The electrostatic model for threefold barriers to internal rotation developed in earlier papers is updated. The barrier in methyl arsine is predicted to be 1.57 kcal/mole. It is also shown that the barrier recently measured for nitrosomethane is in fair agreement with the model result. The effect upon the barrier of methyl substitution in ethanelike molecules is shown to correlate well with the electronegativity of the central atom where substitution occurs. The very different barrier increases caused by methyl substitution in propene and methyl ketene are reproduced and shown to be simply related to structure. An explanation is proposed for the experimentally observed lowering of the barrier in going from acetaldehyde to acetone. The very low barrier in acetic acid is shown to be consistent with delocalization of hydroxyl pi electrons. A low barrier for ethanol is not supported by the model.

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For a review of theories of the barrier and a tabulation of experimental measurements, see J. P. Lowe, Progress in Physical Organic Chemistry, A. Streitwieser and R. W. Taft, Eds. (Interscience Publishers, Inc., New York, 1968), Vol. 6.
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The C‐C‐C angle is 2° smaller in isobutene than it is in propene. One might think that this would push the methyl group closer to the cis‐methylene hydrogen, producing a barrier‐lowering steric repulsion. If this were so, the model (which neglects this effect) should predict a barrier for isobutene which is too high. This does not seem to happen. A detailed look at the structures reveals that the methyl‐group hydrogen approaches only 0.02 Å closer to the methylene hydrogen in isobutene than it did in propene, the angle change being compensated by other changes. Apparently the methyl groups in isobutene try to move apart in a way which doesn’t move them closer to the methylene group.
12.
A recent determination of the structure of methyl ketene (Ref. 14) has required that the charge parameter midway in the C = O bond (see Ref. 2) be changed to −1.0. This causes the predicted value for the barrier in methyl isocyanate to be ≈0.18 kcal/mole compared to the experimental value of 0.08 kcal/mole [
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M. E. Schwartz and L. C. Allen (to be published).
15.
The multipoles associated with electronic charges in the pi and cis‐sp2 orbitals on the hydroxyl oxygen have large but opposing effects upon the barrier. Hence, when they are unequally occupied, a net effect upon the barrier results. See Ref. 2 for a discussion of multipoles.
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