Dynamics of phase transitions in a liquid crystal probed by Raman spectroscopy

Raman spectra of the Schiff 's base liquid crystalline compound 5O5, N-(4- n-pentyloxybenzylidene)-4'-n-pentylaniline, have been recorded as a function of temperature from 22 to 80°C in the 1140-1220 cm^-1 and 1550-1640 cm^-1 spectral regions. From careful deconvolution of the spectral features using Lorentzian profiles, precise values of peak positions, integrated intensities and linewidths of some selected Raman bands were obtained. The variations of the Raman spectral parameters with temperature are discussed in terms of changes in the molecular alignment and its effect on intra-/inter-molecular interactions at the Cr-G, G-SmF, SmF-SmC and SmA-N phase transitions. From a detailed study, it is inferred that the increased orientational/vibrational freedom of the alkyl chains, as well as the delocalization of the electron clouds, is responsible for the spectral anomalies at the Cr-G transition. Loss of positional ordering and twist around the -C₆H₄-N= bond takes place at the SmF-SmC transition. In the SmA-N transition, some evidence for the formation of cybotactic clusters was obtained.     

study of liquid crystalline N -[4-(4- n -alkoxybenzoyloxy)-2-hydroxybenzylidene]methylanilines

New homologous series of N-[4-(4-n-alkoxybenzoyloxy)-2-hydroxybenzylidene]methylanilines [nAHmM(n=1-8/10; m=2: ortho, m=3: meta, m=4: para)] were synthesized. They exhibited a nematic phase except for 1AH3M. The temperature dependence of their Raman spectra was observed in the spectral range of 900–1700 cm^-1. In one group of nAHmM compounds, the Raman band at about 1360 cm^-1 abruptly decreased in intensity and wavenumber when the crystalline solid-liquid crystal phase transition was approached. In another group, the corresponding band increased through the phase transition. The bands have been assigned to the coupling mode between the in-plane CCH deformational vibration and the ring-N stretching vibration. Such a behaviour can be explained by the molecular conformation with different twist angles of the aniline ring in relation to the Schiff's base plane of the molecule. Some nAHmMs exhibited photochromism.     

Temperature-dependent vibrational spectroscopic studies of pure and gold nanoparticles dispersed 4-n-Hexyloxy-4’-cyanobiphenyls

Raman spectra of pure and 2 wt.% gold nanoparticles (GNPs) dispersed liquid crystalline compound 4-n-Hexyloxy-4?- cyanobiphenyls (6OCB) has been recorded as a function of temperature from room temperature (solid crystal) to 80°C (isotropic liquid) in the spectral region of 500–2500 cm^?1. The variation of Raman spectral parameters (peak positions and line width) with temperature is used to explain the changes in molecular alignment and its effect on inter-/intra-molecular interactions at crystal-Nematic (K-N) transition. To understand the change in molecular structure during phase transition and on account of dispersion of gold nanoparticles in pure liquid crystal more precisely, two spectral regions 1000–1500 cm^?1 and 1500–2400 cm^?1 have been selected separately. From the detailed study, it is concluded that increased orientational/vibrational freedom of the molecules as well as delocalisation of electron clouds results in the spectral anomalies at K-N transition. The geometrical structure of 6OCB was optimised using density functional theory (DFT) and theoretical Raman spectra have been obtained for comparison with experimental spectra. The tentative assignment of vibrational modes observed in our region of study was calculated based on potential energy distribution (PED) using vibrational energy distribution analysis (VEDA) calculation.     

Raman spectroscopy of liquid crystalline N-[4-(4-n-alkoxybenzoyloxy)-2-hydroxybenzylidene]-3-aminopyridines

New homologous series of N-[4-(4-n-alkoxybenzoyloxy)-2-hydroxybenzylidene]-3-aminopyridines were synthesized, they exhibited a nematic phase. The temperature dependence of their Raman spectra has been observed in the spectral range 900–1800?cm^?1. Some Raman bands show a marked change in their intensity and frequency through the phase transition from crystalline solid to nematic. These bands are ascribed to the vibrational modes related to the core part of the molecule. Such behaviour can be explained by the change of molecular conformation related to the core. Some members of these series exhibited photochromism in the solid state.     

