which encompasses both intramolecular and intermolecular vibrations. This approach, based on the lattice dynamics method, enables full spectra to be obtained for molecular crystals with allowance for the coupling of intramolecular and lattice vibrations. It is shown that consideration of this coupling leads to splitting of the peaks of intramolecular vibrations and to a significant change in the spectrum in the range of translation and libration molecular vibrations, which agrees qualitatively with experimental results.     

Low frequency vibrational spectra of some amino acids

Far-IR absorption and reflection spectra, as well as laser Mandelshtam–Brillouin and Raman scattering spectra of -glycine, β-alanine, -histidine, -tryptophane single crystals in the 0.2–400 cm⁻¹ range were investigated. It was revealed that the far-IR and Raman spectra of the amino acids under study contain more bands than predicted by factor-group analysis, thus indicating a possible contribution of low-energy intramolecular vibrations and overtones, as well as an emergence of forbidden vibrations. Some of the low-frequency bands have never, to our knowledge, been detected previously.     

Raman and infrared spectra of crystalline chloroform

Raman and far-infrared spectra of polycrystalline samples of chloroform at 20 and 80 K are reported. Crystal field spilttings of the intramolecular fundamentals are observed as well as nine Raman and six infrared lattice modes. Spectra are interpreted in terms of a group theoretical analysis based on the molecular and crystal symmetries.     

Low frequency vibrations in crystalline biphenyl: Model calculations and raman and neutron spectra

Vibrational frequencies of crystalline biphenyl have been calculated in the rigid phenyl approximation using the crystal structure at 110 K. The calculated results explain successfully the experimental infrared and Raman frequencies as well as the polarization data of the Raman bands at 80 K that have been observed in the present study for the first time. The frequency dispersion curves calculated in the b^* direction show that the coupling between the intramolecular CC torsion and the translational lattice vibrations changes drastically as the wave vector changes. The existence of a minimum frequency point located away from the Brillouin zone boundary indicates that the pulse transition near 40 K is a commensurate—incommensurate one in accordance with the results of neutron diffraction that have recently been reported. Incoherent inelastic neutron scattering spectra have also been investigated above and below the phase transition temperature.     

Inelastic neutron scattering studies on dichloro-1,4-benzoquinones

Inelastic neutron scattering (INS) spectra of 2,6-dichloro- and 2,5-dichloro-1,4-benzoquinone were compared with Raman and infra-red (IR) spectra and analysed in detail below 1800 cm(-1). The analysis was based on calculations tending towards simulation of spectra by using GAUSSIAN (HF, DFT/B3LYP and BLYP/6-31G(d,p)), and auntieCLIMAX programs. The correlations between calculated and experimental (either INS or Raman and IR) frequencies enabled to analyse the problem of scaling factors (SFs). The advantages of INS technique was shown in studies of low frequency vibrations with participation of H-atoms. The macroscopic lattice effect at low temperatures on INS spectra is discussed.     

A vibrational molecular force field of model compounds with biological interest. I. Harmonic dynamics of crystalline urea at 123 K

Normal coordinate calculations have been performed for urea and deuterated urea in the crystalline state. We have used the modified Urey–Bradley–Shimanouchi intramolecular potential energy function and a rather sophisticated intermolecular energy function to reproduce I.R. and Raman frequencies with an average error of 2 cm^–1. The general agreement between the calculation and experiment suggests that intermolecular interactions must be taken into account to determine reliable intramolecular parameters of the potential energy function, mainly the barrier to internal rotation around the C? N bond. The intermolecular energy function, which consists of the Buckingham function and an explicit harmonic function for hydrogen-bonding, then has the merit to reproduce quite well the observed frequencies of lattice vibrations.     

