X-ray crystal structure, Raman spectroscopy, and Ab initio density functional theory calculations on 1,1,3,3-tetramethylguanidinium bromide

The salt 1,1,3,3-tetramethylguanidinium bromide, [((CH(3))(2)N)(2)C═NH(2)](+)Br(-) or [tmgH]Br, was found to melt at 135(5) °C, forming what may be referred to as a moderate temperature ionic liquid. The chemistry was studied and compared with the corresponding chloride compound. We present X-ray diffraction and Raman evidence to show that also the bromide salt contains dimeric ion pair "molecules" in the crystalline state and probably also in the liquid state. The structure of [tmgH]Br determined at 120(2) K was found to be monoclinic, space group P2(1)/n, with a = 7.2072(14), b = 13.335(3), c = 9.378(2) ?, β =104.31(3)°, Z = 2, based on 11769 reflections, measured from θ = 2.71-28.00° on a small colorless needle crystal. Raman and IR spectra are presented and assigned. When heated, both the chloride and the bromide salts form vapor phases. The Raman spectra of the vapors are surprisingly alike, showing, for example, a characteristic strong band at 2229 cm(-1). This band was interpreted by some of us to show that the [tmgH]Cl gas phase should consist of monomeric ion pair "molecules" held together by a single N-H(+)···Cl(-) hydrogen bond, the stretching vibration of which should be causing the band, based on ab initio molecular orbital density functional theory type calculations. It is not likely that both the bromide and chloride should have identical spectra. As explanation, the formation of 1,1-dimethylcyanamide gas is proposed, by decomposition of [tmgH]X leaving dimethylammonium halogenide (X = Cl, Br). The Raman spectra of all gas phases were quite identical and fitted the calculated spectrum of dimethylcyanamide. It is concluded that monomeric ion pair "molecules" held together by single N-H(+)···X(-) hydrogen bonds probably do not exist in the vapor phase over the solids at about 200-230 °C.     

Nonionic diethanolamide amphiphiles with unsaturated C18 hydrocarbon chains: thermotropic and lyotropic liquid crystalline phase behavior

The neat and lyotropic liquid crystalline phase behavior of three nonionic diethanolamide amphiphiles with C18 hydrocarbon chains containing one, two or three unsaturated bonds has been examined. This has allowed the effect of degree of unsaturation on the phase behavior of diethanolamide amphiphiles to be investigated. Neat linoleoyl and linolenoyl diethanolamide undergo a transition from a glassy liquid crystal to a liquid crystal at ～-85 °C, while neat oleoyl diethanolamide undergoes a transition at ～-60 °C to a liquid crystalline material before re-crystallizing at -34 °C. Oleoyl diethanolamide then undergoes a third transition from a crystalline phase to a smectic liquid crystalline phase at ～5 °C. In the absence of water, the transition temperature from a smectic liquid crystal to an isotropic liquid decreases with increasing unsaturation. The addition of water results in the formation of a lamellar phase (L(α)) for all three amphiphiles. The lamellar phase is stable under excess water conditions up to temperatures of at least 70 °C. Approximate partial binary amphiphile-water phase diagrams generated for the three unsaturated C18 amphiphiles indicate that the excess water point for each amphiphile occurs at ～60% (w/w) amphiphile.     

In situ Raman spectroscopy of H2 interaction with WO3 films

It is well known that WO(3) interacts efficiently with H(2) gas in the presence of noble metals (such as Pd, Pt and Au) at elevated temperatures, changing its optical behaviors; and that its crystallinity plays an important role in these interactions. For the first time, we investigated the in situ Raman spectra changes of WO(3) films of different crystal phases, while incorporating Pd catalysts, at elevated temperatures in the presence of H(2). The Pd/WO(3) films were prepared using RF sputtering and subsequently annealed at 300, 400 and 500 °C in air in order to alter the dominant crystal phase. The films were then characterized using SEM, XRD, XPS, and both UV-VIS and Raman spectroscopy. In order to fundamentally study the process, the measurements were conducted when films were interacting with 1% H(2) in synthetic air at elevated sample temperatures (20, 60, 100 and 140 °C). We suggest that the changes of Raman spectra under such conditions to be mainly a function of the crystal phase, transforming from monoclinic to a mix phase of monoclinic and orthorhombic achieved via increasing the annealing temperature. The as-deposited sample consistently shows similar Raman spectra responses at different operating conditions upon H(2) exposure. However, increasing the annealing temperature to 500 °C tunes the optimum H(2) response operating temperature to 60 °C.     

