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Nonbonded attraction

Scheme 19a,b Nonbonded attraction controls the regioselectivity (a) and the mode selectivity (b)... [Pg.43]

Inagaki, S., Fujimoto, H. and Fukui, K. (1976). The selection of 02(1Ag)-olefin reaction courses. Intermolecular nonbonded attraction and n bond polarity of olefins. Chem. Lett. 749-752... [Pg.265]

In the previous sections, we saw that, in most cases, a nonbonded attractive or repulsive interaction is enforced by both four electron destabilization and two electron stabilization. Hence, in order to simplify subsequent discussions, we shall adopt the OEMO model with neglect of overlap. Consequently, in the remainder of this work... [Pg.40]

The problems facing the static model can be best understood by reference to the various computed indices for difluoromethane at various FCF angles. Comparing the two structures shown above, we note that the total pi overlap population increases as the angle shrinks. Thus, according to the dynamic model, there is F—F pi nonbonded attraction since the difference in the total pi overlap populations is... [Pg.52]

The sigma lone pair nonbonded interaction in F2C=CH2 can be treated in a similar fashion. The appropriate interaction diagram is shown in Fig. 15 and our conclusions are identical to the ones reached in the case of F2CH2, i.e. nonbonded attraction between the fluorine 2px lone pairs favors angle shrinkage in CF2=CH2. The results of calculations shown above are, once again, in agreement with these predictions. [Pg.54]

Good quality ab initio calculations have not yet been carried out in order to test whether lone pair nonbonded attraction obtains in 1,2-difluoroethane. Results of INDO calculations shown below indicate the presence of a nonbonded attractive interaction on the basis of the partial bond order p (F2px, F2px). By contrast, the presence of a nonbonded repulsive interaction favoring the anti conformation is indicated on the basis of the bond order p(F2px, F2px). [Pg.57]

If the two substituents are not identical, one may still focus on the interaction of the group MO s of the substituents and those of the — CH2— CH2 - fragment. As an example, we will consider 1-fluoropropane. This molecule constitutes system where a hydrogen bond determines conformational preference and merits special attention. This hydrogen bond represents a bonding situation which can be classified under the heading of nonbonded attractive interactions. [Pg.59]

On the basis of the above discussion, we are led to the conclusion that sigma nonbonded attractive interaction in the form of a hydrogen bond will tend to favor a syn conformation opposing the inherent preference of ethane molecules for a staggered conformation. A compromise is expected to be reached in the gauche conformation. However, severe steric effects may force an anti conformational preference. [Pg.61]

Experimental results shown in Table 10 suggest that a hydrogen bond, Le., a strong nonbonded attractive interaction, is instrumental in dictating gauche preference in CH3CH2CH2X and XCH2CH2OH systems where X carries at least one lone pair. [Pg.61]

Arguing as before we predict that cis C1NSO is stabilized more than tram C1NSO by pi lone pair nonbonded attraction. The results of CNDO/2 calculations are shown above and it can be readily seen that pi nonbonded attraction favors the cis isomer. On the other hand, the indices of sigma nonbonded interaction imply that the tram geometry is stabilized more by sigma lone pair nonbonded attraction than the cis. [Pg.66]

The nature of the nonbonded interactions between the pi and sigma fluorine lone pairs has been discussed in detail before. The results of ab initio calculations are shown above. They clearly demonstrate that pi nonbonded attraction obtains in 1,2-difluoroethylene and related systems. The predicted greater stability of the trans isomer is most likely due to an exaggeration of dipolar interaction effects, favoring the trans molecule, by the basis sets used. [Pg.69]

This has been confirmed by ab initio calculations as shown by the appropriate indices of nonbonded attraction shown above. [Pg.72]

The above is an example, therefore, where the apparent dominance of sigma over pi nonbonded attraction is attributable to steric effects. [Pg.73]

Ab initio calculations reveal that Tee is favored over Cee. This constitutes an example where steric effects dominate sigma nonbonded attractive effects. [Pg.73]

Ab initio calculations indicate that the Css geometry is more stable than the Tss one. This constitutes an example where pi nonbonded attractive effects dominate steric effects. [Pg.73]

The next question is which of the two possible cis rotamers, i. e. Cs and Ce, is the stable conformation Clearly, pi nonbonded attraction favors Cs more than Ce while sigma nonbonded attraction, L e. hydrogen bonding, stabilizes the Ce rotamer... [Pg.74]

