Temperature Dependent Inversion of Elution Sequence in the Resolution of α‐Amino Acid Enantiomers on Chiral Diamide Selectors

Great caution should be taken in attempting to derive configurations from the sequence of elution of enantiomers, according to the observations of Gil-Av et al. and Schurig et al. Both groups found a temperature dependence in the elution sequence obtained in the gas-chromatographic separation of enantiomers on chiral stationary phases, which led to lack of separation at the isoenantioselective temperature Tiso. defined by (a) and to a reversed order of elution above and below T.iso. Hydrogen-bond-forming systems (amino acids on diamide phases) and complex-forming selectand-selector systems (oxaspirocyclic compounds on nickel complexes) were investigated. The Tiso values were 130–150°C and 70–90°C, respectively. These findings show, at the same time, that below Tisothe elution sequence is determined by the differences in the interaction enthalpy as given by the Gibbs–Helmholtz equation (b) and that above Tiso it is determined by the interaction entropy.