Anisotropic solvent

Benzene is an isotropic solvent its viscosity is the same in every direction. However, a liquid crystal solvent is an anisotropic solvent its viscosity is smaller in the direction parallel to the long axis of the molecule than the perpendicular direction. Methylhenzene is a small, spherical molecule, so its interactions with either solvent are similar in all directions. 

Running a sample in an anisotropic solvent like D6-benzene or D5-pyridine, can bring about some even more dramatic changes in chemical shifts. We tend to use benzene in a fairly arbitrary fashion, but in some cases, there is a certain empirical basis for the upfield and downfield shifts we observe. 

Liquid crystals are usually excellent solvents for other organic compounds. Non-mesomorphic solute molecules may be incorporated into liquid-crystalline solvents without destruction of the order prevailing in the liquid-crystalline matrix. The anisotropic solute-solvent interaction leads to an appreciable orientation of the guest molecules with respect to the axis of preferred solvent alignment. The consequences may be useful as shown by the use of liquid crystals as anisotropic solvents for spectroscopic investigations of anisotropic molecular properties [166]. Ordered solvent phases such as liquid crystals have also been used as reaction media, particularly for photochemical reactions cf. for example [111, 155, 163] and Section 5.5.9. 

When a dipolar molecule is dissolved in a magnetically anisotropic solvent consisting of disc-shaped molecules, e.g. benzene, the NMR signals of the solute H-atoms are usually shifted upheld with respect to their positions in an isotropic solvent such as 2,2-dimethylpropane (neopentane) or tetraehloromethane cf. the preceding discussion of (Ta in Eq. (6-21). The specific solvent-induced H NMR chemical shift of a solute H-atom signal when the solvent is changed from a reference aliphatic solvent to an aromatic solvent is called aromatic solvent-induced shift (ASIS) and is defined according to Eq. (6-24) [278, 279],... 

Janossy, 1991 Kremer, 1992 Zentel et al., 1992 Eich, 1993). Since the liquid cry.s-talline order is sensitive to temperature changes, this is used to visualize temperature differences of a substrate (e.g. Grabmaier, 1975). For special display effects but even more in science, liquid crystals are employed as anisotropic solvents for aligning solute or probe molecules. Just to mention one applicational field different from optics, liquid crystals are used as high-performance stationary phases in both, gas and liquid chromatography (Pesek and Wiliiamsen, 1992). 

Utilized selective anisotropic solvent adsorption on specific crystal faces to favour the growth of morphologically polar crystals. Some additional reports of the study of crystal modification and nonlinear optical activity include those on anhydrous and hydrated sodium / -nitrophenolate (Brahadeeswaran et al. 1999), derivatives of 2-benzylideneindan-l,3-dione (Matsushima et al. 1992), straight-chain carbamyl compounds (Francis and Tiers 1992), benzophenone derivatives (Terao et al. 1990), a 1,3-dithiole derivative (Nakatsu et al. 1990), o -[(4 -methoxyphenyl)methylene]-4-nitro-benzene-acetonitrile (Oliver et al. 1990) and so-called lambda shaped molecules (Yamamoto et al. 1992). Hall et al. (1988) followed the thermal conversion of the centrosymmetric P2 /c) form of 2,3-dichloroquinazirin to the non-centrosymmetric Pc form by monitoring the development of an SHG signal. Consistent with the earlier observation, the centrosymmetric form was obtained under equilibrium conditions, while the non-centrosymmetric one could be obtained under more kinetic conditions. 

The exposition of the PCM procedures has been limited to the aspects we consider more important for the study of chemical reactions. We would like to point out here that there are other extensions that use more specialists medium models (for example non-homogeneous, Cossi et al., (1994), and anisotropic solvents, Mennucci et al. (1995, 1996)) or that are addressed to the study of specific classes of phenomena (for example electric and magnetic susceptivity, non linear optics Cammi et al., 1995b, 1996). After saying this about PCM, we would like to dispel any false impression... 

Fluorescence polarisation studies on acenaphthylene labelled polyacrylic acid has demonstrated complexation occurs with polyethylene oxide to give a tightly packed and sterically restricted conformation SS. Neutralisation of the acidic groups restored the polarisation of the fluorescence and together with time resolved fluorescence analysis indicated that the motion of the probe is anisotropic. Solvent partitioning has been observed between glycol methacrylate copolymers from the fluorescence anisotropy changes of... 

Poulin P, Cabuil V and Weitz D A 1997 Direct measurement of colloidal forces in an anisotropic solvent Phys. Rev. Lett. 79 4862-5... 

In a magnetic field the direction of orientation is rotated so that the long axes of the molecules are all largely oriented parallel to the magnetic field. This results from the interaction of the magnetic susceptibility of the solvent molecules. The liquid crystals provide an anisotropic solvent for the compound to be studied. The effects of this on the NMR spectrum are considered shortly. 

The integral equation formulation PCM (lEF-PCM) method is a generalization of the PCM method that allows one to deal with anisotropic solvents such as liquid crystals, as well as with isotropic solvents [E. Canc s, B. Mennucci, and J. Tomasi, J. Chem. PhysL, 107, 3032 (1997)]. lEF-PCM has given good results when applied to aqueous ionic solutions [M. Cossi et al., Chem. Phys. Lett., 286,253 (1998)]. 

A crucial step in the manufacture of many complex materials is the orientation of the constituents in a solvent such that they can be deposited on a substrate with a desired orderly structure. Several methods have been considered for an efficient solute orientation in a solvent [123, 124], but they all rely on the presence of a nanoparticle permanent dipole or considerable dielectric contrast between the particle and the medium [125, 126]. A newly proposed method of orienting nanoparticles [53] exploits the coupling between the field-alignment of polar solvent molecules and anisotropic solvent-solute interactions due to solvent... 

A 10% solution of 2-naphthaldehyde in the nematic liquid-crystal mixture 4-cyano-4 -alkylbicyclohexyls (ZLI-1695, Merck) (Figure 2-19) was prepared for the measurements. This nematic phase is very convenient as an anisotropic solvent because of its weak IR spectrum, allowing one to obtain polarization spectra in practically the whole mid-IR region. 

It is straightforward to measure the birefringence as function of temperature [8] as well as strain [9]. One experimental consideration to take into account is the change in sample thickness due to strain. The response can be readily xmderstood in terms of the simple model described in the Introduction. LCEs have been swollen with both isotropic and anisotropic solvents [8, 10, 11]. Isotropic solvents reduce the number density of mesogens and, hence, the critical temperature = pU/5k. They also reduce the crosslink density and the influence of the network. Nematic solvents, on the other hand, up to first order do not reduce the mesogen density, but... 

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