Substrates nonplanar

An important issue in the thermodynamics of confined fluids concerns their symmetry which is lower than that of a corresponding homogeneous bulk phase because of the presence of the substrate and its inherent atomic structure [52]. The substrate may also be nonplanar (see Sec. IV C) or may consist of more than one chemical species so that it is heterogeneous on a nanoscopic length scale (see Sec. VB 3). The reduced symmetry of the confined phase led us to replace the usual compressional-work term —Pbuik F in the bulk analogue of Eq. (2) by individual stresses and strains. The appearance of shear contributions also reflects the reduced symmetry of confined phases. 

To illustrate the effects of nonplanarity of the substrate on fluid structure, a hard-sphere fluid exposed to a periodic array of wedges (see Fig. 13) is... 

In the examples given, there is good evidence for the formation of an unstable epoxide intermediate in the production of monohydroxymetabolites. However, there is an ongoing debate about the possible operation of other mechanisms of primary oxidative attack that do not involve epoxide formation, for example, in the production of 2 OH 3,3, 4,4 -TCB (Figure 6.3). As mentioned earlier, P450s of gene family 1 (CYP 1) tend to be specific for planar substrates, including coplanar PCBs they do not appear to be involved in the metabolism of nonplanar PCBs. On the other hand. 

It is often found that the ratio R (measured, for instance, by gas adsorption methods) of actual metal surface area accessible to the gas phase, to the geometric film area, exceeds unity. This arises from nonplanarity of the outermost film surface both on an atomic and a more macroscopic scale, and from porosity of the film due to gaps between the crystals. These gags are typically up to about 20 A wide. However, for film thicknesses >500 A, this gap structure is never such as completely to isolate metal crystals one from the other, and almost all of the substrate is, in fact, covered by metal. In practice, catalytic work mostly uses thick films in the thickness range 500-2000 A, and it is easily shown (7) that intercrystal gaps in these films will not influence catalytic reaction kinetics provided the half-life of the reaction exceeds about 10-20 sec, which will usually be the case. 

Rate constants of bimolecular, micelle-assisted, reactions typically go through maxima with increasing concentration of inert surfactant (Section 3). But a second rate maximum is observed in very dilute cationic surfactant for aromatic nucleophilic substitution on hydrophobic substrates. This maximum seems to be related to interactions between planar aromatic molecules and monomeric surfactant or submicellar aggregates. These second maxima are not observed with nonplanar substrates, even such hydrophobic compounds as p-nitrophenyl diphenyl phosphate (Bacaloglu, R. 1986, unpublished results). 

E. Kapon, Lateral Patterning of Quantum Well Heterostructures by Growth of Nonplanar Substrates H. Temkin, D. Gershoni, andM. Panish, Optical Properties of Gai xInxAs/InP Quantum Wells... 

Describe how LbL may be used to deposit coatings onto complex, nonplanar substrates. Cite examples for this strategy from the literature. 

Spin coating demands that the substrate be flat, while plasma derived coatings can be applied to nonplanar surfaces (even spherical substrates) efficiently. A detailed description of the... 

Highly nonplanar octaalkyltetraphenylporphyrins (OATPP) have been investigated to determine the influence of the coordinated metal on this distortion. Sparks et al. considered the effect of the central atom on the geometry and optical spectra of a series of OATPPs. The overall goal of the research is to be able to use molecular mechanics as a tool in the careful design of substrate... 

This rearrangement, which accounts for the scrambling, is completely stereospecific.The rearrangements probably take place through a nonplanar cyclobutyl cation intermediate or transition state. The formation of cyclobutyl and homoallylic products from a cyclopropylmethyl cation is also completely stereospecific. These products may arise by direct attack of the nucleophile on 65 or on the cyclobutyl cation intermediate. A planar cyclobutyl cation is ruled out in both cases because it would be symmetrical and the stereospecificity would be lost, iv. The rate enhancement in the solvolysis of secondary cyclobutyl substrates is probably caused by participation by a bond leading directly to 65, which accounts for the fact that solvolysis of cyclobutyl and of cyclopropylmethyl... 

High-level ab initio calculations show that diacyl-hydrazines are intrinsically nonplanar with respect to the CO—N—N—CO torsion, and the corresponding rotational barriers are high.171 The global minimum finds the nitrogen lone pairs approximately perpendicular to one another. Thus, their conformational properties are essentially determined by the conformation of their hydrazine and urea constituents. As a result of restricted rotation about the N—N bond, azapeptides cannot adopt extended conformations. This is likely the reason that they are resistant172 to chymotrypsin-like proteases, which bind their substrates in extended forms. 

A more realistic approximate theory for the SECM with a tip shaped as a cone or spherical segment was presented in Ref. 9. The surface of the nonplanar tip electrode was considered to be a series of thin circular strips, each of which is parallel to the planar substrate. The diffusional flux to each strip was calculated using approximate equations for a disk-shaped tip over a conductive or an insulating substrate. The normalized current to the nonplanar tip was obtained by integrating the current over the entire tip surface. Two families of working curves for conical tips over conductive (Fig. 8A) and insulating substrates (Fig. 8B) illustrate the effect of the tip geometry. 

Since the shape of a nonplanar tip is typically imperfect, one is not motivated to carry out extensive simulations for these complicated systems. All equations describing the approach curves under diffusion control are highly approximate, and no theory has been developed for finite kinetics at either tip or substrate electrode. 

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