Rotational potentials sixfold

The most famous rotational barrier is that in ethane, but because the molecule is nonpolar its barrier is obtained from thermodynamic or infrared data, rather than from microwave spectroscopy. Microwave spectroscopy has provided barrier heights for a few dozen molecules. For molecules with three equivalent potential minima in the internal-rotation potential-energy function, the barriers usually range from 1 to 4 kcal/ mole, except for very bulky substituents, where the barrier is higher. Interestingly, when the potential function has sixfold symmetry, the barrier is extremely low for example, CH3BF2 has a barrier of 14 cal/mole.14... 

For 2,2,3-trimethylbutane (pentamethylethane 32), there is presumably a sixfold rotational potential with six equivalent conformations skewed on either side of perfect staggering. There is a barrier of 6.9kcalmol" to rotation about the central ethane bond . Pentaalkylethanes can also be analysed as trialkylmethanes, remembering... 

Sixfold barriers to internal rotation occur in molecules such as toluene andp-fluoro-toluene whose molecular frame has C2v symmetry about the rotor axis. The simplest spectroscopic model of internal methyl rotation assumes a rigid, threefold symmetric methyl rotor attached to a rigid molecular frame with the C2 axis coincident with the rotor top axis.25 The effective one-dimensional sixfold torsional potential takes the traditional form,... 

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. 

Regardless of the lack of our present knowledge concerning the nature and heights of these two barriers in the polypeptide backbone, two important points seem evident, viz. these barriers are not as high as the barriers to rotation about C—C bonds in hydrocarbons where the barriers are of the order of 3 kcal mole-1 or greater, and the potentials are probably threefold rather than sixfold, because sixfold potentials occur in only a few cases where the symmetry is such that the threefold contribution cancels. 

The derivation of quantitative information concerning the potential barrier to internal rotation from such measured frequencies in multitop molecules is a somewhat questionable procedure, which will not be treated in great depth here. For a two-top molecule the potential energy function can be expanded in a Fourier series (omitting sixfold and higher terms) to obtain... 

It is a general rule for a carbon-carbon bond that the greater the dihedral angle change between conformational minima, the higher the barrier is likely to be, so a onefold or twofold rotational barrier should be higher than a sixfold. The exceptions tend to be the most memorable, but generally the very factors which produce minima at only one or two positions in the rotational cycle are likely to increase the potential between the two minima. 

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