A special method of deformational correction in N.M.R. of molecules dissolved in liquid crystals

A simple method of deformational correction for the N.M.R. spectral parameters of molecules dissolved in liquid crystals is described. The method is applicable to the different isotopically substituted derivatives of the molecules belonging to the cubic point groups. Its validity is verified for the dipolar and quadrupolar couplings observed in the molecules CH₄, CD₄ and CH₃D dissolved in the liquid crystals Merck Phase IV, Merck ZLI 1167 and their mixtures.     

2H-N.M.R. studies of flexibility and orientational order in nematic liquid crystals

Abstract

Deuteron magnetic resonance spectroscopy (²H-N.M.R.) has been used to investigate the effect of the nematic environment on the flexibility and orientational order of two perdeuteriated cyanobiphenyl homologues: 4-methyl-4′-cyanobiphenyl (1CB-d ₁₁) and 4-n-pentyl-4′-cyanobiphenyl (5CB-d ₁₉). The systems studied were low concentrations of 1CB-d ₁₁ and 5CB-d ₁₉ dissolved in the nematic phases of 5CB, N-(-4-ethoxybenzylidene)-4′-n-butylaniline (EBBA), Merck ZLI-1132 (1132) and a 55wt% mixture of 1132: EBBA. The spectra are dramatically different in these environments. Previous studies on small solutes have suggested that in the 55wt% 1132: EBBA mixture (at 301.4 K) the dominant orienting mechanism depends on the size and shape of the molecule which suggests that it is a short range repulsive interaction. This interaction has been modelled by treating the liquid crystal as an elastic continuum and the solute as a collection of van der Waals spheres which stretch the liquid crystal in the two dimensions perpendicular to the director. The distortion of the liquid crystal depends on the dimensions of the solute, and the elastic energy is described in terms of a Hooke's law force constant, k. The model is extended to include flexible liquid crystal molecules and quadrupolar couplings are calculated for each conformation of the 5CB chain. Statistical averaging over all conformations gives an excellent fit to the experimental spectrum. The results for 1CB and 5CB show that in the other nematic phases contributions from additional mechanisms must be included. Previous studies of ²H₂ and other solutes indicate that the additional mechanism is the interaction between the solute molecular quadrupole moment and the mean electric field gradient of the liquid crystal.     

Probing director fluctuations in nematic solutions by N.M.R. The breakdown of the simple RD + DF model for tolan in nematic Phase V

Abstract

Frequency dependent relaxation measurements have been carried out on the para deuterons of tolan-d ₁₀ (diphenylethyne), dissolved in Merck Phase V. The results are compared with those of three smaller probe molecules, studied previously. The simple RD + DF model which works well for the smaller probes, cannot give a satisfactory explanation for the relaxation behaviour, in particular the strong relaxation dispersion, of the para deuterons of tolan. The simple RD + DF model does not account properly for the effects of director fluctuations if the degree of orientation of the probe molecule studied is too high; this appears to be the case for tolan.     

Solvent dependence of the molecular structures of furan and thiophene determined by NMR of partially oriented molecules

The molecular r_α structures of furan and thiophene dissolved in liquid crystals Merck ZLI 1167 and Phase IV were determined from the linear combinations of dipolar H? H and C? H coupling constants derived from the ¹H and ¹H? ¹³C satellite NMR spectra. Significant variations with the liquid crystal solvent are observed for both molecular structures. The deformations are particularly large in the case of thiophene and, surprisingly, in the normally well behaved ZLI 1167 liquid crystal.     

Linear dichroism of molecules with cubic symmetry

Linear dichroism (LD) of chromophores which belong to the cubic point group of symmetry has been observed. The molecules CrO²⁻₄, Fe(CN)³⁻₆ and Mo(CO)₆ were dissolved in several lyotropic liquid crystals. These systems can be macroscopically aligned which is a prerequisite in LD spectroscopy. The observed LD is ascribed to distortions of the cubic symmetry caused by the uniaxial constraints in the liquid crystals. Contributions to the LD due to the inherent optical birefringence of the systems were taken into account. Furthermore, NMR of CrO²⁻₄ and ESR of Mn²⁺ in various lyotropic liquid crystals strongly support that the symmetry is distorted.     

Measurement of the rotational viscosity, γ1 of nematic liquid crystals under high pressure

Abstract

A method for the measurement of the rotational viscosity, γ₁ of nematic liquid crystals under high pressure is described. First measurements for the liquid crystals 4'-methoxybenzylidene-4-n-butylaniline, a broad range nematic mixture of substituted cyclohexyl-phenyls and a re-entrant nematic mixture are presented.     

