Methyl group, model

In order to confirm the appropriateness of the models for the artificial DNA, we first verify the model dependence of the inter-base pair interaction. We adopt three models, (a) a hydrogen atom model (dP=H) and (b) a methyl group model (dP=CH3) (c) real model (dP=5 -deoxyribose) (Fig. 25.2). These models are often used in the analyses of the inter-base pair interaction between the bases in the natural B-DNA, where it is known that the backbone molecules do not contribute much to the stability of the duplex structure. We here also make the same assumption for the artificial DNA. 

Fig. 25.3 The interaction energy between (a) hydrogen atom model (dP=H) and (b) methyl group model (dP=CH3). In the figure, Int E of LC-BOP is obtained by and LC-BOP-I-ALL is...

As in the case of the free bases, the substitution of a nuclear hydrogen atom by a methyl group induces a bathochromic shift that decreases in the order of the position substituted 4->5->2- Ferre et al. (187) have proposed a theoretical model based on the PPP (tt) method using the fractional core charge approximation that reproduces quite correctly this Order of decreasing perturbation. 

The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface As the molecular model m Figure 6 3 shows one of the methyl groups on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst The bottom face of the double bond is more exposed and both hydrogens are transferred from the catalyst surface to that face... 

Construct a molecular model corresponding to the Fischer projection of rneso 2 3 dibro mobutane Convert this molecular model to a staggered conformation in which the bromines are anti to one another Are the methyl groups anti or gauche to one another in this staggered con formation" ... 

The most stable conformation of acetone has one of the hydrogens of each methyl group eclipsed with the carbonyl oxygen Construct a model of this conformation... 

Manufacture and Processing. Mononitrotoluenes are produced by the nitration of toluene in a manner similar to that described for nitrobenzene. The presence of the methyl group on the aromatic ring faciUtates the nitration of toluene, as compared to that of benzene, and increases the ease of oxidation which results in undesirable by-products. Thus the nitration of toluene generally is carried out at lower temperatures than the nitration of benzene to minimize oxidative side reactions. Because toluene nitrates at a faster rate than benzene, the milder conditions also reduce the formation of dinitrotoluenes. Toluene is less soluble than benzene in the acid phase, thus vigorous agitation of the reaction mixture is necessary to maximize the interfacial area of the two phases and the mass transfer of the reactants. The rate of a typical industrial nitration can be modeled in terms of a fast reaction taking place in a zone in the aqueous phase adjacent to the interface where the reaction is diffusion controlled. 

The peroxide decomposes at elevated temperatures to give free radicals, which then abstract a hydrogen atom from the methyl group. The radicals formed then combine to form a hydrocarbon linkage. Results obtained by reacting model systems with benzoyl peroxide and analysing the reaction products are consistent with this type of mechanism. ... 

Ethynylation of 3j -hydroxy-16a-methyl-5a-androstan-17-one in a mixture of diethylene glycol dimethyl ether and diethylene glycol monoethyl ether in the presence of potassium hydroxide produces two isomeric 17-ethynyl derivatives. This result is not unexpected since molecular models suggest that the steric influence of the 13/ -methyl group is nearly offset by the 16a-methyl group. The presence of a 16a-acetoxy group in the estrone series also leads to the formation of epimeric 17-ethynyl compounds (61) and (62) on reaction with acetylenedimagnesium bromide. 

FIGURE 3.6 The gauche and anti conformations of butane shown as ball-and-spoke models (left) and as Newman projections (right). The gauche conformation Is less stable than the anti because of the van der Waals strain between the methyl groups. 

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