Barriers, torsional

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

It is noteworthy that it is not obligatory to use a torsional potential within a PEF. Depending on the parameterization, it is also possible to represent the torsional barrier by non-bonding interactions between the atoms separated by three bonds. In fact, torsional potentials and non-bonding 1,4-interactions are in a close relationship. This is one reason why force fields like AMBER downscale the 1,4-non-bonded Coulomb and van der Waals interactions. 

Torsional barriers are referred to as n-fold barriers, where the torsional potential function repeats every 2n/n radians. As in the case of inversion vibrations (Section 6.2.5.4a) quantum mechanical tunnelling through an n-fold torsional barrier may occur, splitting a vibrational level into n components. The splitting into two components near the top of a twofold barrier is shown in Figure 6.45. When the barrier is surmounted free internal rotation takes place, the energy levels then resembling those for rotation rather than vibration. 

Table 6.7 gives a few other examples of torsional barrier heights. That for ethylene is high, typical of a double bond, but its value is uncertain. The barriers for methyl alcohol and ethane are three-fold, which can be confirmed using molecular models, and fhose of toluene and nifromefhane are six-fold. The decrease in barrier heighf on going fo a higher-fold barrier is fypical. Rofafion abouf fhe C—C bond in toluene and fhe C—N bond in nifromefhane is very nearly free. 

The torsional strain is a sinusoidal function of the torsion angle. Torsional strain results from the barrier to rotation about single bonds as described for ethane on p. 56. For molecules with a threefold barrier such as ethane, the form of the torsional barrier is... 

The effect of introducing -hybridized atoms into open-chain molecules was discussed earlier, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are noted when sp centers are incorporated into six-membered rings. Whereas the fiee-energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecyclohexane and to 4.9 kcal/mol in cyclohexanone. ... 

Propane, the next higher member in the alkane series., also has a torsional barrier that results in hindered rotation around the carbon-carbon bonds. The barrier is slightly higher in propane than in ethane—a total of 14 kj/mol (3.4 kcal/mol) versus 12 kj/mol. 

Rotations around torsional barriers induce changes in chain conformation. For conjugated systems like polydiacetylenes, flow-induced changes in chain conformation can have a profound influence on the photon absorption and electronic conductivity properties of the material [73]. Flow-induced changes in molecular conformation form the basis for several technically important processes, the best known examples are the production of oriented fibers by gel spinning [74], the compatibility enhancement [75] and the shear-induced modification of polymer morphology [76]. 

The structures, energies, torsional barriers and vibrational spectra of three rotamers of tetrasulfane, H2S4, have been examined by Drozdova, Miaskiewicz and Steudel at the MP2/6-311G level [34]. Surprisingly, the cis-trans conformation (motif -l-H— symmetry Ci) is found to be most stable, followed by the all-cfs form (h—t symmetry C2), while the helical all-... 

Chain flexibility of the POP skeleton is very high and the corresponding torsional barriers for these macromolecules are estimated to be well below 1 kcal/ bond/repeat [451]. 

Table 5 Energy values obtained for the ground and first singlet and triplet excited states of trans-biacetyl in two extremal conformations of the methyl groups, as well as torsional barriers.

However, the nature of the skeletal bonds and the elements involved can have a powerful influence on the torsional barrier for individual skeletal bonds. 

The energetical description of rotations around bonds with high torsional barriers (e.g. the C=C double bond) demands the evaluation of the influence of higher cosine terms. Rotations around single bonds with sixfold symmetric torsional potentials have very low barriers (18) they occur in alkylsubstituted aromatic compounds (e.g. toluene), in nitro-alkanes and in radicals, for example. 

Torsional barrier the energy barrier to rotation of a single bond. 

We now discuss systematic hyperconjugative effects on orbital composition and stabilization, torsion barriers, and spectroscopic properties, for the CH2=CHX species summarized in Table 3.22. 

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. 

Figure 3.64 Torsion-barrier interactions of NH2CH2F. Left panel the torsional potential (solid line) and leading nN-oCF+ stabilization (dashed line) in the range 4> = 0-150°, where the amine group does not spontaneously undergo inversion. Right panel orbital contours of the strong nN-uCF+ interaction at = 0°.

Fig. 3.64 dramatically ilustrates how the hyperconjugative preference for strong vicinal donor-acceptor interactions may overcome pyramidalization barriers and other expected constraints on a torsion-barrier profile. 

Hydrogen-bond modulation of torsion barriers in amides... 

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