Alcohols, ethers, carbohydrates, and related compounds part V. The Bohlmann torsional effect

The Bohlmann effect, in its original context, occurred when, on a carbon attached to an amine nitrogen, there was a C-H bond anti-coplanar to the nitrogen lone pair. A negative hyperconjugation between the lone pair and that C-H bond (a transfer of electron density from the lone pair into the C-H sigma* orbital) led to the bond becoming weaker, stretching, and having its stretching frequency in the infrared shifted lower by 100-150 cm(-1). In the generalized Bohlmann effect, one does not need specifically a hydrogen, but an alkyl group (for example) will serve as well. Quantum mechanical calculations (at any level) show this effect clearly, both for the hydrogen case and for the sp(3) carbon as the attached electron-accepting atom. The C-H bond stretches by about 0.005 Angstrom, and a C-C bond similarly stretches by about 0.007 Angstrom. If one replaces the amine nitrogen with an ether (or alcohol) oxygen, a similar effect is noted. The bond (C-H or C-C) anti to a lone pair on oxygen stretches, and its stretching frequency is reduced. The reduction of the stretching frequency in the ether or alcohol case is partly compensated by the increase in stretching frequency resulting from the electronegativity of the attached oxygen, so the spectroscopic effect is less dramatic in ethers than in amines. A similar effect can be shown to occur in fluorides, but it has no stereochemical consequences. An exo-Bohlmann torsional effect was also uncovered in this work. An alkyl group on the oxygen or nitrogen atom, on the other side away from the area where the Bohlmann effect is occurring, suffers a reduction in the size of its torsional barrier. This is still one further example of the "effects" that complicate organic chemistry, which must be taken into account if molecular mechanics is to give a good representation of molecular structural behavior.