Point dipoles

Hydrocarbons normally have very small dipole momen Why (Hint Consider the relationship betwe electronegativity differences and dipole momer established above for hydrogen halides.) Does sing methylene possess a small dipole moment Explain. W1 direction do you expect singlet methylene s dipole point Explain. In what direction does it point ... 

Distance vdw dipole—dipole point charges net charges... 

In the 1,3-dipolar cycloaddition reactions of especially allyl anion type 1,3-dipoles with alkenes the formation of diastereomers has to be considered. In reactions of nitrones with a terminal alkene the nitrone can approach the alkene in an endo or an exo fashion giving rise to two different diastereomers. The nomenclature endo and exo is well known from the Diels-Alder reaction [3]. The endo isomer arises from the reaction in which the nitrogen atom of the dipole points in the same direction as the substituent of the alkene as outlined in Scheme 6.7. However, compared with the Diels-Alder reaction in which the endo transition state is stabilized by secondary 7t-orbital interactions, the actual interaction of the N-nitrone p -orbital with a vicinal p -orbital on the alkene, and thus the stabilization, is small [25]. The endojexo selectivity in the 1,3-dipolar cycloaddition reaction is therefore primarily controlled by the structure of the substrates or by a catalyst. 

Now we can apply the process of combination shown in Figure 16-14 to BeF2. In the linear, symmetric BeFj molecule, the two bond dipoles point in opposite directions. Since the two bonds are equivalent, there is a complete cancellation, as shown in Figure 16-15. Hence the molecule has no net dipole the molecular dipole is zero. 

We consider a collection of molecular dipoles in thermal equilibrium. It is assumed that all the molecules are identical and they can take on any orientation. Because of thermal energy each molecule undergoes successive collisions with the surrounding molecules. In the absence of an applied electric field, the collisions tend to maintain a perfectly isotropic statistical orientation of the molecules. This means that for each dipole pointing in one direction there is statistically a corresponding dipole pointing in the opposite direction, as described by Fig. 1.2. 

In the most common LB films with the Y-type structure, the center of inversion exists, and hence they are not suitable for pyroelectric usages. On the other hand, since LB films with X- or Z-type structure have no center of symmetry, it is possible to construct the polar pyroelectric film with permanent dipoles pointing toward one direction. Similar structures can also be formed in hetero LB films with two different amphiphiles stacked altematingly. The first report on the pyroelectric LB film with X-or Z-type structure appeared in 1982 by Blinov et al. [12], It was followed by those of the alternate LB films by Smith et al. [13] and Christie et al. [14]. The polarized structure of the fabricated LB film can be checked by the surface potential measurements using the Kelvin probe [15], the Stark effect measurements [12], or the sign inversion of the induced current between heating and cooling processes. 

If equal bond dipoles act in opposite directions in three-dimensional space, they counteract each other. A molecule with identical polar bonds that point in opposite directions is not polar. Figure 1.5 shows two examples, carbon dioxide and carbon tetrachloride. Carbon dioxide, CO2, has two polar C=0 bonds acting in opposite directions, so the molecule is non-polar. Carbon tetrachloride, CCI4, has four polar C—Cl bonds in a tetrahedral shape. You can prove mathematically that four identical dipoles, pointing toward the vertices of a tetrahedron, counteract each other exactly. (Note that this mathematical proof only applies if all four bonds are identical.) Therefore, carbon tetrachloride is also non-polar. 

Figure 1 Examples of electrostatic interactions, (a) A schematic side view of a surface, with two anions (indicated by the As) binding to it. (b) The same as in (a), but now showing the surface-adsorbate dipoles (large ovals) due to adsorption of the anions. Since the dipoles point in the same direction, they repel each other, (c) Similar, to (b), except that the adsorbates are now cations, (d) When cations and anions are mixed, their dipoles point in opposite directions, and they attract each other...

In the head-to-head transition state, the component dipoles point in the same direction and the net moment is expected to be greater than that of the head-to-tail transition state where the component dipoles lie in opposite directions. Therefore, the solvent effect appears due to the polarity differences in the transition states leading to the head-to-head and head-to-tail dimers. 2-Methylbenzothiophene oxide gave the corresponding... 

Of course, cations will be surrounded by clusters of the water molecules, with the negatively charged end of water s molecular dipole pointing towards the cation. Anions will be surrounded by clusters of water molecules, with the... 

Just as a metal may play the role of a halogen atom in the adsorption of alkali ions discussed in the previous section, it may in other cases act similarly to the chlorine atom in HC1 and form a covalent bond. Hydrogen atoms adsorbed on the surface of platinum may serve as an example. The dipoles point with their positive ends away from the metal and may... 

In other cases the adsorption of hydrogen atoms results in surface hydride dipoles pointing with their negative ends away from the metal, as is observed in the adsorption on nickel,... 

The free molecules of metallic oxides can best be described as having covalent links, possessing dipoles (58). Similarly oxygen atoms adsorbed on metallic surfaces form covalent bonds, sharing two pairs of electrons with the metal (59) or with one specific atom or two atoms of the metal. Their dipoles point with the negative ends away from the metal. We may, to give an example, express the situation of the adsorption of oxygen on silver by... 

Similar covalent bonds may be formed between metal surfaces and many other atoms, including atoms forming part of molecules or radicals. In many cases the dipoles point with their negative ends away from the metal surface. In other cases, however, as with C2H2 and C2H4 on nickel, they form dipoles pointing with their positive poles away from the surface (60). 

The same picture holds for physical adsorption on metal surfaces. The polarization of the adsorbed molecules causes dipoles pointing with their positive ends away from the metal surface. The work function of the metal will be lowered by this effect, and it seems as if the increase of the normal nonselective photoelectric emission of metals by the adsorption of water molecules (122) or molecules of organic substances such as pyridine, propionic acid, and benzene (123) or alcohol, diethyl ether, and acetone (124) is caused by this effect. The explanation, which, many years ago, was given by the author (125), viz., polarization by positive hydrogen ions which should still be present, may seem to be unnecessary and obsolete. 

Photoelectric measurements at higher temperatures have often shown that chemisorbed hydrogen forms dipoles pointing with their positive ends away from the surface, also in those cases where in later investigations at low temperatures (liquid-air temperatures) contact potential measurements revealed dipoles of reversed direction. It is, therefore, not to be excluded that both types of chemisorption may occur at the same metal surfaces and it is to be expected then that the latter type, which in the case of iron is called 4-type chemisorption ... 

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