Gauche effect


 * Main article: alkane stereochemistry

The term "gauche" refers conformational isomers (conformers) where two vicinal groups are separated by a 60° torsion angle.

In stereochemistry, gauche interactions hinder bond rotation. For example, sighting along the C2-C3 bond in staggered butane, there are two possible relative potential energies. The two methyl groups can be in an anti-bonding relationship, or offset at sixty degree dihedral angles. In the latter configuration, the two methyls are said to be in a gauche relationship, and the relative potential energy of each methyl-methyl gauche interaction is 0.9 kilocalories per mole. In general a gauche rotamer is less stable than an anti rotamer.

Gauche effect
The Gauche effect characterizes any gauche rotamer which is actually more stable than the anti rotamer. This effect is present in 1,2-difluoroethane (H2FCCFH2) for which the gauche conformation is more stable by 2.4 to 3.4 KJ/mole. Another example is 1,2-dimethoxyethane.



Key in the explanation of the gauche effect in difluoroethane is the increased p orbital character of both C-F bonds due to the large electronegativity of fluorine. As a result electron density builds up above and below to the left and right of the central C-C bond. The resulting reduced orbital overlap can be partially compensated when a gauche conformation is assumed, forming a bent bond.

The molecular geometry of both rotamers can be obtained experimentally by high resolution infrared spectroscopy augmented with in silico work. In accordance with the model described above, the carbon - carbon bond length is higher for the anti rotamer (151.4 pm vs. 150). The steric repulsion between the fluorine atoms in the gauche rotamer causes increased CCF bond angles (by 3.2°) and increased FCCF dihedral angles (from the default 60° to 71°).

In the related 1,2-difluorodiphenylethane (two protons replaced by phenyl) the threo isomer is found (by X-ray diffraction and from NMR coupling constants) to have an anti conformation between the two phenyl groups and the two fluorine groups and a gauche conformation is found for both groups for the erythro isomer. According to in silico results this conformation is more stable by 0.21 kcal/mole.

A gauche effect has also been reported for a molecule featuring 4 successive fluor syn substituents, the last one introduced by deoxofluorinating agent bis(2-methoxyethyl)aminosulfur trifluoride :



A related effect is the cis effect in alkenes.