Rotational barriers table

Further investigation of allylpotassium complexes have shown that 2-isopropylallyl potassium does not show diastereotopism of the methyl groups at temperatures as low as —155 °C54,59. Therefore, the activation barrier for interconversion is on the order of 4 kcal mol 1 or lower. Both crotyl (19) and prenyl (20) potassium complexes are further examples of the preference for allylpotassium compounds to exist as symmetric it species. The potassium has the appropriate atomic radius to reach both C(i) and C(3). No increase in stabilization is gained upon addition of solvent. Allylcesium behaves in the same manner. In general, the theoretically calculated rotational barriers (Table 9) are higher... 

The comparison of the averaged estimates of the rotational barriers (Table 26.6) with experimental data (2.6 kcal/mol) for the dimethyl ether obtained from rotational spectra [44] outlines the excellent consensus, which is a trustworthy validation of the chosen theoretical approach. 

The rotational barrier in methylsilane (Table 3.4, entry 5) is significantly smaller than that in ethane (1.7 versus 2.88 kcal/mol). This reflects the decreased electron-electron rqjulsions in the eclipsed conformation resulting from the longer carbon-silicon bond length (1.87 A) compared to the carbon-carbon bond length (1.54 A) in ethane. 

Table A4.6 gives the internal rotation contributions to the heat capacity, enthalpy and Gibbs free energy as a function of the rotational barrier V. It is convenient to tabulate the contributions in terms of VjRTagainst 1/rf, where f is the partition function for free rotation [see equation (10.141)]. For details of the calculation, see Section 10.7c.

Table A4.6 Contributions to the thermodynamic properties due to internal rotation as a function of V, the rotational barrier, and... 

TABLE 9. Calculated energies and rotational barriers of r/3 and / allyllithium and allylalkali metal compounds... 

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...

Table 5.27. Methyl rotation barriers Ai+b for various H-bonded andprotonated acetamide X complexes (cf. Fig. 5.64), with comparison NRT bond orders bco and bcs and bond lengths Rco and Rq n of the amide moiety in each complex...

Figure 5.65 The dependence of the acetamide methyl-rotation barrier (AT ) on NRT bond-order differences in the amide group (Ab = bco - cn) for various H-bonded complexes of the pseudo-cA (occH(in) = 0°) rotamer (see Table 5.27).

Experimentally, the methyl rotational barrier is found to be smaller for the cis isomer relative to the tram isomer. The experimental methyl rotational barrier as well as ab initio values are shown in Table 12. 

Table 12. Methyl rotational barriers in cw- and trans-2-butene...

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 ... 

Table 16. Methyl rotational barrier difference between double rotor (CH3-rotor (CH3-X-H) molecules -X-CH3) and single...

Finally, experimental measurement as well as ab initio computation show that the methyl rotational barrier is also higher in the cis than the tram conformation. These results are shown in Table 17. 

Table 17. cis - trans energy differences and methyl rotational barriers for methyl-vinyl-ether... 

Table 38. Rotational barriers for substituted ethyl cations...

As X becomes increasingly electronegative along a row of the Periodic Table, the conformational minimum (cis) will be increasingly stabilized relative to the maximum (trans) by Och-ffcx conjugative interactions. Consequently, the methyl rotational barrier should increase as X becomes more electronegative. 

TABLE 17. Nitro group rotational barriers (cm 1) for the set of nitro compounds used in the parameterization of MM2 and MM3 as obtained by the two force fields and from experiment43. Reproduced by permission of Elsevier Science from Ref. 43... 

TABLE 22. A comparison of rotational barriers and conformational energies (kcal mol-1) for several amino compounds and a single nitro compound between several commonly used force fields and experiment (all experimental and calculated data are taken from Reference 60 unless otherwise noted) die last entry provides the Average Absolute Error (kcal mol-1) between theory and experiment60. Reproduced by permission of John Wiley Sons, Inc. from Ref. 60... 

The effects of the substituents on nitrogen on rotational barriers were discussed by Yoder and Gardner (34) for formamides and acetamides. The pertinent data, given in Table 5, suggest that the barriers to rotation of formamides are not affected by the bulkiness of the alkyl group on nitrogen, but such a conclusion... 

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). 

Table 11 Rotational barriers (kcal mol" ) in allylic radicals."...

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]/... 

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