Measurement of splay elastic constant and rotational viscosity of an induced nematic binary system

We investigated the temperature dependence of the splay elastic constant (K₁₁) and rotational viscosity (γ₁) of a binary liquid crystal system comprising 5-trans-n-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4-cyano-4′-n-undecyloxy-biphenyl (11OCB), exhibiting induced nematic phase. Both the splay elastic constant and rotational viscosity increased following lowering of the temperature. In the vicinity of smectic A-nematic (SmA-N) transition, both the relaxation time and rotational viscosity exhibited a strong pretransitional behaviour. The critical exponent (ν) presented here led to valuable qualitative information about the pre-transitional behaviour of the rotational viscosity data near the SmA-N phase transition. The extracted ν values ranging between 0.336 and 0.352 are in fair agreement with those predicted in the de Gennes model as compared to that by the mean-field model.     

Ion conductive poly(ethylene oxide-dimethyl siloxane) copolymers

A series of poly(ethylene oxide-dimethyl siloxane) copolymers, — [SiMe₂O(CH₂CH₂O)_n]_m — (n = 2, 3, 4, 5, 6.4, 8.7, 13.3), were synthesised by the reaction of polyethylene glycol with dimethyl dimethoxy/diethoxysilane. Corresponding ion-conductive polymers were prepared by complexing these copolymers with salts (sodium tetrafluoroborate or ammonium adipate). The highest conductivity of these systems at room temperature was 3 × 10⁻⁴ S cm⁻¹ and 6 × 10⁻⁵ S cm⁻¹, respectively. The glass transition temperature of these copolymers is reported and is seen to be dependent on the length of the ether units. The effects of siloxane content, salt concentration, and temperature on the conductivity are discussed. © 1996 John Wiley & Sons, Inc.     

X–ray Single Crystal Structure and Raman Spectrum of Ammonium Hexafluoroarsenate

The structure of ammonium hexafluoroarsenate, NH₄AsF₆, has been determined by X‐ray diffraction using a single crystal grown from saturated solution in anhydrous HF. NH₄AsF₆ crystallizes rhombohedral with the KOsF₆ structure type, with a = 7.459(3) Å, c = 7.543(3) Å (at 200 K), Z = 3, space group (No. 148). No phase transition was observed in the 100 K–296 K temperature range. The structure is dominated by regular AsF₆ octahedra and disordered NH₄⁺ cations. Raman spectrum of a single crystal of NH₄AsF₆ shows the bands at 372 cm^?1, 572 cm^?1, 687 cm^?1 (AsF₆^?) and at 3240 cm^?1 and 3360 cm^?1 (NH₄⁺).     

Fluorokomplexe von Platin(II): Synthese,NMR‐ und Schwingungsspektren von Tetrafluoroplatinat(II) und Difluorooxalatoplatinat(II)