Nanostructural studies on monoelaidin-water systems at low temperatures

In recent years, lipid based nanostructures have increasingly been used as model membranes to study various complex biological processes. For better understanding of such phenomena, it is essential to gain as much information as possible for model lipid structures under physiological conditions. In this paper, we focus on one of such lipids--monoelaidin (ME)--for its polymorphic nanostructures under varying conditions of temperature and water content. In the recent contribution (Soft Matter, 2010, 6, 3191), we have reported the phase diagram of ME above 30 °C and compared with the phase behavior of other lipids including monoolein (MO), monovaccenin (MV), and monolinolein (ML). Remarkable phase behavior of ME, stabilizing three bicontinuous cubic phases, motivates its study at low temperatures. Current studies concentrate on the low-temperature (<30 °C) behavior of ME and subsequent reconstruction of its phase diagram over the entire temperature-water composition space (temperature, 0-76 °C; and water content, 0-70%). The polymorphs found for the monoelaidin-water system include three bicontinuous cubic phases, i.e., Ia3d, Pn3m, and Im3m, and lamellar phases which exhibit two crystalline (L(c1) and L(c0)), two gel (L(β) and L(β*)), and a fluid lamellar (L(α)) states. The fluid isotropic phase (L(2)) was observed only for lower hydrations (<20%), whereas hexagonal phase (H(2)) was not found under studied conditions. Nanostructural parameters of these phases as a function of temperature and water content are presented together with some molecular level calculations. This study might be crucial for perception of the lyotropic phase behavior as well as for designing nanostructural assemblies for potential applications.     

Landau theory description of observed isotropic to anisotropic phase transition in mixed clay gels

A characteristic new cooperative dehydration transition, in 1:1 Laponite-MMT cogel, was observed at T(c) ≈ 60 °C, a temperature at which the storage modulus (G(')) and depolarization ratio (D(p)) showed sharp increase, and the isotropic cogel turned into an anisotropic one. The dehydration dynamics could be described through power-law relations: G(') ～ (T(c)-T)(-γ) and D(p) ～ (T(c)-T)(-β) with γ ≈ β = 0.40 ± 0.05. The x-ray diffraction data revealed that the crystallite size decreased from 17 nm (at 20 °C) to 10 nm (at 80 °C) implying loss of free and inter-planar water. When this cogel was spontaneously cooled below T(c), it exhibited much larger storage modulii values which implied the existence of several metastable states in this system. This phase transition could be modeled through Landau theory, where the depolarization ratio was used as experimental order parameter (ψ). This parameter was found to scale with temperature, as ψ ～ (T(c)-T)(-α), with power-law exponent α = 0.40 ± 0.05; interestingly, we found α ≈ β ≈ γ.     

Density, DSC, X-ray and NMR measurements through the gel and lamellar phase transitions of 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC): observation of slow relaxation processes and mechanisms of phase transitions

Dialkyl lecithin dispersions in water exhibit two phase transitions upon cooling from the lamellar phase (L(α)). At the main transition (T(M)) the L(α) phase changes to a ripple (gel) phase (P(β')) which then transforms to a second gel phase (L(β')) at the "pretransition" (T(P)). We have made accurate density measurements through the various phases for two lecithins having unequal chains: 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC). The measurements were carried out over five heat/cool cycles from 5 to 55 °C, followed by cooling back to 5 °C. The samples were then held at 50 °C for 24 hours, followed by a further three cool/heat cycles. For SMPC we observe an increase in density of the gel phases over the first 5 cycles, followed by much smaller changes after incubation at 50 °C. The lamellar phase also shows an increase in density, albeit much smaller. This parallels the behaviour of 1,2-di-palmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-di-myristoyl-sn-glycero-3-phosphatidylcholine (DMPC) reported earlier (Jones et al., Liquid Crystals 32, 1465 (2005)). For MSPC we observe a decrease in density within the gel phases while T(P) almost disappears after the first cycle. The lamellar phase shows little evidence of any change with each cycle. Within the lamellar phases there is a marked reduction in density on approaching T(M), which is attributed to the formation of transitory gel phase domains. Additional measurements by DSC and X-ray diffraction show that the changes in densities are not accompanied by large changes in transition enthalpies or phase structures. NMR data indicate that the pretransitional event within the L(α) phase is accompanied by ordering of the alkyl chains. The results indicate that the exact nature of the lipid alkyl chains could play a key role in the formation of gel phase patches within membrane bilayers. Their detailed chemical structures merit more attention than by simply assuming a uniform "bending energy" to describe the behaviour.     

