Structures, alternating

Once the basic performance of the design has been evaluated, changes can be made to improve the performance in other words, we optimize. These changes might involve the synthesis of alternative structures, i.e., structural optimization. Thus we simulate and... 

Davies and Warren" found that when 1,4-dimethylnaphthalene was treated with nitric acid in acetic anhydride, and the mixture was quenched after 34 hr, a pale yellow solid with an ultraviolet spectrum similar to that of a-nitro-naphthalene was produced. However, if the mixture was allowed to stand for 5 days, the product was i-methyl-4 nitromethylnaphthalene, in agreement with earlier findings. Davies and Warren suggested that the intermediate was 1,4-dimethyl-5 nitronaphthalene, which underwent acid catalysed rearrangement to the final product. Robinson pointed out that this is improbable, and suggested an alternative structure (iv) for the intermediate, together with a scheme for its formation from an adduct (ill) (analogous to l above) and its subsequent decomposition to the observed product. 

In the reaction of 4-substituted 2-aminoselenazoles with ethyl propiolate and dimethylacetylene dicarboxylate. the major products obtained from such a condensation are substituted 7H-selenazolo[3,2-a]pyrimidin-7-ones (5) and not the alternative isomeric substituted 5H-selenazoles[3,2-a]pyrimidin-5-ones (6). Distinction between the alternative structures was based on infrared, ultraviolet, and NMR data (Scheme... 

X-ray studies indicate that the vinyl chloride polymer as normally prepared in commercial processes is substantially amorphous although some small amount of crystallinity (about 5% as measured by X-ray diffraction methods) is present. It has been reported by Fuller d in 1940 and Natta and Carradini in 1956 that examination of the crystalline zones indicates a repeat distance of 5.1 A which is consistent with a syndiotactic (i.e. alternating) structure. Later studies using NMR techniques indicate that conventional PVC is about 55% syndiotactic and the rest largely atactic in structure. 

Figure 8.18 (a) Proposed pentagonal dodecahedral structure of Ti8C 2. (b) The same structure viewed as a Tig cube with each face capped by a C2 group, (c) An alternative structure (see text). 

Short-lived molecules may often be identified by their infrared spectra measured at extremely low temperatures. In most cases, the experimental spectrum will be incomplete, although a few characteristic lines or bands are often sufficient to decide among alternative structures. 

Melles and Backer " found, from a study of the oxidation of substituted thiophenes with perbenzoic or peracetic acid, that sulfones could be obtained from polysubstituted methyl- and phenyl-thiophenes and that the presence of electron-attracting groups, such as nitro, hindered the oxidation. Oxidation of thiophene - led to a product which was formed through a Diels-Alder reaction between the intermediate thiophene sulfoxide (211) and thiophene sulfone (212) and for which two alternative structures, (213) or (214), were suggested. Similar sesquioxides were also obtained from 2- and 3-methylthiophene and 3-phenylthiophene. The structures were not proved. Bailey and Cummins synthesized thiophene-1,1-dioxide... 

Two alternative structures for the simple Helmholz double layer (Fig. 20.11) have been provided ... 

It is possible to write a simple Lewis structure for foe S042- ion, involving only single bonds, which follows foe octet rule. However, Linus Pauling and others have suggested an alternative structure, involving double bonds, in which foe sulfur atom is surrounded by six electron pairs. 

It is to be especially emphasized that problems relating to choice among various alternative structures are usually not solved directly by the application of the rules resulting from the quantum mechanics nevertheless, the interpretation of valence in terms of quantities derived from the consideration of simpler phenomena and susceptible to accurate mathematical investigation by known methods now makes it possible to attack them with a fair assurance of success in many cases. 

The Relative Stability of Alternative Structures of Ionic Crystals.—... 

Efforts to provide such a treatment for simple alternative structures, such as the sodium chloride and cesium chloride structures and the fluoride and rutile structures, have been made with the aid of the Bom potential expression and modifications of it. Assuming that all ions repel each... 

Ethylenimine is known to spontaneously form copolymers with /3-propiolactone274) or cyclic imides275). Crystalline copolyamides of alternating structure were obtained with the latter monomer. 

The model process Eq. (15) has been studied by means of the MINDO/3 method to clarify the energetic conditions during the formation of cyclic reactive intermediates in cationic propagation of alkoxy-substituted monomers. The enthalpies of formation in the gas phase AH°g of both the alternative structures e and /were supplemented by the solvation energies Eso]v for transition into solvent CH2C12 with the assistance of the continuum model of Huron and Claverie which leads to heats of formation in solution AH° s. Table 13 contains the calculated results. 

To characterize the energetic and electronic conditions of the alternative structures the following quantum chemical values were used (see Table 16) ... 

The oxygen atoms of the homo cyclic oxides occupy axial positions in solid S7O and SsO. In fact, recent ab initio MO calculations at the G3X(MP2) level of theory have demonstrated that these isomers are, by 7 and 9 kj mol respectively, more stable than the alternative structures with the oxygen atoms in equatorial positions [89]. In the case of SsO, however, the energy difference is only 1 kJ mol and the conformation with the oxygen atom in the equatorial position is more stable. This result agrees with the observation that SsO exists as two isomeric forms in the solid state depending on the crystallization conditions (see above). 

Westerhout EM, Ooms M, Vink M, Das AT, Berkhout B (2005) HIV-1 can escape from RNA interference by evolving an alternative structure in its RNA genome. Nucleic Acids Res 33 796-804... 

In order to obtain compounds with Ti-O-P and Zr-O-P units, the hexaethoxy-derivative, NsPaCOEOg, was treated with titanium and zirconium tetrachlorides. In each case, hygroscopic solids of the type NaPaCOEOiOaMCU (M = Ti or Zr) and ethyl chloride were obtained. The degree of polymerization of these solids was 1.6—1.8, and on the basis of their i.r. and n.m.r. spectra, two alternative structures, (46) and (47), were proposed. In an alternative route to the same type of compound, N3P3CI6 was treated with tetra-n-butoxytitanium in o-xylene. Butyl chloride was liberated and a solid was obtained which has been assigned the structure (48). Its thermal decomposition was studied by differential thermal analysis. 

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