Fluorine Complexes of Platinum(II): Synthesis, NMR and Vibrational Spectra of Tetrafluoroplatinate(II) and Difluorooxalatoplatinate(II) From the platinum(IV) compounds (n‐Bu₄N)₂[PtF₄(ox)] und cis‐(n‐Bu₄N)₂[PtF₂(ox)₂] on exposure to ultraviolet light at —196 °C the new platinum(II) fluorine complexes (n‐Bu₄N)₂[PtF₄] ( 1 ) and (n‐Bu₄N)₂[PtF₂(ox)] ( 2 ) are formed by elimination of a single oxalate ligand. With the synthesis of 1 the series of the tetra halogeno platinates(II) is completed now. With Cs⁺ and bis‐(triphenylphosphine)iminium(PNP⁺) as cations tetrafluoroplatinate(II) can be precipitated as pale yellow salts. Under exclusion of air all compounds are stable at —30 °C for several days, but they decompose and become black at room temperature in air within some hours. The infrared spectrum (60 K) of 1 exhibits the antisymmetric PtF stretching vibration at 515 and two deformation vibrations at 255 and 230 cm^—1. In the Raman spectrum (293 K) of (PNP)₂[PtF₄] the symmetric PtF stretching vibrations appear at 595 and 565 cm^—1. The calculated valence force constant is f_d(PtF) = 3.09 mdyn/Å. The NMR shifts are δ(¹⁹⁵Pt) = 6592 ( 1 ) and 5099 ( 2 ) and δ(¹⁹F) = —428 ( 1 ) and —393 ppm ( 2 ) with the coupling constants ¹J(PtF) = 1747 ( 1 ) and 1385 Hz ( 2 ).     

Synthesis,characterisation, mesomorphic investigation and temperature-dependent Raman study of a novel calamitic liquid crystal: methyl 4-(4′-n-alkoxybenzylideneamino)benzoate

A new series of Schiff base calamitic liquid crystal; methyl 4-(4′-n-alkoxybenzylideneamino)benzoate (MABAB), H_2n+1C _n OC₆H₄C(H)=NC₆H₄COOCH₃ (n = 6, 8, 10, 12, 14, 16) has been synthesised and characterised by elemental analyses, Fourier transform infrared spectroscopy (FT-IR), ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy. The mesomorphic properties of these compounds were studied by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). All members of the series exhibit enantiotropic smectic A (SmA) mesophase. Temperature-dependent micro-Raman study of one of the members, MABAB-10 has been employed to identify phase transitions and the molecular rearrangement therein. Analysis of Raman marker bands; C–H in-plane bending, C–C stretching of phenyl rings and –C(H)=N– linking group of core confirms the transitions clearly as observed through DSC and POM. An in situ Raman measurement of C–H in-plane bending mode has also been performed to visualise the molecular changes more clearly. The Raman study gives an evidence of induced co-planarity of rings at Cr→SmA phase transition. The density functional theoretical (DFT) optimisation of monomer, dimer and rotational conformer of MABAB-10 also support the induced co-planarity at Cr→SmA phase transition.     

Polymerization of [o-(trimethylgermyl)phenyl] acetylene and polymer characterization

[o-(Trimethylgermyl)phenyl]acetylene was polymerized in the presence of WCl₆, W(CO)₆-hv, etc., to give polymers whose weight-average molecular weights reached ca. 7.0 X 10⁵ at the highest. When the MoOCl₄-n-Bu₄Sn-EtOH (1 : 1 : 1) catalyst was used, the polydispersity ratio of the polymer obtained was 1.08, and the number-average molecular weight increased in direct proportion to monomer conversion; these indicate that this polymerization is a living polymerization. The polymer had the structure ? [CH?C(C₆H₄-o-GeMe₃)]_n ? and was a dark purple solid (λ_max = 551 nm, ε_max = 6100 M^-1 cm^-1 in THF) soluble in organic solvents such as toluene and chloroform. The onset temperature of weight loss of the polymer in TGA in air was ca. 230°C, and the glass transition temperature was above 180°C. The Po₂ of the present polymer is 105 barrers—larger than the value of natural rubber and fairly close to that of poly(dimethylsiloxane). © 1993 John Wiley & Sons, Inc.     