Lyotropic liquid crystalline phase behaviour in amphiphile-protic ionic liquid systems

Approximate partial phase diagrams for nine amphiphile-protic ionic liquid (PIL) systems have been determined by synchrotron source small angle X-ray scattering, differential scanning calorimetry and cross polarised optical microscopy. The binary phase diagrams of some common cationic (hexadecyltrimethyl ammonium chloride, CTAC, and hexadecylpyridinium bromide, HDPB) and nonionic (polyoxyethylene (10) oleyl ether, Brij 97, and Pluronic block copolymer, P123) amphiphiles with the PILs, ethylammonium nitrate (EAN), ethanolammonium nitrate (EOAN) and diethanolammonium formate (DEOAF), have been studied. The phase diagrams were constructed for concentrations from 10 wt% to 80 wt% amphiphile, in the temperature range 25 °C to >100 °C. Lyotropic liquid crystalline phases (hexagonal, cubic and lamellar) were formed at high surfactant concentrations (typically >50 wt%), whereas at <40 wt%, only micelles or polydisperse crystals were present. With the exception of Brij 97, the thermal stability of the phases formed by these surfactants persisted to temperatures above 100 °C. The phase behaviour of amphiphile-PIL systems was interpreted by considering the PIL cohesive energy, liquid nanoscale order, polarity and ionicity. For comparison the phase behaviour of the four amphiphiles was also studied in water.     

Structural organization and phase behaviour of 1-butyl-3-methylimidazolium hexafluorophosphate: an high pressure Raman spectroscopy study

The complexity of the phase diagram of a representative room temperature ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF(6)]) is explored by means of Raman spectroscopy at high pressure (up to 1000 MPa) and high temperature (from room temperature to 100 °C) conditions. The first experimental evidence of the existence of a second crystalline phase for this salt at high pressure conditions is provided. By comparing the low frequency vibrational bands for the liquid state and the two observed crystalline phases, we confirm the scenario that considers the crystal polymorphism in this class of materials as a consequence of the rotational isomerism of the butyl chain. Furthermore the pressure dependence of other vibrational bands indicates the existence of a structural rearrangement across p≈ 50 MPa at ambient temperature.     

HPLC-MS/MS enantioseparation of triazole fungicides using polysaccharide-based stationary phases

The enantiomeric separation of 21 triazole fungicides was carried out on four polysaccharide-derived chiral stationary phases in the reversed phase separation mode using high performance liquid chromatography coupled with tandem mass spectrometry. All fungicides were detected in electrospray ionization (ESI) positive mode with selected reaction monitoring (SRM). Complete enantioseparation was achieved for 21 fungicides except for difenoconazole based on cellulose tris (3,5-dimethylphenylcarbamate) and cellulose tris (3-chloro-4-methylphenyl carbamate) columns by optimizing experimental conditions including mobile phase and column temperature. Mobile phase was 0.1% formic acid aqueous solution mixed with methanol or acetonitrile in different proportions. Among all the fungicides, 15 with two enantiomers and three with four stereoisomers (bitertanol, bromuconazole, and cyproconazole) were successfully separated at 25°C. Enantioseparation for the other three fungicides (propiconazole, triadimenol, and difenoconazole) with four stereoisomers could be achieved by changing the column temperature from 10 to 40°C. Propiconazole and triadimenol were enantioseparated on baseline at 40 and at 35°C, respectively, and difenoconazole was enantioseparated partially with the R(s) > 1.1 at 25°C. Moreover, linearities and limits of detection (LODs) of 21 fungicides except for difenoconazole were studied, showing coefficients of determination (R(2)) higher than 0.99 and LODs lower than 2.5 μg/L.     

Vibrational spectra and structure of cyclopentane and its isotopomers

The infrared and Raman spectra of vapor, liquid, and solid state cyclopentane and its d(1), 1,1-d(2), 1,1,2,2,3,3-d(6), and d(10) isotopomers have been recorded and analyzed. The experimental work was complemented by ab initio and density functional theory (DFT) calculations. The computations confirm that the two conformational forms of cyclopentane are the twist (C(2)) and bent (C(s)) structures and that they differ very little in energy, less than about 10 cm(-1) (0.1 kJ/mol). The bending angle for the C(s) form is 41.5° and the dihedral angle of twisting is 43.2° for the C(2) form. A reliable and complete vibrational assignment for each of the isotopomers has been achieved for the first time, and these agree very well with the DFT (B3LYP/cc-pVTZ) computations. The ab initio CCSD/cc-pVTZ calculations predict a barrier to planarity of 1887 cm(-1), which is in excellent agreement with the experimental value of 1808 cm(-1).     