NMR in chiral polypeptide liquid crystals: the problem of amines

It is shown that lyotropic liquid crystal mixtures made of poly-γ-benzyl-l-glutamate (PBLG) dissolved in N,N-dimethylformamide (DMF) are efficient anisotropic NMR solvents to distinguish the enantiomers of chiral amines through the effects of the differential ordering of enantiomers. This type of solvent overcomes problems often encountered when dissolving amines into the more conventional PBLG/CHCl₃ or PBLG/CH₂Cl₂ liquid crystals. Furthermore, it is shown that perdeuterobenzyl chloride is an excellent achiral deuterated derivatizing agent for enantiomeric excess measurements of chiral amines in conjunction with the PBLG/DMF solvent.     

Sorption and partial molar volume of gases in poly (dimethyl siloxane)

Sorption of N₂, O₂, Ar, CH₄, CO₂, C₂H₄, and C₂H₆ in poly (dimethyl siloxane) liquid and rubber and the dilation of the polymers due to sorption of the gases are studied at 25°C under pressures up to 50 atm. In the liquid, the sorption isotherms for low-solubility and high-solubility gases are described by Henry's law and the Flory–Huggins equation, respectively. Gas sorption in the rubber, which contains a 29 wt % silica filler, follows the dual-mode sorption model, though marked hysteresis is observed in the sorption of O₂ and CH₄. The dilation isotherms increase linearly or exponentially in both polymers with increasing pressure. Considering that gas molecules adsorbed into micropores of the filler particles do not participate in the dilation, partial molar volumes of the dissolved gases in the rubber are determined from data of sorption and dilation. The values are nearly equal to the partial molar volumes in the liquid (48–60 cm³/mol).     

Dimethyl sulfoxide as an O-donor ligand in tetravalent actinide complexes

Four new perchlorate complexes of tetravalent actinides with dimethyl sulfoxide (DMSO) molecules (An⁴⁺ = Th, U, Np, Pu) are synthesized and studied. According to the X-ray diffraction data, compounds [Th(DMSO)₉](ClO₄)₄ · 2CH₃CN (I), [U(DMSO)₈](ClO₄)₄ · CH₃CN (II), [Np(DMSO)₈](ClO₄)₄ · CH₃CN (III), and [Pu(DMSO)₈](ClO₄)₄ · CH₃CN (IV) crystallize in the triclinic crystal system (space group P1). The crystals of compounds II–IV are isostructural. The absorption spectra of the complexes in the IR and visible regions are measured. All compounds exhibit a decrease in the frequencies of stretching vibrations ν(SO) over the spectrum of free DMSO, indicating the formation of the O-bonded complexes of An⁴⁺. The optical spectra of the crystalline compounds exhibit shifts of the bands of electronic f-f transitions of the An⁴⁺ ions relative to the hydrated ions: the bathochromic shifts for the U and Np complexes and the hypsochromic shift for the Pu complex. The first coordination sphere of the actinide atoms in the studied complexes is highly stable.     

NMR relaxation of rigid molecules in the nematic phase of a discotic liquid crystal

Abstract

Frequency and temperature dependent NMR relaxation measurements were performed on deuteriated benzene, pyrene and triphenylene dissolved in the nematic phase of a discotic liquid crystal. The results show a strong frequency dependence of the spectral densities. Based on the symmetries of the system and the usual model for director fluctuations this frequency dependence should be equal for J ₁ and J ₂. From fitting the commonly used model of rotational diffusion and director fluctuations to the data we see that this is not the case for benzene and triphenylene, even though the fits themselves are satisfactory. Values for the elastic constants, effective viscosity and translational diffusion in similar discotic liquid crystals do not account quantitatively for the frequency dependence of benzene. For both pyrene and triphenylene quantitative comparison was impossible due to lack of translational diffusion data. We also find that the so-called cut-off wave-length is of the order of the dimensions of the liquid crystal molecules, just as in ordinary nematics.     

Molecular solutes in nematic liquid crystals: Orientational order and electric field gradients

The difference between liquid-crystal and gas-phase values for the nuclear quadrupole coupling constant in D₂ and HD is used to obtain the mean electric field gradient in various liquid crystals. Order parameters for small molecules dissolved in liquid crystals are calculated assuming that the orientational order arises from the interaction of the molecular quadrupole moment with the average field gradient. The results obtained are in good agreement with experimental values for hydrogen and several other solutes.     