Experimentally, it is known that the cis isomer in 1-substituted propenes is more stable and has a lower rotational barrier. Some pertinent data are shown in Tables 13—14. In most cases, the experimental results agree with our predictions. An interesting trend obtains in the alkyl vinyl ether series. Specifically, two types of nonbonded attraction can obtain in these molecules ... [Pg.75]

These two types of nonbonded attractive interactions are illustrated below ... [Pg.75]

Indices of nonbonded attraction calculated by the CNDO/2 method are shown above, and indicate that both pi and sigma nonbonded attraction obtain in the cis isomer with the former being maximized in the Cs conformation and the latter being maximized in the Ce conformation. [Pg.76]

However, one is unable to predict anything about the rotational barriers in these molecules by considering only the dipolar effect. Consequently, the only rationale capkble of explaining both the geometric as well as the conformational preferences observed appears to be nonbonded attraction. [Pg.77]

This conclusion is further supported by an inspection of the indices of nonbonded attraction as calculated by the INDO method. [Pg.80]

Arguing as before, we expect pi nonbonded attraction to favor the cis geometry for a fixed conformation. Furthermore, sigma nonbonded attraction in the form of a hydrogen bond is expected to make Cpv the most stable conformation in the cis series. A complete study of all possible torsional isomers of DADCE will be reported in the future. [Pg.81]

Sigma nonbonded interactions in dimethyl ether can be analyzed in the same way as before and the conclusion is that sigma nonbonded attraction is greatest for the Cee conformation. We expect pi nonbonded attraction to dominate sigma nonbonded attraction and the final result is that the C conformation will be the most stable torsional isomer of R—X-R molecules. [Pg.87]

The indices of nonbonded attractions for the Css and Cee conformations of dimethyl ether are shown below and confirm that the greater stability of the Cs conformation relative to the Cee conformation is due to pi nonbonded attraction ... [Pg.91]

This result is perfectly consistent with our discussion since the so called crab conformation benefits from the stabilizing 1,5-nonbonded attractive interaction which obtains in the conformation of CH3-X-CH3 molecules. [Pg.91]

Recent ab initio calculations have shown, in agreement with the conclusions reached by OEMO theory, that the most stable conformation of methyl vinyl ether is the Cs conformer176). The calculated relative energies of the four conformations of methyl vinyl ether at the STO-3G and 4-31G levels are shown above along with the appropriate indices of nonbonded attraction. As can be seen, the greater stability of the Cs conformation can be attributed to an attractive 1,5 pi nonbonded interaction. [Pg.95]

Sigma nonbonded attraction also obtains in the isopropyl cation and, arguing as before, we conclude that sigma nonbonded attractive interactions are maximized in the Cee conformation. The 2-propyl cation constitutes a good example where pi and sigma nonbonded interactions reinforce each other. [Pg.97]

The appropriate interaction diagrams are similar to that of Fig. 28. Reasoning as before, we conclude that pi nonbonded interactions favor the staggered conformation. Sigma nonbonded interactions, on the other hand, favor the eclipsed conformation. We expect pi nonbonded attractive interactions to dominate sigma interactions and the resulting order of stability is predicted to be > Cee. [Pg.98]

The results of INDO calculations of planar and distorted difluorobenzene in which the C—F bonds have been displaced a few degrees above and below the mean plane, are shown above. As can be seen, the planar geometry maintains pi nonbonded attractive interaction. Sigma lone pair interactions will be similar to those which obtain in 1,2-difluoroethane and appear to be repulsive in nature. [Pg.100]

These results mean that polyhalobenzenes may resist the intuitively expected tendency toward deformations in order to relieve the steric repulsion of bulky vicinal halogens such as Cl, Br, and I, pi nonbonded attraction being the reason. [Pg.101]


See other pages where Nonbonded attraction is mentioned: [Pg.36]    [Pg.39]    [Pg.46]    [Pg.46]    [Pg.62]    [Pg.63]    [Pg.65]    [Pg.71]    [Pg.71]    [Pg.71]    [Pg.72]    [Pg.72]    [Pg.72]    [Pg.73]    [Pg.73]    [Pg.74]    [Pg.74]    [Pg.75]    [Pg.75]    [Pg.88]    [Pg.99]   
See also in sourсe #XX -- [ Pg.58 ]




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