Nonionic diethanolamide amphiphiles with isoprenoid-type hydrocarbon chains: thermotropic and lyotropic liquid crystalline phase behaviour

The thermotropic and lyotropic liquid crystalline phase behaviour of a series of diethanolamide amphiphiles with isoprenoid-type hydrocarbon chains (geranoyl, H-farnesoyl, and phytanoyl) has been investigated. When neat, both H-farnesoyl and phytanoyl diethanolamide form a smectic liquid crystalline structure at sub-zero temperatures. In addition, all three diethanolamides exhibit a glass transition temperature at around -73 °C. Geranoyl diethanolamide forms a lamellar crystalline phase with a lattice parameter of 17.4 ? following long term storage accompanied by the loss of the glass transition. In the presence of water, H-farnesoyl and phytanoyl diethanolamide form lyotropic liquid crystalline phases, whilst geranoyl diethanolamide forms an L(2) phase. H-farnesoyl diethanolamide forms a fluid lamellar phase (L(α)) at room temperature and up to ～ 40 °C. Phytanoyl diethanolamide displays a rich mesomorphism forming the inverse diamond (Q(II)(D)) and gyroid (Q(II)(G)) bicontinuous cubic phases in addition to an L(α) phase.     

Phase behaviors of Gemini cationic surfactants/n-butanol/water systems

Phase diagrams for ternary system of the Gemini cationic surfactants, N,N-long chain alkyl-2-hydroxyl-N,N,N,N-tetramethyl diammonium dichloride (G_nCl₂) with butanol and water have been drawn based on experimental data at 25 °C. The phase diagrams show that L phase and different liquid crystalline phases are existent in the ternary system at different components. Electric conductivity of the L phase has been studied. Small-angle X-ray scattering (SAXS), ²H (deuterium) quadrupolar splitting (²H NMR) and the polarizing-light microscope were employed to confirm the characteristic texture structures and the microstructure of three different liquid crystalline phases.     

The tetragonal structure of nanocrystals in rare-earth doped oxyfluoride glass ceramics

Rare-earth doped oxyfluoride glasses and nanocrystalline glass ceramics have been prepared and studied by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) aiming at investigating the structure and the symmetry of the nanocrystal as well as the site of the rare-earth ion. To solve the problem encountered by previous researchers due to glass host interference, we etched off glass matrix and released the fluoride nanocrystal, which is more convenient for EDS measurement. A tetragonal phase model with the chemical formula as PbREF(5) proved by quantitative EDS and XRD analyses has been proposed in this paper for the first time. Two specific crystalline phases with the same space group have been observed at 460 °C-500 °C and 520 °C-560 °C, respectively. Moreover, a super "pseudo-cubic" cell based on our tetragonal model may give a good explanation to the probable previous cubic-symmetry misunderstanding by researchers. Additionally, the thermodynamic mechanism of phase transition and the thermal stability related to the structure of nanocrystals in glass ceramics have been studied and supported by ab initio calculations and experimental methods. The structure and thermal stability of the nanocrystal and clear environment of the rare-earth ion reported here have far-reaching significance with respect to the optical investigations and further applications of rare-earth doped oxyfluoride glass ceramics.     

Lithium hydroxide phase transition under high pressure: an ab initio molecular dynamics study.

The high-pressure phase transition in the deuterated lithium hydroxide crystalline state has been studied by Car-Parrinello molecular dynamics simulations, in the constant-pressure, constant-temperature ensemble. The recently developed metadynamics approach has been applied to encourage the system to transform into different phases in an affordable simulation time. A previously not completely characterized high-pressure phase has been obtained. The structural and spectroscopic properties have been studied and compared with the neutron scattering, infrared and Raman measurements. It has been found that the calculated structure differs slightly from the experimental hypothesis, and that the presence of strong hydrogen bonds is the source of the red shift and of the characteristic features of the OD-stretching bands in both IR and Raman spectra.     

Collective and non-collective excitations in antiferroelectric and ferrielectric liquid crystals studied by dielectric relaxation spectroscopy and electro-optic measurements

