Most favorable orientation

When both the 1,3-dipoIe and the dipolarophile are unsymmetrical, there are two possible orientations for addition. Both steric and electronic factors play a role in determining the regioselectivity of the addition. The most generally satisfactory interpretation of the regiochemistry of dipolar cycloadditions is based on frontier orbital concepts. As with the Diels-Alder reaction, the most favorable orientation is that which involves complementary interaction between the frontier orbitals of the 1,3-dipole and the dipolarophile. Although most dipolar cycloadditions are of the type in which the LUMO of the dipolarophile interacts with the HOMO of the 1,3-dipole, there are a significant number of systems in which the relationship is reversed. There are also some in which the two possible HOMO-LUMO interactions are of comparable magnitude. 

Properties of Different Solvents. In discussing molecular dipoles in Sec. 25, we estimated the force of attraction between an atomic ion and a dipole having the most favorable orientation and found this attraction to be very strong. In any ionic co-sphere those molecular dipoles which have a favorable orientation will bo attracted, while those that have the opposite orientation will be repelled. Since the former are more numerous the solvent in the co-sphere is, on the whole, attracted toward the ion. Since the liquid is not incompressible, we must expect that this will lead to a contraction in each co-sphere. In any ionic solution the sum of the contractions that have taken place in the co-spheres of the positive and negative ions will be apparent if we measure accurately the volume of the solution. 

Exact molecular parameters of oxathietanes and thiazetidines have not been assessed, although the nonplanar puckered-ring structure appears to be the most favorable orientation. ... 

Make an atomic-orbital model of benzenol, showing in detail the orbitals and electrons at the oxygen atom. From your model, would you expect one, or both, pairs of unshared electrons on oxygen to be delocalized over the ring What would be the most favorable orientation of the hydrogen of the hydroxyl group for maximum delocalization of an unshared electron pair ... 

Our calculations show that the endofullerenes CH4 C6oH36 and He4 C6oH36 have similar properties. For both systems, the structure of symmetry T, in which the C-H bonds or the o-He lines (0 is the center of the He4 tetrahedron) are directed toward the centers of the hydrogenated faces [C6H6], corresponds to a PES minimum and to the most favorable orientation of the guest in the cage (Figure 1). 

Table II shows that the approach of undistorted methane toward the Ni surface is repulsive for all orientations of methane. For a C-Ni surface (perpendicular) distance of 2.4 A, and a 3-fold adsorption site, the three symmetric CH. orientations are (in order of increasing energy) 3 H s down, 1 H down, and 2 H s down. The latter is unfavorable due to interactions of hydrogens with the Ni-Ni bridge site. Thus, these data show the expected repulsive interaction between the saturated methane molecule and the surface. Figure 3b shows the most favorable orientation of the hydrogens for tetrahedral CH. ...

In practice, this value of R is of limited practical value, since it cannot apply to the nominal aperture of the sieve. As sieving progresses, the smaller apertures become ineffective since all the particles finer than these apertures will have passed through the sieve. The largest aperture in the sieve therefore controls the sieving operation and the final particle to pass through the sieve will only do so when presented to this aperture in its most favorable orientation, i.e. for a 75 pm sieve, the true end-point could be 100 pm or more. 

The cyanide approach has been directed so that the alkoxide produced is anti to the halide. In this position the alkoxide is situated in the most favorable orientation for backside nucleophilic reaction at the carbon bearing the bromo group to give an epoxide ... 

Representing the reaction in this way does not clearly indicate how the two isomers arise. A better appreciation of what is going on is obtained by considering the orientation required for the molecular orbitals of the two components to interact in a pericyclic process. The most favorable orientation has the faces of the tt systems parallel to one another, as shown in the following structures. 

Some compounds, like short-chain fatty acids, can be partly soluble in both water and oil. This dual solubility is because such molecules are amphiphilic or amphipathic that is, they have one part that has an affinity for the oil (the nonpolar hydrocarbon chain), and one part that has an affinity for the water (the polar group). The energetically most favorable orientation for these molecules is at the oil-water interface, so that each part of the molecule can reside in the solvent for which it has the greatest affinity see Figure 8). 

As a increases, the maximum value of q remains in the planed = 90°, but the value of 0 for maximum q shifts over toward 0 = 90 that is, toward the direction of the stream lines. Also, as a increases, the height of this maximum increases —that is, the number of molecules whose a axes lie in or near the most favored orientation becomes steadily greater as the orienting forces increase relative to the disorientation produced by Brownian movement. 

Fig. 2.23. Two most favored orientations of styrene for enantioselective dihydroxylation by (DHQD)PYDZ catalyst. The bridging structure is 3,5-pyridazinyl. Reproduced from J. Am. Chem. Soc., 121, 1317 (1999), by permission of the American Chemical Society.

Figure 7-1 Current questions in biological electron transfer. A small monoheme cytochrome may pre-ori-entate by means of complementary electrostatic fields projecting from itself and its redox partner. Having formed a collision complex, it may be able to rotate and translate across part of the molecular surface of the redox partner before it finds the most favorable orientation for electron transfer.

Proton magnetic resonance studies have shown that the furanose ring in 49 assumes that conformation (49a) in which C-3 is exo and C-4 is endo to the plane defined by C-2, C-5, and 0-5. This conformation is stabilized by the acetal groups, each of which may assume its most favorable orientation at the same time. When R-R " = Me (diacetal 3 p. 207), this rearrangement is favored, as a 2,3-isopropyli-dene acetal is more stable than a 1,2- or 1,3-isopropylidene acetal of 4,6-O-isopropylidene-L-sorbofuranose. This driving force is absent in the ethylidenation reactions, and thus, in the latter, product 49 is formed only with difficulty. 

If functional groups are protected in the guest s most favorable orientation, reactivity depends on the inner phase rotational mobility of the guest. 

The as-fabricated room temperature tensile properties ofthe composites are summarized in Table 5. It is important to recognize that, due to their high porosity, the matrices in all of these systems have elastic moduli and strengths an order of magnitude less than those of the fibers [145], hence the composites are highly orthotropic in most manifestations, e.g. laminates [143, 146], See [147] for an extensive review ofthe mechanics of brittle matrix composites. The properties in Table 5 represent the response to loading in the fiber direction. The fiber direction orientation, denoted as 0°/90°, is the most favorable orientation the strength and stiffness in any other orientation, denoted as off-axis , is substantially lower. 

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