Molecules oriented

Nuclear Magnetic Resonance, Volume 31 The Royal Society of Chemistry, 2002 

An overview of the apphcation of NMR to the field of liquid crystals over the past 50 years has been presented and the important aspects of proton and deuterium NMR have been delineated. The use of liquid crystals as solvents in NMR spectroscopy has been reviewed. Reviews on NMR studies of orientational order with 20 references, rotational diffusion of liquid crystals in the nematic phase and NMR spectroscopy in liquid crystals and membranes are available. A review on the application of spectroscopic methods to liquid crystalline phases has appeared during the period under report and it includes a discussion of the NMR methods. NMR and other methods used to determine the order parameters of nematics have been reviewed.  

A molecular dynamics simulation has been performed on 4-n-pentyl-4(-cyanobiphenyl (5CB) in the nematic phase. Order parameters and dipolar couplings have been calculated and used to test theoretical models. Theoretical models have also been developed to explain the shielding of a noble-gas atom in an anisotropic environment and applied to explain the medium-induced shielding of the noble gases Xe and Ne in the nematic liquid crystal 4(-ethoxybenzylidene-4-n-butylaniline (EBBA).  

Transverse nuclear relaxation due to director fluctuations in liquid crystals for the slow motion regime has been considered this leads to an analysis in detail of the transverse magnetization decay in different kinds of experi- 

---

An interesting aspect of two-photon spectroscopy is that some polarization infonnation is obtainable even for randomly oriented molecules in solution by studymg the effect of the relative polarization of die two photons. This is readily done by comparing linearly and circularly polarized light. Transitions to A states will absorb linearly polarized light more strongly than circularly polarized light. The reverse is true of transitions to B ... 

Figure C 1.5.13. Schematic diagram of an experimental set-up for imaging 3D single-molecule orientations. The excitation laser with either s- or p-polarization is reflected from the polymer/water boundary. Molecular fluorescence is imaged through an aberrating thin water layer, collected with an inverted microscope and imaged onto a CCD array. Aberrated and unaberrated emission patterns are observed for z- and xr-orientated molecules, respectively. Reprinted with pennission from Bartko and Dickson [148]. Copyright 1999 American Chemical Society.

Deliberately oriented polystyrene is available in two forms filament (mono-axially oriented) and film (biaxially oriented). In both cases the increase in tensile strength in the direction of stretching is offset by a reduction in softening point because of the inherent instability of oriented molecules. 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Below a temperature of Toi 260 K, the Ceo molecules completely lose two of their three degrees of rotational freedom, and the residual degree of freedom is a ratcheting rotational motion for each of the four molecules within the unit cell about a different (111) axis [43, 45, 46, 47]. The structure of solid Ceo below Tqi becomes simple cubic (space group Tji or PaS) with a lattice constant ao = 14.17A and four Ceo molecules per unit cell, as the four oriented molecules within the fee structure become inequivalent [see Fig. 2(a)] [43, 45]. Supporting evidence for the phase transition at Tqi 260 K is... 

For fibres and filaments such orientation is desirable, but for solid objects where impact strength is often more important than tensile strength such orientation is usually unwelcome. It can also have further unwanted effects. This arises from the fact that oriented molecules are basically unstable and will at the first opportunity try to coil up. Thus on heating samples up to temperatures near severe distortion can occur leading to warped mouldings. 

When carbon black (fine carbon black) is dispersed homogeneously and its volume fraction

less than nearly 0.2, the SH layers of adjacent carbon particles are still separated from each other in matrix mbber. In this case, the molecules in the SH layer shde, orient, and extend along the extension direction and finally produce strands of oriented molecules under large extension, as shown in Figure 18.8. In this situation, as the molecules inside the SH layer are extended much more... 

FIGURE 18.8 Formation of the strands of oriented molecules in the sticky hard (SH) layer under large... 

Super-network of the strands of oriented molecules interconnected at carbon particles produced under large extension supports the increasing stress (stress upturn) and the great stress of the system at break. 

According to the macroscopic model, the adsorption potential shift is due to the removal of some solvent molecules, s, from the surface region and accommodating there the oriented molecules of adsorbate, B."" Using the assumptions listed in Ref 114, the dependence for A% is of the form... 

Saupe, A., Englert, G. High-resolution nuclear magnetic resonance spectra of oriented molecules. Phys. Rev. Lett. 1963, 11, 462 64. 

Biochemical EPR samples are almost always collections of randomly oriented molecules (frozen) aqueous solutions in which each paramagnetic molecule points in a different direction. In order to generate simulations of these powder EPR spectra we have to calculate the individual spectrum for many different orientations and then add these all up to obtain the powder pattern. Numerical procedures that generate sufficient spectra to approximate a powder pattern are collectively known as walking the unit sphere algorithms. Here is the basic procedure ... 

This chapter considers the distribution of spin Hamiltonian parameters and their relation to conformational distribution of biomolecular structure. Distribution of a g-value or g-strain leads to an inhomogeneous broadening of the resonance line. Just like the g-value, also the linewidth, W, in general, turns out to be anisotropic, and this has important consequences for powder patterns, that is, for the shape of EPR spectra from randomly oriented molecules. A statistical theory of g-strain is developed, and it is subsequently found that a special case of this theory (the case of full correlation between strain parameters) turns out to properly describe broadening in bioEPR. The possible cause and nature of strain in paramagnetic proteins is discussed. 

Wasserman, E., Snyder, L.C., and Yager, W.A. 1964. ESR of the triplet states of randomly oriented molecules. The Journal of Chemical Physics 41 1763-1772. 

In latex technology, a submicro scopic aggregation of oriented molecules in polymer technology it is synonymous with crystallite. The term is also applied to the aggregates of soap molecules formed in emulsion polymerisation. Micro... 

Projections of molecular axes onto the surface plane form chain-like structures in which the chains with identically oriented molecules alternate (with the exception of oxygen molecules). The Davydov splitting of spectral lines represents the main spectroscopic manifestation of adsorbed structures with several orientationally inequivalent molecules in the unit cell of a two-dimensional adsorbate lattice. Many... 

Fig. 3.1. A planar lattice consisting of n Bravais sublattices of identical oriented molecules.

As seen from Chapter 2, adsorbed molecules often form monolayers with chain orientational structures in which the chains with identically oriented molecules alternate (Fig. 2.4). Consider the Davydov splitting of vibrational spectral lines in such systems. Let molecular orientations be specified by the angles 6> and spherical coordinate system with the z-axis perpendicular to the lattice plane ... 

Figure 3.27 shows the Mossbauer spectrum that results from splitting of the 57Fe excited state, a quadrupole doublet, for a sample containing randomly oriented molecules such as found in polycrystalline solids or frozen solutions. The two doublets are separated in energy by the quadrupole splitting, A Eq, defined by the... 

Figure 3.27 Typical Mossbauer spectrum for a sample containing randomly oriented molecules.

The highly oriented molecules in thin organic films such as Langmuir-Blodgett (LB) films and self-assembled monolayers (SAM) [1] are essential for some molecular functions. Non linear optical and opto-electronic properties are two of the most important and interesting functions of these molecular assemblies. In the past more than thirteen years, simulation of the primary process of photosynthesis using such molecular assemblies and its application to molecular photodiodes [2,3] have been one of the main subjects of our laboratory. 

---