Syntheses,crystal structures and thermal behavior of three Mn(II) complexes with 4-acyl pyrazolone derivatives

Three new manganese complexes, [Mn(PPeAP-INH)(CH₃OH)₂]_n(CHCl₃) (1) [PPeAP-INH=N-(1-phenyl-3-phenylethyl-4-ethylene-5-pyrazolone isoniazid, INH = isoniazid], [Mn₂(PM4MbP-INH)₂(CH₃OH)₄](CH₃OH) (2·CH₃OH), [PM4MbP-INH=N-(1-phenyl-3-methyl-4-(p-methylbenzoylene)-5-pyrazolone) isoniazid)] and [Mn₂(PPePeP-INH)₂(CH₃OH)₄] (3) [PPePeP-INH=N-(1-phenyl-3-phenylethyl-4-phenylethylene-5-pyrazolone) isoniazid] have been prepared, aiming at the formation of extended coordination polymer with Mn(II) ions. Formation of the corresponding manganese-ligand complexes has been monitored and characterized by chemical and spectroscopic techniques. X-ray diffraction analyses of available single crystals revealed 1D polymeric patterns of supramolecular system. Among them, the dimers of 2 and 3, bridged by two methanol molecules, are extended by hydrogen bond of O–H···N to form the 1D chain. In these three new complexes which are all octahedral coordination environments, the N(5) atom of the pyridine heterocycles plays a very important role in building the bridged polymeric chain.     

Synthesis of solvates of the copper(II) complex with chiral caryophyllane-type morpholino oxime (HL). Structure of Cu(HL)Cl<Subscript>2</Subscript> · CHCl<Subscript>3</Subscript>

Paramagnetic Cu(HL)Cl₂ · 0.25CHCl₃ (I) and Cu(HL)C1₂ · 0.25CH₂C1₂ (II), where HL is the optically active morpholino oxime obtained from the terpenoid caryophyllene, were synthesized. The crystals of Cu(HL)Cl₂ · CHCl₃ (III) were isolated. According to X-ray diffraction data, the crystals of III are composed of acentric mononuclear complex molecules Cu(HL)Cl₂ and solvate molecules CHCl₃. In the complex molecules, the Cu ion coordinates two N atoms of the bidentate chelating ligand HL and two C1 atoms at the vertices of a distorted tetrahedron. The translationally identical molecules of the complex combined by H-bonds form chains along the axis x.     

Semiclassical calculation of energy transfer in polyatomic molecules. XII. Organic molecules

A semiclassical method for energy transfer to the torsional motion of polyatomic molecules is presented. It is shown that a purely classical treatment of the torsional motion is problematic due to the zero-point vibrational energy which may migrate into other modes. State-to-state cross sections for the excitation of the CH₃ torsion in CH₃OH colliding with ⁴He are presented as a function of initial kinetic energy.     

Highly polarized absorption and emission from polymer-stabilized smectic guest-host systems

ABSTRACT

The optical properties of luminescent molecules dissolved in liquid crystals have led to their proposed use in luminescent thin-film polarizers. These molecules are typically required to fulfil a range of criteria related to their absorption and emission properties, degree of alignment and stability; however, concurrently satisfying these requirements has proven a barrier in their practical use. We obtained highly polarized absorption and emission luminescent thin-film polarizer using reactive liquid crystalline monomers and highly dichroic luminescent dyes, both of which have polymerizable end groups. In situ photopolymerization of the liquid crystalline mixtures in the highly ordered SmB phase resulted in the formation of cross-linked polymeric networks in which the anisotropic absorption and emission of the film were fixed. The as-obtained product exhibited a high dichroic ratio (DR = 30) with a large fluorescence quantum yield (?_F = 0.77). The device is both cheap and easy to fabricate and has the potential to be used in practical electro-optic applications.     

Differential dichroic spectroscopy in liquid crystals using unpolarized radiation

An unpolarized, differential technique is described for obtaining the dichroic spectra of uniaxial liquid crystals from a single spectral scan. Using this method, we investigate the near infrared dichoic spectrum of n-heptyl cyanobiphenyl (7CB). The temperature dependence of the orientational order parameter has been derived close to the nematic-isotropic transition and this is in good agreement with earlier Raman data. The differential method is also advantageous for dichroic studies on oriented guest molecules in liquid crystals. Illustrative results are presented for p-nitrotoluene dissolved in a nematic medium.     