External,internal and semi-internal vibrations in molecular solids: spectroscopic criteria for identification

A new method to identify the nature of vibrations in molecular crystals as external or internal is proposed. The criterion is that in isotropic mixed crystals of protonated and corresponding perdeuterated compounds, the external motions (phonons) are in the amalgamation limit, while the internal modes, and the “semi-internal” methyl torsions, are usually in the separated band limit. This criterion is supported by experimental isotopic mixed crystal studies of many molecular crystals. Previous criteria, like temperature and isotope shifts, are shown to be less reliable. Anthracene and biphenyl Raman spectra, as well as literature data, are used as illustrations.     

Lattice dynamics of TTF–CA across the neutral–ionic transition

We present the temperature evolution of polarised Raman and IR reflectivity spectra of Tetrathiafulvalene–Chloranil (TTF–CA) in the lattice phonon region (10–200 cm⁻¹). Quasi Harmonic Lattice Dynamics (QHLD) calculations prove to be essential in discussing the assignment of the phonons and their evolution across the temperature induced neutral–ionic phase transition.     

Assignment of the in-plane molecular vibrations of the electron-donor molecule BDT-TTP based on polarized Raman and infrared spectra,in which BDT-TTP is 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene

Infrared and Raman spectra of 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (BDT-TTP) and 1,3,4,6-tetrathiapentalene-2,5-dione (TTP-DO) are reported. The vibrational modes of TTP-DO are assigned with the aid of the depolarization ratio of solution Raman spectra, polarized reflection spectra and polarized Raman spectra. A D2h symmetry is assumed for the BDT-TTP molecule and its in-plane fundamental vibrations are assigned with the aid of the polarization ratio and the correlation with TTP-DO, tetrathiafulvalene (TTF), tetramethyltetrathiafulvalene (TMTTF) and bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF). Normal coordinate calculation with a modified internal valence force field was carried out for the in-plane fundamental vibrations of TTP-DO and BDT-TTP. Ab initio calculations of the normal modes of BDT-TTP0 and BDT-TTP+ are compared with the empirical analysis.     

Far- and mid-infrared of crystalline 2,2'-bithiophene: ab initio analysis and comparison with infrared response

Infrared intramolecular vibrations and lattice modes in the crystalline phase of 2,2'-bithiophene (2T) are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. For the first time, the far- and mid-infrared responses have been calculated from the Gamma-point modes and the Born effective charge tensors of the 2T crystalline phase. The relative good agreement between the calculated and experimental infrared spectra allows us to assign the origin of the main features of the experimental spectra, which is of particular interest in the far-infrared domain. These assignments are useful for understanding all the properties of the 2T crystalline phase in which phonon-phonon and electron-phonon interactions play an important role.     

Inter- and intramolecular hydrogen bonding in phenol derivatives: a model system for poly-L-tyrosine

The ultrafast dynamics of solutions of phenol and two phenol derivatives--hydroquinone (1,4-benzenediol) and pyrocatechol (1,2-benzenediol)--have been studied with Optically Heterodyne-Detected Optical Kerr-Effect (OHD-OKE) spectroscopy. The solvents, methanol and acetonitrile, were selected to provide strong and weak solvent-solute hydrogen-bonding interactions, respectively, while pyrocatechol features an intramolecular hydrogen bond. Together these provide a series of model systems for polypeptides such as polytyrosine, which facilitate the direct study of inter- and intramolecular hydrogen bonding. A broad contribution to the Raman spectral density of the methanol solutions at frequencies between 150 and 300 cm(-1) has been observed that is absent in acetonitrile. This contribution has been assigned to solvent-solute hydrogen-bond stretching vibrations. The OHD-OKE response of poly-L-tyrosine has been measured and was found to contain a similar contribution. Density functional theory geometry optimizations and normal mode calculations have been performed using the B3LYP hybrid functional and 6-311++G** basis set. These have yielded a complete assignment of the low-frequency Raman and far-infrared spectra of pyrocatechol for the first time, which has provided information on the nature of the intramolecular hydrogen bond of pyrocatechol.