Hydrogen bonding effects on the skeletal optical and the longitudinal acoustical modes in long chain molecules and polymers

Raman spectra of stearyl alcohol (n-C₁₈H₃₇OH) indicate that vibrational bands in both the skeletal optical (1000-1200 cm^?1) and longitudinal acoustical (0–500 cm^?1) regions are considerably perturbed by intermolecular hydrogen bonding. The zone interior skeletal stretching mode reflecting phonon dispersion in the v₄ branch of the infinite polyethylene chain is found at 1105 cm^?1, approximately 20 cm^?1 higher than in the corresponding n-alkane. Similarly the single nodal longitudinal acoustical mode (LAM-1) is found shifted by 12 cm^?1 to higher frequency when the expected “mass effect” produced by the ? OH group is considered. This shift is further increased to 16 cm^?1 at ?100°C indicating a further perturbation on this accordion mode due to the increased strength of the hydrogen bond at the low temperatures. The positions of the higher multinodal vibrations, LAM-3 and LAM-5, are also perturbed by the hydrogen bond but by differing amounts. The observation of a low-frequency Raman-active LAM in polytetrahydrofuran [(? CH₂CH₂CH₂CH₂O? )_n] is discussed in conjunction with the expected effects of hydrogen bonding at the lamellar surface.     

OsII-Phthalocyaninate(2−): Darstellung und Eigenschaften von (Halogeno)-(carbonyl)phthalocyaninato(2−)osmat(II)

Os^II Phthalocyaninates(2?): Synthesis and Properties of (Halo)(carbonyl)phthalocyaninato-(2?)osmate(II) Soluble, blue tetra(n-butyl)ammonium (halo)(carbonyl)phthalocyaninato(2?)osmate(II), (ⁿBu₄N)[Os(X)(CO)Pc^2?] (X = Cl, Br, I) is obtained by the reaction of [Os(THF)(CO)Pc^2?] (THF: tetrahydrofurane) with (ⁿBu₄N)X in THF. In the cyclovoltammograms there are three reversible electrode processes at ?1.21 ± 0.01, 0.18 ± 0.04 and 0.65 ± 0.01 V assigned to the three redox pairs Pc^2?/Pc^3?, Os^II/Os^III and Pc^2?/Pc^3?. In the electronic absorption spectra only the intense B and Q regions are observed at ～ 15800 resp. 27500, 33000 cm^?1. The infrared and resonance Raman spectra closely resemble those of other phthalocyaninates(2?) of low valent osmium. In the infrared spectrum v(C? O) is detected at 1896 ± 4 cm^?1 and v(Os? X) at 260 (X = Cl), 175 (X = Br) or 143 cm^?1 (X = I).     

Far-infrared spectrum,conformational stability,barriers to internal rotation,normal coordinate calculations,and vibrational assignment for chloroacetaldehyde

The far infrared spectrum [350 to 25 cm^–1] of gaseous chloroacetaldehyde, ClCH₂CHO, has been recorded at a resolution of 0.10 cm^–1. The first excited-state transition of the asymmetric torsion of the more stable near s-cis [chlorine atom s-cis to the aldehyde hydrogen atom] conformer has been observed at 26.9 cm^–1, with seven additional upper state transitions falling to higher frequency. Additionally, the fundamental torsional transition of the s-trans conformer has been observed at 58.9 cm^–1 with two excited states also falling to higher frequency. From these data, the asymmetric torsional potential coefficients have been determined to be:V ₁=414±11;V ₂ = 191±3;V ₃=–203±5;V ₄=44±1 andV ₆=–26±1 cm^–1. The s-cis to s-trans barrier is 500±5 cm^–1 (1.43±0.01 kcal mol^–1) with the s-cis conformer being more stable by 267±19 cm^–1 (0.76±0.05 kcal mol^–1) than the s-trans form. The Raman [4000 to 100 cm^–1] and infrared (4000 to 400 cm^–1] spectra of the gas have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained. Complete vibrational assignments are proposed for both conformers based on band contours, depolarization values, and group frequencies. The assignments are supported by ab initio Hartree-Fock gradient calculations employing the 3–21G^* basis set to obtain the frequencies and the potential energy distributions for the normal vibrations for both rotamers. Additional ab initio calculations at the MP4/6-31G^* level have been carried out to determine the structural parameters for both conformers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.This contribution taken in part from the thesis of C. L. Tolley which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.     