Orientational ordering of 4-substituted phenylcyclohexanes studied by carbon-13 two-dimensional N.M.R.

Abstract

An N.M.R. method combining the techniques of separated local field spectroscopy (SLF) and variable angle spinning (VAS) is valuable in the investigtion of nematic liquid crystals. Rapid sample spinning causes the nematic director to align along the spinning axis, resulting in narrow peaks in the C-13 N.M.R. spectrum. SLF is a two-dimensional N.M.R. method which produces a first order splitting pattern for each carbon signal from which C–H dipolar coupling constants can be determined. The order parameters for all segments of the liquid crystal molecule can then be calculated. Results for three 4′-cyanophenylcyclohexanes are considered here. These compounds are trans-substituted at the 4 position of cyclohexane ring with n-pentane (PCH5), 1-pentene (3d ₁CP) and 3-pentene (1d ₃CP), respectively.     

Vibrational population lifetimes of polyatomic molecules in liquids

CH-stretching modes were first excited by picosecond infrared pulses and the generated excess population was monitored by anti-Stokes scattering of subsequent ultrashort probe pulses. Experimental data are reported on five molecules: CHCl₃, CH₂Cl₂, CH₃CCl₃, CH₃CH₂OH, and CH₃I in the neat liquid and/or in solutions of CCl₄. The observed time constants vary between 1 and 100 ps depending upon the individual molecule and surrounding. Theoretical calculations show that rotational coupling, Fermi resonance, Coriolis coupling, and resonance energy transfer can strongly effect the vibrational population lifetime. The relevance of these processes is quite different for the various molecules investigated.     

Inelastic neutron scattering from matrix isolated species

Matrix isolation is an experimental method in chemistry that is widely used for the preparation of samples for spectroscopic studies. It makes it possible to stabilize species which are unstable at room temperature, to isolate molecules in solids from each other, and to carry out molecular spectroscopy at low temperatures. Matrix isolated molecules are studied by a variety of techniques. In this paper the application of inelastic neutron scattering (INS) to matrix isolation is reviewed. Molecules that contain hydrogen atoms are diluted in inert gases such as argon, krypton and nitrogen, and then condensed inside a liquid helium cryostat of the type that is in use at many neutron spectrometers. For this work we developed a technique for the vapor deposition of inert gases with dopants that have low vapor pressure. INS focuses on some aspects of matrix isolation which are not appropriately covered by other spectroscopies, mainly solid state aspects of matrix isolated molecular aggregates and of the matrix itself. Neutron scattering is used to observe optically forbidden excitations such as methyl librations, tunnelling transitions, phonons, and rotational transitions involving a nuclear spin flip. Moreover, a direct correlation of spectroscopic data with the powder diffraction pattern of the matrix is possible with this technique. Localized modes and phonon densities of states can be observed in the same sample and may then be related to the respective diffraction pattern. The vapor deposited samples can be characterized in this way, and possible structural faults in rare-gas lattices revealed which are not usually recognized by other techniques. The structure of molecular aggregates has been elucidated by neutron spectroscopy of their low frequency internal intermolecular modes. HCN forms long linear chains, CH₃CN antiparallel dimers. Both species may, be understood as intermediates for the formation of the respective crystals. The structure of the matrix cages, in which single molecules are embedded, is explored by recording rotational and translational localized modes of these molecules. The single particle rotations of HCl, H₂O, NH₃, and CH₄ were studied directly. Tunnel splittings and librational spectra were recorded from molecules with methyl groups [CH₃CN, CH₃I, CH₃COCH₃, C₃H₈, C₄H₁₀, Sn(CH₃)₄, and others]. The mutual influence of translational modes of the guest and phonons of the host can be studied, since the neutron spectra are directly connected with the phonon density of states of the system. The librational spectrum of N₂ in Ar and a local mode of H₂ in solid D₂ are presented and compared with theoretical calculations.     

Crystal structure and solid state IR-LD analysis of a mononuclear Cu(II) complex of 4-aminopyridine

A new mononuclear Cu(II) complex of 4-aminopyridine (4AP), [Cu(4AP)₄]Cl₂·2CH₃OH (1), was obtained and structurally characterized by single-crystal X-ray diffraction methods. The complex is square-planar involving four monodentate 4AP molecules coordinated through the N_py atoms. A solid-state linear-dichroic spectroscopic analysis (IR-LD) as a suspension in a nematic liquid crystal was carried out over the 4000–400?cm^?1 range and determined stereochemical parameters compared crystallographic data. Detailed IR characteristics are reported.