Donor rotational barrier

These isomerization processes may be dependent on the nature of the solvent. For example, the rotational barrier of the tetrazathiapentalenes 15.15 (ca. 16 kcal moF ) is influenced by the donor or acceptor ability of the substituents X and Y through the S N short contacts.Solvents with acidic protons increase the magnitude of the barrier, whereas solvents that are good Lewis bases decrease the size of the barrier, owing to solvation of the transition state. 

Table 3.23 summarizes the rotation barriers and leading vicinal cr-cr interactions for methyl rotors CH3—X(X = CH3, NH2, OH) as well as higher group 14 congeners H3M—MH3(M = Si, Ge). Figure 3.59 shows orbital contour diagrams for syn and anti orientations of selected vicinal donor-acceptor NBOs in these species. We now discuss some qualitative trends of torsion barrier potentials in terms of these examples. 

Table 3.23. Rotation barriers (Ecc — Estg) and leading a-cr donor acceptor stabilizations (—A Eaa ) in anti and syn orientations for methyl rotors (CH3—X, X = CH3, NH2, OH) and higher ethane-like congeners...

From such comparisons one can judge that the syn/anti difference is sensitive to antibond polarity (which shifts the nodal plane relative to the adjacent donor NBO) and to the bond angles at which donor and acceptor NBOs are canted toward one another. Compared with C—H bonds, rotation barriers involving polar A—X bonds... 

In comparison with previous plots of this section, the no-crco anomeric interaction of Fig. 3.65 can be seen to be a rather typical example of hyperconjugative donor-acceptor interactions. Consequently, there seems to be no valid reason to invoke a special effect for the conformational preferences of sugars, obscuring their essential conformity with a unified donor-acceptor picture of ethane-like rotation barriers. 

Most of the data in Table 12 come from the work of Shvo et al. (78). Careful band-shape analysis and solvent-effect studies permitted evaluation of the rate constants and AG values at 298 K, which renders the discussion of substituent effects more meaningful than usual. The authors obtained reasonably linear Hammett plots when correlating log km with Or (79) for X and Y, holding one of these substituents constant. They also found that the dihydropyridine system may act as an unusually efficient donor, giving a AG of 17.6 kcal/mol with X, Y = H, CN, the only barrier below 25 kcal/mol reported for any donor-substituted cyanoethylene. However, with other acceptor combinations the dihydropyridine moiety is not so outstanding, and this illustrates the difficulty of measuring donor and/or acceptor effects by rotational barriers alone (vide infra). 

The experimental result seems to support this model. Table 11 lists values for rotational barriers in some allyl radicals (Sustmann, 1986). It includes the rotational barrier in the isomeric 1-cyano-l-methoxyallyl radicals [32]/ [33] (Korth et al., 1984). In order to see whether the magnitude of the rotational barriers discloses a special captodative effect it is necessary to compare the monocaptor and donor-substituted radicals with disubstituted analogues. As is expected on the basis of the general influence of substituents on radical centres, both captor and donor substituents lower the rotational barrier, the captor substituent to a greater extent. Disubstitution by the same substituent, i.e. dicaptor- and didonor-substituted systems, do not even show additivity in the reduction of the rotational barrier. This phenomenon appears to be a general one and has led to the conclusion that additivity of substituent effects is already a manifestation of a special behaviour, viz., of a captodative effect. The barrier in the 1-cyano-l-methoxyallyl radicals [32]/... 

Benzylic radicals offer themselves to a similar analysis. Some barriers to rotation have been determined (Conradi et ai, 1979). The barrier to rotation of 9.8 + 0.8 kcal mol for the a-cyano-a-methoxybenzyl radical [21] (Korth et al., 1985) could not be interpreted rigorously in terms of a captodative effect because estimates had to be made for the effect of a single captor or donor substituent on the rotational barrier. Within these limitations the barrier does not reflect more than an additive substituent effect. 

The theoretical work showed that the rotational barriers of the vinylidene ligand increased with X from n acceptor, a donor to n donor properties. Ligands (X) with... 

In the ethane case, however, the AIM analysis helps in understanding the overlap of the bonds and the location of the electrons as derived from the density picture, but it does not tell us anything about the origin of the rotational barrier. For that, we need methods that quantitatively give us energies that can be associated with the effects of donor-acceptor bonding (hyperconjugation) and electron-electron repulsion (Pauli repulsion) as noted above. 

Rotations in the monomer 9b, dimer 9c and trimer 9d do not seem to influence the electronic structure and the coupling between donor and acceptor. The rotational barriers comply with these found in the < xTTF-oPPV -C60 triads (0.34 kcal/mol). 

Barriers of 12.5-15.5 kcal/mol for neutral CpMo(CO)(MeC=CMe)-(SR) complexes are quite similar to rotational barriers in cationic complexes (74). Given the 7r acidity of CO and the tt basicity of SR-, these barriers are surprisingly small. Sulfur donor ligands tend to be electronically flexible, and the soft thiolate may facilitate alkyne rotation by simultaneous rotation of the thiolate substituent. 

If the donor-free syn adduct (48) is generated by adding a Lewis acid, then it can rapidly isomerize to the anti adduct (50) at 25 °C. Available evidence indicates that the rotational barrier about the C bond in (48) and (50) is very small. A possible explanation is that pir-pir bonding with the 3p-orbital on aluminum lowers the C=C double bond character. Furthermore, o--bond hyperconjugation in the transition state for rotation (49) reduces its energy and hence the barrier to rotation. That the facile isomeriza-... 

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