Anisotropic reactivity

The presence of a polar axis confers anisotropic activity to organic crystals (Curtin Paul, 1981 Desiraju, 1984b). Common polar (noncentrosymmetric) space groups adopted by organic crystals areP2i2i2i and P2i- While chiral crystals must have polar directions, polar crystals need not be chiral. Anisotropic reactivity is seen for instance in the reaction of ammonia with p-bromobenzoic anhydride, which crystallizes with a polar axis the polar axis directs the reaction, p-bromobenzoic anhydride is chiral as well as polar. Chirality of the crystalline anhydride has been exploited to resolve a racemic gaseous amine the chiral crystal preferentially reacts with one of the enantiomers of the amine. Thus when p-bromobenzoic anhydride crystals are exposed to vapours of racemic phenylethylamine, the resulting amide contains one of the enantiomers in excess. 

This section is concerned with the rates of re-orientation of typical reactants in solution and upon both theoretical analyses and experimental studies of such. Molecules which may display orientationally anisotropic reactivity are indicated and the modifications to the rate of mutual diffusion and chemical reaction are considered. 

Recently, Steiger and Keizer [259b] have discussed the theory of reactions between anisotropically reactive species in considerable detail. They illustrated their analysis by using a diffusion equation approach to solve for the rate coefficient for reaction between species which displayed dipolar reactivity. The rate coefficient was reduced by approximately 15% from the Smoluchowski value [eqn. (19)]. Berdnikov and Doktorov [259c] have also analysed the rate of reactions between a spherical reactant having a reactive site, which is a spherical shell of semi-angle 60, and a spherical symmetric reactant. Again, these reactants were not allowed to rotate. Approximate analytic expressions were obtained for the rate coefficient, which was a factor feit less than the Collins and Kimball expression, where... 

The studies of Hasinoff [53] on the recombination rate of carbon monoxide and the heme units after photodissociation of carboxy ferrous microperioxidase come close to satisfying the requirements for observing the effects of anisotropic reactivity and rotational diffusion on the rate of a translational diffusion-limited reaction. In Chap. 2, Sect. 5.6, the details of this study were briefly mentioned. Hasinoff found that the rate of recombination was substantially diffusion-limited in all three aqueous solvents used at 260 K, but at higher temperatures, the rate of reaction of the encounter pair, feact, was a significant factor in determining the overall rate of recombination (see Fig. 9). The observed rate coefficient of recombination, feobs, was separated into the rate coefficient of diffusive formation of encounter pairs, feD, and the rate coefficient of reaction of encounter pairs, fcact, with the Collins and Kimball expression, eqn. (26)... 

Hasinoff noted that the rate coefficient of formation of encounters pairs, fcD, was smaller than predicted from the Smoluchowski—Stokes—Einstein rate coefficient [eqn. (29)]. In aqueous glycerol, this reduction was by 0.14 times, in aqueous polyethylene glycol by 0.30 times, and in aqueous ethylene glycol by 0.11 times. Hasinoff compared these reductions in rate of diffusive rate of formation of encounter pairs with three theories of anisotropic reactivity due to Weller [262], Schmitz and Schurr [257] and... 

It would be interesting to study the effect of changing the size of peptides associated with the heme unit, but only in the plane of the heme unit. The rotational relaxation time would be markedly affected but not the anisotropic reactivity of the heme unit. 

Microfabrication was effected by anisotropic reactive ion etching (RIE) using a plasma [34], Back-side structures were realized by wet-chemical etching. A transparent plate is bonded on top of the microstructures. 

Surface-enhanced PBS films can be used in selective anisotropic reactive-ion etch processes to reproducibly delineate high aspect ratio features with submicron resolution. 

From the sphere-dimer studies, two major conclusions emerge. The first is that the trajectory method can be extended to structured reactants with anisotropic reactivity and anisotropic direct forces and hydrodynamic interactions. The second major conclusion is that complicated electrostatic interactions between species with anisotropic reactivity can "steer" the approaching particles into favorable orientations and enhance the reaction rate. For these model studies, rate enhancements up to 20% have been obtained. The second conclusion is likely to be of considerable relevance to molecular biology. In the third and final series of simulations, the Brownian dynamics trajectory method is applied to a particular biological system. 

Stochastic Gating and Anisotropic Reactivity V. Concluding Remarks... 

Both macroporous and mesoporous silicon are similarly exploited to achieve various types of separation and/or seed layers by high-temperature anneal and formation of active layer by epitaxial growth or epitaxial free processing (Depauw et al. 2008, 2009 Brendel et al. 2010). Similar studies of void shape evolution and SON (silicon-on-nothing) structure formation from masked anisotropic reactive ion etching resulting in square arrays of holes during H2 anneal were shown in Sudoh... 

Two techniques are principally used to transfer the upper image into the base layer, flood ultra-violet exposure and anisotropic reactive ion etching (RIE) although wet processing is also used. Hood exposure is used in conjunction with positive resist base layers using the upper layer as a mask followed by development. The use of RIE requires some resistance of the thin top layer so intermediate layers are... 

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... 

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