Vicinal interactions

Gronert [42] and Schleyer [43] are not aware of our theory [41]. Branched alkanes are stabilized by the C-C bond polarization by two antiperiplanar C-H bonds. The polarization is favored by the orbital phase continuity. We can predict the relative stabilities of alkanes only by counting the number of the vicinal bond trios. Neither the Gronert nor the Schleyer model contains any vicinal interactions. 

Following the procedure for vicinal interactions illustrated in Example 1.4 and Section 3.3.1, we can associate each geminal ctab— obc donor-acceptor interaction with an arrow-pushing diagram and partial admixture of an alternative resonance diagram, as shown in Fig. 3.78. The formal two-electron transfer from gab to obc results in the NBO configuration... 

It is inherently surprising that geminal interactions are typically weaker than vicinal interactions, because the former involve orbitals that are in closer spatial proximity. The reasons for this counterintuitive distance dependence can be seen by decomposing the geminal Fock-matrix element into individual atomic hybrid contributions. 

In the ketoses, there is a side chain attached to the anomeric carbon atom. The more-stable furanose anomer is, therefore, the one in which the side chain and the hydroxyl group on the neighboring carbon atom are trans to each other,16 In this anomer, there is little vicinal interaction, but there is one in the other anomer, as well as in the pyranose forms, and this interaction increases when the bulk of the side chain is increased. 

These observations are in accord with the established idea that weak aromatic transitions can gain or lose intensity through vibronic vicinal interactions which practically do not affect the backbone peptide chromophores. The results are compiled in Table II. 

Sulfur-hydrogen scalar coupling has been observed for a solution of 2moldm 2,5-dihydrothiophene 1,1-dioxide (butadiene sulfone) 90, in acetone. A partially resolved triplet, of total width 18 Hz, collapses to a singlet, width 7 Hz, when proton decoupling is applied. The major coupling is attributed to a vicinal interaction between the sulfur and olefinic protons, by comparison of the carbon spectrum of diethyl ketone (the carbonyl carbon resonance exhibits a septet) and the sulfur spectrum of sulfolane (the sulfur peak shows no structure). This was confirmed by selectively decoupling at the two relevant proton frequencies in turn. 

From this type of analysis, Leonard and Foster conclude that a change in the mean intrinsic residue coefficient best accounts for the lack of correlation between the monochromatic rotations and the dispersive parameters, and they tentatively ascribe this change to alterations in tertiary structure which might affect vicinal interactions, side-chain hydrogen bonds, and so... 

The simple at-a-glance predictive correlation that emerges from Tables 3 and 4 is as follows with the peptide backbone drawn in the plane of paper, substituents in the same half space (i.e., ///reconfiguration) are all matched to the same helical handedness. This correlation can be rationalized by analyzing the basic conformers in /3-amino acids.183184 263 264 Such an analysis reveals a strongly preferred backbone torsion around the Ca—Cp bond, driven in part by the tendency to minimize repulsive vicinal interactions (Figure 36)249 and possibly by internal electrostatic considerations.184 The sc conformation, favored for monosubstituted /32- or /33-residues (especially if the substituents are bulky)263... 

Methoxy- or methylthio-5-iodopyrimidines (9) are effectively coupled with methyl acrylate to form the product (10). Reaction of the 5-bromo analog is slower, and is sensitive to vicinal interaction in 4,6-disubstituted pyrimidines, most of the substrate was unreacted (only a 10% yield) (86S555). A free amino group in the 4-position (11) is tolerated in the preparation of the alkenylated product (12) (82CPB2410). 

Cinchonidine, being a bulky molecule, reduces the accessible active platinum surface as it adsorbs and should causes some deactivation with respect to racemic hydrogenation. The decrease in formation rate of the main product after the maximum can be a result of poisoning by adsorbed spectator species, which inhibit enantiodifferentiating substrate-modifier interaction. Adsorbed cinchonidine in parallel mode (active form) provides an enantioselective site (Figure 7.8) and when the reactant is adsorbed in the vicinity, interaction between reactant and modifier leads to such orientation that hydrogenation towards the main product (e.g. B or 1-R enantiomer) is preferred. However, when the tilted form (Figure 7.8) of... 

In the competing endo/exo orientations, the reaction can reach the exo TS at the cost of an increased repulsion between the reacting centres and a slightly increased deformation of the maleonitrile fragment this cost is partly recovered by new bond formation. Since vicinal interactions are more favourable to the endo than to the exo mode, the endo mode emerges to be favoured also in the absence of further endo-orienting effects. 

Figure 28 presents the PE spectra " of bicyclo[l.l.l]pentane 10 and of di-bicyclo-[l.l.l]pentyl 11, the latter in its conformation. These molecules are again examples of the fact that vicinal interaction terms must be included in an EBO model, for even a qualitative discussion of their electronic structure, and thus of their PE spectra. This... 

On the other hand, vicinal interactions are also very common because they correspond to the interactions of orbitals at two directly connected atoms. The stereoelectronic aspects of such interactions are immediately obvious when stability and reactivity change as a function of relative arrangement of the interacting bonds in space (i.e. via rotation around the bridging bond). Vicinal interactions are responsible for the majority of stereoelectronic effects discussed in this book. 

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