1.3- Dipoles, generation

Similar cycloadditions between thiirene dioxides and 1,3-dipoles generated in situ give heterocycles which result from either loss of sulfur dioxide or from the three-membered ring opening of the initially formed adduct (e.g. 174). Such cycloadditions with nitrilium imides (173a) and nitrile ylids (173b) are illustrated in equation 69175. 

Van der Waals forces arise from attractions between transient dipoles generated by the rapid movement of electrons on all neutral atoms. Significantly weaker than hydrogen bonds but potentially extremely numerous, van der Waals forces decrease as the sixth power of the distance separating atoms. Thus, they act over very short distances, typically 2-4 A. 

Dipolar cycloadditions of the unusual dipolarophiles 9-arylidenefluorenes 446 with the dipoles generated from isatin 432a and cyclic amino acid proline 433a were carried out under four different conditions to yield a series of novel dispiro oxindole derivatives 50a-f via [3+2] cycloaddition (Scheme 100) <2002T8981>. 

And consequently, all three dipoles generate derivatives of the same heterocyclic system, viz., isoxazoles, in the reactions with C,C multiple bonds. 

Malacria and coworkers346 prepared phyllocladane and kaurane types of diterpenes by means of [3 + 2]/[2 + 2 + 2]/[4 + 2] cascade reaction sequences. A representative example of such a reaction sequence has been outlined in equation 171. The five-membered ring of 598 was built by a 1,3-dipolar cycloaddition between 596 and an all-carbon 1,3-dipole generated from 597. The reaction of 598b with 568h afforded benzocyclobutene 599. The intramolecular [4 + 2] cycloaddition afforded diastereomers 600 and 601 in a 5 1 ratio. It is noteworthy that the exocyclic double bond in 598b neither participates in the [2 + 2 + 2] cycloaddition reaction nor isomerizes under the reaction conditions applied. 

Ionic contact adsorption on metallic electrodes alters the potential profile across the compact layer at constant electrode potential. If anions are adsorbed on the metal electrode at positive potentials, the adsorption-induced dipole generates a potential across the inner Helmholtz layer (IHL) as illustrated in Fig. 5-29. The electric field in the outer part (OHL) of the compact layer, as a result, becomes dififerent fi om and frequently opposite to that in the inner part (IHL) of the compact layer. 

An alternative is to describe the surface using a symmetric model. In the symmetric model, the center of the slab consists of a mirror plane. The atoms in the middle layers are typically fixed at bulk geometries and the layers above and below are allowed to relax. One advantage of a symmetric model is that any dipole generated by surface features will be automatically canceled. There is a cost involved, however, because it is typically necessary to include more layers in a symmetric slab than in an asymmetric slab. A symmetric slab with nine layers is depicted in Fig. 4.12. Note that, in this example, three layers are allowed to relax on each side of the slab. Recall that in our earlier model of an asymmetric slab, three layers were allowed to relax on one side of a five-layer slab. So in order to carry out calculations in which three layers are allowed to relax, one would need to employ nine layers in a symmetric model, compared to the five layers needed for an asymmetric one. 

As shown in Fig. 7.1, at large distances, the system can be considered as a neutral hydrogen atom plus an isolated proton. The field of the proton polarizes the hydrogen atom to induce a dipole. The interaction between the proton and the induced dipole generates a van der Waals force. The van der Waals force can be treated as a classical phenomenon by introducing a phenomenological polarizability a ... 

In order to obtain high conversion efficiencies, optimization of the short-circuit photocurrent (z sc) and open-circuit potential (Voc) of the solar cell is essential. The conduction band of the TiO is known to have a Nernstian dependence on pH [13,18], The fully protonated sensitizer (22), upon adsorption, transfers most of its protons to the TiO surface, charging it positively. The electric field associated with the surface dipole generated in this fashion enhances the adsorption of the anionic ruthenium complex and assists electron injection from the excited state of the sensitizer in the titania conduction band, favoring high photocurrents (18-19 inA/cm ). However, the open-circuit potential (0.65 V) is lower due to the positive shift of the conduction-band edge induced by the surface protonation. 

It was found that basic alumina worked well as the basic catalyst for the in situ dipole generation from the AT-acyl pyridinium salt. A three-component mixture of phenacyl bromide (1 mmol), pyridine (1.2 equiv.) and the acetylene (1.2 equiv.) was thoroughly mixed in basic alumina (1 g) and then irradiated for 8 min at 80% power in a domestic microwave. The products were formed in 87-94% yields when running the reaction under solvent-free conditions and in 60-71% yields when using anhydrous toluene as the solvent. 

The tandem intermolecular 1,3-azaprotio cyclotransfer-intramolecular cycloaddition of divinyl sulfones 19 with oxime 193 resulted in the formation of bridged cycloadducts 194 and 195 (Scheme 42). In this reaction, the divinyl sulfone behaves either as a bifunctional dipole-generating component or as a bis-dipolarophile <1996TL4597, 1991T8297>. 

When dimethyl maleate was chosen to trap the 1,4-dipole generated in the reaction of PTAD with 1-phenyl-4-vinylpyrazole, the expected... 

The electro-chemical potential fi = /ic + fic, consists of two terms, the electro-static part, fie, and the chemical part, fic, as discussed in the literature13-15. Since the electro-chemical potential is a constant throughout the film thickness, Afi = 0, then A/te =— A/ c. The electro-static part corresponds to the electro-static potential, and reflects the dipole generated by charge transfer at the interface, as seen in the step in the vacuum level in the lower part of Fig. 8.2. The chemical part, in the present case, corresponds to the gradient in the number of particles , which is just the number of electrons transferred (per unit area) from the metal to the polymer at the interface. Since electrons are transferred to the polymer, the gradient in the number of particles works against, and just equals (cancels) the step in the electro-static potential. 

In the second kind of adsorption (Fig. 2B), the dipole exposes to the solution a charge opposite of that of the surface. Correspondingly, the dipole generates a field opposite of that generated by the surface charge, and the adsorption is expected to decrease the repulsive force between the two plates. However, if the surface charge is small, then the dipoles of the adsorbed molecules alone generate repulsion. 

The solutions for y>(z) and m(z) of the system of eqs 3 and 4 depend not only on the surface charge density but also on the surface dipole densities. The boundary conditions are related to the surface charge and the surface dipoles generated by the association of the cations and H+ with some of the Na acidic surface sites per unit area of the surface and of the anions and OH with some of the Nb basic surface sites per unit area. The details are given elsewhere,11 and the results will only be briefly reviewed here. 

Gunner, M.R., Saleh, M. A., Cross, E., Ud-Doula, A., and Wise, M. (2000) Backbone dipoles generate positive potentials in all proteins origins and implications ofthe effect, Biophys. J. 78, 1126-1144. 

In a ferromagnetic material, the magnetic dipoles generated from unpaired electrons tend to align in the same direction, even in the absence of an external magnetic... 

Table II summarizes the azomethine ylide 1,3-dipoles generated by the desilylation route.

Azomethine ylide 1,3-dipoles generated through the deprotonation route are listed in Table IV. 

---