Kinetics of the ene reactions of 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione with β‐pinene and 2‐carene: Temperature,high pressure,and solvent effects

The data on temperature, solvent, and high hydrostatic pressure influence on the rate of the ene reactions of 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione ( 1 ) with 2‐carene ( 2 ), and β‐pinene ( 4 ) have been obtained. Ene reactions 1 + 2 and 1 + 4 have high heat effects: ∆H_r‐n ( 1 + 2 ) −158.4, ∆H_r‐n( 1 + 4 ) −159.2 kJ mol⁻¹, 25°C, 1,2‐dichloroethane. The comparison of the activation volume (∆V^≠( 1 + 2 ) −29.9 cm³ mol⁻¹, toluene; ∆V^≠( 1 + 4 ) −36.0 cm³ mol⁻¹, ethyl acetate) and reaction volume values (∆V_r‐n( 1 + 2 ) −24.0 cm³ mol⁻¹, toluene; ∆V_r‐n( 1 + 4 ) −30.4 cm³ mol⁻¹, ethyl acetate) reveals more compact cyclic transition states in comparison with the acyclic reaction products 3 and 5 . In the series of nine solvents, the reaction rate of 1+2 increases 260‐fold and 1+4 increases 200‐fold, respectively, but not due to the solvent polarity.     

Dilation and plasticization of polystyrene by carbon dioxide

We have used an optical interference technique to measure the dilation of polystyrene films in the presence of carbon dioxide or helium at pressures up to 20 atm. Dilation isotherms (plots of dilation versus gas pressure at constant temperature) were obtained for three samples of polystyrene which had widely differing molecular weights. The dilation isotherms have the same general shape as sorption isotherms, which means that all of the sorbed gas molecules contribute to volume dilation and non can be thought of as occupying molecular-sized voids in the polymer. Using sorption results from the literature we show that the partial molar volume of CO₂ at 35°C is about 39 cm³ mol^?1 and appears to be independent of polystyrene molecular weight. For a polystyrene sample with M_n = 3600, the partial molar volume of sorbed CO₂ increases to 44 and 50 cm³ mol^?1 at 45 and 55°C, respectively. The sorption of CO₂ in polystyrene is shown to depress the glass transition temperature of the mixture, consistent with theoretical predictions. The shape of the dilation and sorption isotherms are consistent with the depression of the glass transition temperature.     

Electronic raman transition from an optically pumped excited state

From the intensity behaviour of a 29 cm^?1 Raman shift of α-Al₂O₃:Cr³⁺ as a function of the incident power, it is concluded that the shift is due to an electronic Raman transition between the components of the excited state ²E. Population of the excited states is obtained through a pumping mechanism induced by the laser radiation λ_$\text{o}$ = 514.5 nm which at the same time serves as a Raman probe.     

Unsuccessful search for a liquid-liquid transition in compression isotherms of polystyrene

In a search for a liquid-liquid transition in the melt of polystyrene, we have measured and analyzed a number of compression isotherms to 1800 kg/cm² in a relevant temperature range (1.2T_g < T < 1.4T_g). Volume data were recorded at pressure intervals of 10 kg/cm², i.e., at pressure intervals 5-20 times smaller than used in previous work. For the analysis the data were fitted to the Tait equation. Deviations between fits and data are small, typically 0.0004 cm³/g, but nonrandom. From the absence of any systematic shifts of the nonrandom deviation patterns with temperature, we conclude that the deviations are not manifestations of a pressure-dependent liquid-liquid transition, and that there is, in fact, no evidence for the existence of such a transition in our data.     

