Geometric steric factor

It is convenient to consider a model of an anisotropic recombination region the reflecting recombination sphere (white sphere) with black reaction spots on its surface [77, 78], The measure of the reaction anisotropy here is the geometrical steric factor Q which is a ratio of a black spot square to a total surface square. Such a model could be actual for reactions of complex biologically active molecules and tunnelling recombination when the donor electron has an asymmetric (e.g., p-like) wavefunction. Note the non-trivial result that at small Q, due to the partial averaging of the reaction anisotropy by rotational motion arising due to numerous repeated contacts of reactants before the reaction, the reaction rate is K() oc J 1/2 rather than the intuitive estimate Kq oc Q. 

Except consideration of purely geometrical (steric) factors, we shall derive mathematical expressions describing the response of a water molecule, librating between other molecules. Hence, we consider a one-dimensional hat well fitted to strongly simplified to water/ice structure. We emphasize that such an attempt is made for the first time.28... 

These reactants possess anisotropic reactivity the reaction occurs at a certain mutual orientation, which does not take place at each collision. In the first approximation, these reactants can be considered as spheres each of which has a small reaction spot. The reaction occurs when the spheres collide by their spots. The reaction occurs without an activation energy. The size of the spot in the form of a circumference on the sphere-reactant can be characterized by the angle q, the relative surface area of the spot on the sphere is equal to sin (0/2), and the probability of collision with the favorable mutual orientation of two identical particles is sin (0/2). This is the geometric steric factor = sin (0/2) or - sin (0A/2)sin (0B/2) if the sizes of the spots differ for A and B. After collision the particle-reactants exist near each other for some time and turn relatively to each other. The rate of turn of the particles depends, naturally, on viscosity because the coefficient of rotational diffusion depends on viscosity... 

Oxygen chelates such as those of edta and polyphosphates are of importance in analytical chemistry and in removing Ca ions from hard water. There is no unique. sequence of stabilities since these depend sensitively on a variety of factors where geometrical considerations are not important the smaller ions tend to form the stronger complexes but in polydentate macrocycles steric factors can be crucial. Thus dicyclohexyl-18-crown-6 (p. 96) forms much stronger complexes with Sr and Ba than with Ca (or the alkali metals) as shown in Fig. 5.6. Structural data are also available and an example of a solvated 8-coordinate Ca complex [(benzo-l5-crown-5)-Ca(NCS)2-MeOH] is shown in Fig. 5.7. The coordination polyhedron is not regular Ca lies above the mean plane of the 5 ether oxygens... 

For complex organic molecules, geometric considerations alone lead one to the conclusion that only a small fraction of bimolecular collisions can lead to reaction. One can represent the fraction of the collisions that have the proper geometric orientation for reaction by a steric factor (Ps). Except for the very simplest reactions, this factor will be considerably less than unity. On the basis of simple collision theory, it is not possible to make numerical estimates of Ps, although it may occasionally be possible to make use of one s experience with similar reactions to determine whether Ps for a given... 

Similar geometric optimization has been reported for bicyclo[3.2.2]nona-6,8-diene (BND). The double bond situated in the opposite direction to the methylene group was found to be more exo-pyramidalized than the other double bond and the electron density (qi, HOMO) of the former double bond in HOMO of the molecule higher than that of the latter double bond. The exo and endo faces of exo-pyramidalized double bonds proved not to be equal and the electron density was found to be higher on the endo faces. The endo molecular complexes with bromine have been found by the HF/321G method to be more stable than their exo congeners this was attributed to electronic and steric factors. As a result, endo-facial stereoselectivity of bromination ( ) predominates.21 A related theoretical study of facial selectivity and regioselectivity of the electrophilic addition of chlorine to exo-tricyclo[4.2.1.02,5]nona-3,7-diene (exo-TND) has also been reported.22... 

It is hard to see how the authors can reconcile such different exigencies. As regards the steric factors the third condition renders meaningless the condition of resemblance to steroids. The steroids and the purine-pyrimidine pairs are too different from each other to serve simultaneously as a standard for geometrical analogies. As to the electron transfer factors, we have shown in a series of publications that they are unrelated to the presence or absence of carcinogenic activity in aromatic hydrocarbons.27 >81 24... 

Our own geometry optimizations confirmed the preference for equatorial methyl groups, but we could discern no significant difference in the spin-coupled description of axial and equatorial P—CH3 bonds. This seems to suggest, at least for these systems, that the geometric preference is more closely linked to steric factors than to details of the bonding. 

Only two amino acids, tyrosine and histidine, form stable derivatives as the result of peroxidase-catalyzed iodination. All the tyrosine and histidine residues in a protein are not identical with respect to their reactivity or their geographic position. The residue which will be iodinated by lactoperoxidase must have the proper geometric position, while other methods of halogenation are influenced only by reactivity. The reactivity depends upon the microenvironment of the residue. There is an inverse relationship between the extent of tyrosine iodination and the dielectric constant of the environment of the tyrosine. Tyrosine iodination increases with decreasing dielectric constant. Steric factors also influence iodination since the relatively large iodine atom may be blocked in either the production of monoiodotyrosine or the formation of diiodotyrosine. 

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