On the interaction of geminal fluorines and geminal cyano groups. ‘anomalous’ ordering of rotational barriers in fluoroethanes

Intramolecular chiral discrimination between meso and d, l isomers

The problem of short-range (contact) interactions between chiral molecules is modelled by that of intramolecular interaction within a molecule formed by combining chiral moieties. The example used is 2,3-dicyanobutane in its active and meso isomers. The energy difference between the two conformational minima, which is the analogue of the intermolecular discrimination energy, is found by ab initio molecular orbital calculations to be 3.1 kJ mol -1 in favour of the meso species. A simpler calculation with the help of a pairwise additivity scheme (see Craig, Radom & Stiles 1975, preceding paper) gives 2.9 k J mol -1 . These discriminations are much greater than any found previously for dispersive and electrostatic forces, but on account of the stronger binding of the chiral moieties are expected to be greater than in intermolecular contacts.

A pairwise additivity scheme for conformational energies of substituted ethanes

Ab initio molecular orbital calculations are used to explore additivity in the conformational energies of poly-substituted ethanes in terms of conformational energies of ethane and appropriate mono- and 1,2-di-substituted derivatives. Such relations would allow complex calculations for poly-substituted ethanes to be replaced by much simpler ones on a small number of parent molecules. General expressions for the linear combinations are derived from the assumption that interactions between vicinal substituents are pairwise additive and depend only on the vicinal dihedral angle. The additivity scheme is tested for 15 ethanes, di-, tri- or tetrasubstituted by cyano and methyl groups and for a smaller number of fluoroethanes. Additivity applies to within 0.1- 0.3 k J mol -1 in the methylethanes and mostly to within about 0.7- 0.8 kJ mol -1 in cyanoethanes. Large deviations are found among the geminally substituted fluoroethanes. It is suggested that the additivity approximation is most successful in the absence of strongly interacting geminal groups. Predictions are made of conformational energies of ten hexa(cyano- and methyl-) substituted ethanes.