Intermolecular quadrupolar

Intermolecular quadrupolar 2 Fluctuation of the electric field gradient, moving multipoles Common for />1 In free Ions In solution [la... 

Figure 12 Discotic mesogens that, in certain mixture combinations, showed modified mesomorphism on account of intermolecular quadrupolar interactions.

The quadrupolar quasi-static coupling in NF3 — 7.068 MHz— is very close to the value obtained from microwave measurements — 7.07 MHz — this feature may be interpreted as a clue that intermolecular effects are negligible in this compound unless, by a fortuitous coincidence, thes intermolecular effects are cancelled by some other mechanism, such as a molecular deformation in the solid (the coupling is very sensitive to slight variations of the pyramidal angle, as discussed below). 

Quadrupolar relaxation is frequently used to obtain information about molecular motion and intermolecular interactions in liquids and solutions. 

While eq N) is the largest principal component of the electric field gradient (EFG) tensor at the site of the quadrupolar nucleus. All the information about the structure of the molecule and interactions is in the eq N) term. In the gas-phase (at low symmetry positions) the EFG is entirely of intermolecular origin. In the condensed phases (liquids and solids), intermolecular interactions may have a substantial influence on the observed EFG. 

The application of relaxation time measurements to study segmental motion (in polymers) as well as diffusional chain motion is very well documented but is still a subject of study, particularly using the frequency dependence of relaxation times to test the detailed predictions of models (McBriety and Packer 1993). The anisotropy of reorientation can also be studied conveniently, and recent interest in motion of molecules on surfaces (e.g. water on porous silica) has been investigated with great sueeess (Gladden 1993). Since the dipolar interaction is usually both intermolecular and intramolecular, the relaxation of spin- /2 nuclei (e.g. H) in the same molecule as a quadrupolar nucleus (e.g. H) can permit a complete study of reorientation and translation at a microscopic level (Schmidt-Rohr and Spiess 1994). 

A few solvents exhibit intermediate properties. For example, both BTF (Ref. 22) and F-626 (Ref. 23 see Section 3.1) are able to dissolve appreciable quantities of both fluorous and non-fluorous solutes. Other solvents are often mistakenly assumed to be fluorous. Of these, the most important are perfluoroarenes such as hexafluo-robenzene. The arene tt clouds and sp carbon-fluorine bonds lead to significant intermolecular bond dipoles, induced dipoles and quadrupolar interactions with non-fluorous molecules. ... 

The first attempt to develop a statistical model of the cholesteric phase was by Goossens who extended the Maier-Saupe theory to take into account the chiral nature of the intermolecular coupling and showed that the second order perturbation energy due to the dipole-quadrupolar interaction must be included to explain the helicity. However, a diflUculty with this and some of the other models that have since been proposed is that in their present form they do not give a satisfactory explanation of the fact that in most cholesterics the pitch decreases with rise of temperature. 

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