VIBRATIONAL SPECTRA AND STRUCTURE OF ALKOXY AND CHLOROALKOXY DERIVATIVES OF ANTIMONY(V)

Abstract

Vibrational spectra of (CH₃O) _n SbCl_5–n . n = 1: 1; n = 2: 2: n = 3: 3: n = 4: 4; n = 5: 5; have been recorded. According to ir and Raman data 1–5 are centrosymmetrical bridged dimers. The Raman spectra of 3–5 exhibit v(Sb–O) doublets of terminal CH₃O at 530–541 and 550–570 cm^?1; vibrations of the 4-membered Sb₂O₂ ring, observed in the 500–517 cm^?1 region of the ir spectra of 1–5, are absent. The v(C–O) bands of bridged and terminal CH₃O are shifted to higher wave numbers (60 and 31 cm^?1, respectively) in the series 1 → 5. The stability of the dimers increases in the series 1 < 2 < 3 < 4 ? 5. At 100–120°C and in CH₃CN solutions dimers of 1–3 dissociate to monomers (v(Sb–O) 537–540 cm^?1, ir data). The monochloride, 4, is partially dissociated in CH₃CN. On solution of the tetrachloride, 1, in benzene a dimer-monomer equilibrium has been observed, with the dimeric form being predominant.     

Effect of the reference chain dimension on the viscoelastic and stress–strain behavior of poly(n-butyl methacrylate) networks

The linear viscoelastic and stress-strain behavior of poly(n-butyl methacrylate) networks at a content of crosslinking agent (ethyleneglycol dimethacrylate) of c? 0–1 × 10^?4 mole/cm³ was investigated in the main transition and rubberlike region in the temperature interval from 20 to 150°C. The dependence of the unperturbed chain dimensions on temperature was determined from thermoelastic measurements in the rubberlike region; this dependence was unaffected by the content of crosslinking agent. Application of time–temperature superposition to the linear viscoelastic behavior did not give a continuous superimposed curve in the proximity of the rubberlike region; superposition within the whole time region required introducing the change of the unperturbed chain dimensions with temperature. This correction was sufficient for a sample with a higher content of the crosslinking agent. However, for loose networks (c< 0.1 × 10^?4 mole/cm³) it was insufficient, because of another relaxation mechanism in the region of high temperatures. It was found that the intensity and temperature dependence of this relaxation mechanism, which is probably due to a change of the number of entanglements with temperature, are connected with the magnitude and the temperature dependence of the C₂ constant of the Mooney-Rivlin equation.     

Raman and infrared spectra,conformational stability,barriers to internal rotation,r 0 structural parameters,ab initio calculations,and vibrational assignment for vinyl chloroformate

The Raman (3200 to 10 cm^–1) and infrared (3500 to 50 cm^–1) spectra of vinyl chloroformate, H₂C=CHOC(O)Cl, have been recorded for both the gas and solid. Additionally, the Raman spectrum of the liquid has been recorded, and depolarization ratios have been obtained. These data have been interpreted on the basis that the only stable conformation present at ambient temperature is thetrans-trans rotamer, where the firsttrans refers to the vinyl moiety relative to the O—CCl bond and the second to the C—Cl bond relative to the=C—O bond. Using harmonic rigid asymmetric top calculations, the infrared vapor phase contours for the C=O and the C=C stretch were predicted for thetrans-trans and for thecis-trans conformer, and were compared with experiment. For both fundamentals thetrans-trans hybrid reproduces the experimental contour, whereas thecis-trans contours fail to do so for both fundamentals. From far-infrared spectrum of the vapor obtained at 0.1 cm^–1 resolution, the C(O)Cl andO-vinyl torsional fundamentals have been observed at 132 and 61 cm^–1, respectively. Ther ₀ structural parameters have been obtained from a combination of ab initio calculated parameters with appropriate offset values and the fit of the microwave rotational constants for the two naturally occurring chlorine isotopes. The structure, barrier to internal rotation, and vibrational frequencies have been determined from ab initio Hartree-Fock gradient calculations, using the 3-21G^* and 6-31G^* basis sets. These results are compared to those obtained experimentally and to similar quantities for some related molecules.