The molecule immobilized in an electric field

ELECTRIC PHENOMENA 12.2 THE MOLECULE IMMOBILIZED IN AN ELECTRIC FIELD 

The electric field intensity 5 at a point represents the force acting on a unit positive point charge (probe charge) S = —VK, where V stands for the electric field potential energy at this point. When the potential changes linearly in space 

The last term can be easily calculated ff om the positions of the nuclei. The first term requires the calculation of the one-electron integrals. Note, that the resulting formula states that the forces acting on the nuclei follow from the classical Coulomb interaction involving the electronic density p, even if the electronic density has been (and has to be) computed from quantum mechanics. 

instead of a unit charge, we shift the charge Q, then the energy will change by — Qx. 

It is seen that if we change the direction of the shift or the sign of the probe charge, then the energy will go up (in case of the stone we may change only the direction). 

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ELECTRIC PHENOMENA The Molecule Immobilized in an Electric Field (4(g))... 

Because a layer of immobile ions and water molecules exists at the surface of the particle it is not easy to directly measure the surface potential of particles. Instead, a closely related potential known as the zeta potential is usually measured. The zeta potential can be determined by measuring the particle s velocity in an electric field. The zeta potential is the potential at the plane of shear between the immobilized surface layer and the bulk solution. This plane is typically located only a few Angstroms from the surface so that there is little difference between the zeta potential and the surface potential. In practice, the zeta potential can be used in place of the surface potential in Equation (5.11) to predict the interparticle forces as a function of separation distance with little... 

The mechanism by which analytes are transported in a non-discriminate manner (i.e. via bulk flow) in an electrophoresis capillary is termed electroosmosis. Eigure 9.1 depicts the inside of a fused silica capillary and illustrates the source that supports electroosmotic flow. Adjacent to the negatively charged capillary wall are specifically adsorbed counterions, which make up the fairly immobile Stern layer. The excess ions just outside the Stern layer form the diffuse layer, which is mobile under the influence of an electric field. The substantial frictional forces between molecules in solution allow for the movement of the diffuse layer to pull the bulk... 

Many polymeric materials consist of dipoles (chemical bonds which have an unbalanced distribution of charge in a molecule) and traces of ionic impurities. If a polymer containing polar groups is heated so that an immobile dipole becomes mobile, an increase in permittivity is observed as the dipole starts to oscillate in the alternating electric field. This effect is referred to as a dipole transition and has a characteristic relaxation time (t) associated with it (76). When exposed to an electric field, the dipoles tend to orient parallel to the field direction and the ions move toward the electrodes, where they form layers. The dipole relaxation time... 

Nonpolar and dipolar altitudinal rotors (compounds 2 and 3 in Fig. 17.3) have been synthesized. 19F NMR spectroscopy showed that the barrier to rotation in 3 was extremely low in solution. Both systems have then been immobilized on Au(l 11) surfaces and studied with a variety of techniques.57 The results obtained indicated that for a fraction of molecules the static electric field from the scanning tunneling microscopy (STM) tip could induce an orientation change in the dipolar rotor but not in the nonpolar analog (for a recent example of an azimuthal molecular rotor controlled by the STM tip, see Reference 58). Compound 3 can exist as three pairs of helical enantiomers because of the propeller-like conformation of the tetra-arylcyclobutadienes. For at least one out of the three diastereomers, an asymmetric potential energy surface can be predicted by molecular dynamics simulations on application of an alternating electric field.55... 

Ions in aqueous solution are surrounded by a shell of water molecules in tetrahedral or octahedral co-ordination that are relatively immobile because of the intensity of the electric field in the vicinity of the ion. Three regions of solvent around an ion may be labelled. In region 1 (see Fig. 16), all the water molecules are aligned by the field of the central ion forming a solvation shell. Between the distances and R is region 2, known as the Gurney... 

To have a reasonable residence time in the column, an analyte must show some degree of compatibility (solubility) with the stationary phase. Here, the principle of like dissolves like applies, where like refers to the polarities of the analyte and the immobilized liquid. Polarity is the electrical field effect in the immediate vicinity of a molecule and is measured by the dipole moment of the species. Polar stationary phases contain functional groups such as —CN, —CO, and —OH. Hydrocarbon-type stationary phases and dialkyl siloxanes are nonpolar, whereas polyester phases are highly polar. Polar analytes include alcohols, acids, and amines solutes of medium polarity include ethers, ketones, and aldehydes. Samrated hydrocarbons are nonpolar. Generally, the polarity of the stationary phase should match that of the sample components. When the match is good, the order of elution is determined by the boiling point of the eluents. 

If an electric potential gradient is applied in the solution, polyelectrolyte molecules will move in the direction of the electrode of opposite charge. This is called electrophoresis. If the polyelectrolyte is immobilized, the solvent will move in the electric field, a process called electroosmosis. These principles are applied in several laboratory techniques but will not be discussed here. 

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