The dynamics of different molecular modes in four antiferroelectric liquid crystal substances have been studied by a combination of spectroscopic methods.The fastest motion is the reorientation around the molecular long axis, here found in the low GHz range by time domain spectroscopy. The reorientation around the short axis has a characteristic frequency of about 10kHz and is detected by frequency domain spectroscopy in the homeotropic configuration. As for the collective excitations, the Goldstone and soft modes, characteristic of the ferroelectric phase, have counterparts in the antiferroelectric phase which appear very different. There are two characteristic peaks in the spectrum, one at high frequency, about 100kHz, the other at low frequency, about 10 kHz. The latter has often been mistaken for short axis reorientation and both have been attributed to soft modes. By combining different experimental techniques and different geometries it can be shown that neither is a soft mode, but both are collective modes of different character: the high frequency mode corresponds to fluctuations where molecules in neighbouring layers are moving in opposite phase, the low frequency mode to phase fluctuations in the helicoidal superstructure. In materials exhibiting a C* phase in addition to the C*a or C* gamma phases, an additional strong peak appears in at least one lower-lying phase adjacent to the C* phase. We show that this peak, which we call a hereditary peak, has nothing to do with the antiferroelectric or ferrielectric order, but is just the Goldstone peak from a coexisting C* phase. In the same way, a Goldstone mode peak from the C* gamma phase may appear in the underlying C* a phase. In a general way, narrow phases like C* gamma, being bounded by first order transitions on both sides (C* a -C* gamma -C*) are likely to show non-characteristic (hereditary) peaks from both adjacent phases.     

Structure of CO2 adsorbed on the KCl(100) surface

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.     

High temperature structures and orientational disorder in compressed solid nitrogen

An experimental study of the phase transitions at high temperature in compressed solid nitrogen has been performed using Raman spectroscopy. Knowledge of the equilibrium phase diagram in the region of the ordered epsilon phase and the two disordered delta and deltaloc phases, at pressures between 10 and 20 GPa, has been extended up to 500 K. The Raman scattering line shape and line width of the active vibrons has been measured accurately, along isobaric scans, across the phase transitions. Analysis of the width and of its different behavior with increasing temperature in the three phases led to more precise conclusions about the nature of the disorder in the different phases. Observation of an evident shoulder in the nu2 band of the deltaloc phase suggests the possibility that sites of two different symmetries may be occupied by the disk molecules in this structure.     

3D interconnected ionic nano-channels formed in polymer films: self-organization and polymerization of thermotropic bicontinuous cubic liquid crystals

Thermotropic bicontinuous cubic (Cub(bi)) liquid-crystalline (LC) compounds based on a polymerizable ammonium moiety complexed with a lithium salt have been designed to obtain lithium ion-conductive all solid polymeric films having 3D interconnected ionic channels. The monomer shows a Cub(bi) phase from -5 to 19 °C on heating. The complexes retain the ability to form the Cub(bi) LC phase. They also form hexagonal columnar (Col(h)) LC phases at temperatures higher than those of the Cub(bi) phases. The complex of the monomer and LiBF(4) at the molar ratio of 4:1 exhibits the Cub(bi) and Col(h) phases between -6 to 19 °C and 19 to 56 °C, respectively, on heating. The Cub(bi) LC structure formed by the complex has been successfully preserved by in situ photopolymerization through UV irradiation in the presence of a photoinitiator. The resultant nanostructured film is optically transparent and free-standing. The X-ray analysis of the film confirms the preservation of the self-assembled nanostructure. The polymer film with the Cub(bi) LC nanostructure exhibits higher ionic conductivities than the polymer films obtained by photopolymerization of the complex in the Col(h) and isotropic phases. It is found that the 3D interconnected ionic channels derived from the Cub(bi) phase function as efficient ion-conductive pathways.     

Vibrational spectroscopic study on polymorphism of erucic acid and palmitoleic acid: γ1→α1 and γ→α reversible solid state phase transitions

The infrared and Raman spectra of four polymorphic phases (α, α1, γ and γ1) of erucic acid (cis-13-docosenoic acid) and those of two polymorphic phases (α and γ) of palmitoleic acid (cis-9-hexadecenoic acid) were investigated. The γ and γ1 phases of erucic acid were analyzed on the basis of crystal structures determined by us. There were large spectral differences between γ and γ1 phases, which could be ascribed to the differences in the conformation of cis-olefin groups and the subcell structure. Two types of reversible solid state phase transitions (γ→α and γ1→α1 transitions) were followed by the infrared and Raman spectra. It was concluded that the mechanism of the γ→α phase transition of erucic and palmitoleic acids is essentially the same as that of oleic acid previously reported by us [J. Phys. Chem. 90, 6371 (1986)], i.e. this phase transition is of order-disorder type accompanied by a conformational disordering at the methyl-terminal chain. Spectral changes on the γ1→α1 transition suggested that a similar structural change took place during this transition but there were large structural differences between α and α1.     

Raman spectra of titanium dioxide

First-order Raman spectra have been recorded at room temperature for the anatase and rutile phases of polycrystalline titanium dioxide using an argon ion laser as exciter. The high-temperature rutile phase was found to be stabilized at temperatures below 450°C. Anatase transforms to rutile phase at ～750°C. All the Raman active fundamentals predicted by group theory are observed.