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.