Partially orientated molecules

These conclusions indicate that the Gibbs film, for such systems at any rate, is but one molecule thick and it is evidently important to consider how far we are justified in making the assumption that in all cases both for pure liquids and for solutions the Gibbs layer may be considered as similar in character, viz. unimolecular in thickness and consisting of orientated or partially orientated molecules. 

Fig 6. Schematic diagram of polarization measurements of a. Completely uniaxially oriented molecules, b. Two-dimensional system with partially oriented molecules, and c. Three-dimensional system with randomly oriented molecules. Axis of chromophoric group of the molecule lies along the double-headed arrows. The intensities of the incident exciting light and the fluorescence emission are represented by and //, respectively. The vertical and horizontal components of // are represented by J and /j respectively... 

Another classification of methods for measuring anisotropies is that of partially oriented molecules, the most important examples of which are the liquid crystals. The utility of this method lies in the fact that many molecules can be dissolved in liquid crystals without destroying the liquid crystal phase.12-15 The solute molecules are able to translate relatively freely in one direction-along the optic axis. As a result of this motion the intermolecular dipole-dipole interactions are averaged to zero but the intramolecular interactions are not. Also since the solute molecules cannot rotate equally in all directions, there will be an anisotropic contribution to the observed shielding. 

Finally, a recent publication by Warren and Moore - proposed the use of ML to measure residual dipolar coupling of partially oriented molecules for which only small data set are available. 

Shift anisotropy. If the chemical shift depends upon molecular orientation, as it does in an aromatic ring, then this will also contribute to V t) as a result of motionally induced fluctuations in the electron shielding field. The contribution increases with and typically contributes 0.02 Hz to 1/Ti, in high-field spectrometers. Again, if it arises in partially oriented molecules then it affects lineshape, this time asymmetrically. This is discussed further in chapters 5 and 7. 

New applications and perspectives of ID and 2D NMR spectroscopy for study of chiral organic compounds in the non-isotropic phases have been reviewed. The implementation of NMR of partially oriented molecules (e.g., molecules embedded in thermotropic liquid crystalline phases) opens new opportunities and is an excellent tool for revealing structures of molecules in solution. In details, this approach can be used for distinguishing of racemates and enantiomers, establishing of enantiomeric excess, and searching of absolute configuration and conformational changes. In these... 

The direct dipole—dipole interaction is a function (1) of the time average over the anisotropic solute tumbling motion and (2) of the internuclear distances. The nmr spectroscopy in liquid crystals may therefore provide information on both the average solute orientation and the molecular geometry. Moreover, information on the absolute signs of the indirect coupling constants, and on the anisotropic contributions to the chemical shift and the indirect coupling, respectively, may be obtained from the nmr-spectra of partially oriented molecules. 

In this equation Azy = z. — Zj Axjj = Xj — Xj and Ay j = y — yj, where Xj, yj, Zj are the coordinates of the ith nucleus in the molecular fixed coordinate system. The angular brackets < take account for the fact that the internuclear distances are averages over the internal molecular motions. The above equation provides the basis for the determination of the molecular geometry from an analysis of the nmr spectra of partially oriented molecules. 

According to the above discussion a quantitative evaluation of the absorption spectra in oriented solvents is much simpler than an analysis of magnetic resonance spectra of partially oriented molecules. However, this is only valid if the axes of preferred solute orientation (or the principal axes of the order matrix) coincide with the directions of the optical transition moments. The situation may be much more complicated if molecules of lower than Cjv symmetry are considered. In these cases the principal axes system of the order matrix and the directions of the OO-transitions moments do not coincide. An example for such a molecule is chrysene [7]. In these cases the liquid crystal method yields only the relative polarization directions. A way out of these difficulties could be a systematic study of substituted molecules. It is well known that the substituents such as methyl groups do not shift the polarization direction appreciably but such groups may have large effects on the average orientation of the molecule in the liquid crystal. 

Photoselection and photoorientation This elegant method allows one to obtain a set of partially oriented molecules in a completely isotropic medium. It makes use of the fact that the photoexcited ensemble of molecules is always anisotropic, and thus exhibits LD (transient dichroism). Moreover, if the excitation is followed by chemical transformation, and the environment is rigid enough to prevent molecular rotation, a permanent alignment of both reactant and product is obtained. Such photooriented samples can be studied by conventional LD techniques. Particularly attractive media for use in photoorientation are low-temperature rare gas matrices, which are inert and transparent in both the UV-visible and IR regions. 

In partially oriented systems (in liquid crystal solution or in strong electric fields) only linear combinations of tensor elements, such as — or Jaa — Jbb + Jcc)l may be obtained. The spin Hamiltonian for partially oriented molecules, spectral analysis, determination of NMR constants, and results from many applications have... 

Experiments were carried out on partially oriented molecules in the nematic phase, except for MeHgNOj, in which values measured in the nematic and lyotropic phases have been reported. 

It is occasionally desirable to retain a small proportion of molecular orientation, in order to quantitate the dipolar interactions present, whilst minimizing their contribution to the linewidth. Partial orientation may be achieved by using a nematic solvent. In large, magnetically anisotropic molecules it may occur naturally at the highest magnetic fields. 

This chapter discusses stereospecific intermolecular interactions in monolayers at the air-water interface, where surface-active molecules (surfactants) are partially oriented with respect to each other by the cooperative combination of interionic, hydrophobic, and hydrogenbonding forces. We believe that these reports should be of particular interest in relation to stereospecificity in assemblies such as micelles, vesicles, and bilayer membranes, where their significance has been largely ignored. 

It has already been observed in the case of pure liquids such as substituted hydrocarbons that the surface tension was, if not completely defined by the non-polar portions of the molecule, at least not so markedly affected by substituent groups as we should anticipate if no orientation existed. Again, although vaporisation and condensation at a liquid surface such as water at high temperatures takes place with great speed, yet the life of a molecule on the surface is probably long enough to permit of such adjustment as orientation requires. Even more conclusive in favour of the hjq)othesis of at least partial orientation is the evidence derived from a consideration of the latent heats of evaporation and the... 

Experimentally in an isotropic phase and without any partial orientation of the solute molecules, it is only possible to observe the average value or trace of the second rank J tensor. In the current literature this quantity is commonly referred to as the scalar coupling , which is a somewhat misleading expression. Obviously, it would be more correct to call it the isotropic coupling as can be seen in several papers however to follow the common usage in this work these expressions will be used indistinctly. 

The dielectric constant e of a gas sample depends on the total dipole moment induced in response to an applied electric field. The dipole moment is the vector sum of the partially oriented permanent dipoles pt which individual molecules i may possess, plus the field-induced dipoles a, E arising from the polarizability a of the molecules i, plus all interaction-induced dipoles fiik, plus the field-induced dipoles which arise from the interaction-induced polarizability ptk [93]. The dielectric constant e depends, therefore, on the density g of the gas, according to... 

The water molecules that are in immediate contact with dissolved nonpolar groups are partially oriented. They form a cagelike structure around each hydrophobic group. When particles surrounded by such hydration layers are 1-2 nm apart, they sometimes experience either a fairly strong repulsion or an enhanced attraction caused by these hydration layers.21 64 66,72 Direct experimental measurements have shown that these effects extend to distances of 10 nm21,63 and can account for the previously mentioned long-range van der Waals forces. 

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