Existence of a low isoenantioselective temperature in complexation gas chromatography

The gas chromatographic enantiorecognition of the two enantiomeric pairs of Z- and E-2-ethyl-dioxaspiro[4,4]nonane (chalcogran), respectively, critically depends on whether the chiral selector nickel(II) bis[3-(heptafluorobutanoyl)-(1R)-camphorate] is chemically linked via a methylene spacer to poly(dimethylsiloxane) (Chirasil-Nickel 1), or is only dissolved in poly(dimethylsiloxane) (Chira-Nickel 2a). On 2a, the enantiomers are separated at ambient temperature with a large apparent enantioseparation factor αapp whereas on Chirasil-Nickel 1 only a low αapp is observed whereby the elution order of the enantiomers is reversed. Concise temperature-dependent studies show that on Chirasil-Nickel 1 a low isoenantioselective temperature, Tisoenant, of 80 °C is experimentally observed as the result of enthalpy/entropy compensation. For thermodynamic measurements, the method of the retention-increment R′ has been employed. This concept separates achiral contributions to retention due to the poly(dimethylsiloxane) matrix, which are identical for enantiomers, from enantioselective contributions to retention, which are due to complexation of enantiomers with the chiral selector. From Van't Hoff plots, concise thermodynamic data of enantioselectivity, −ΔD,L(ΔG), −ΔD,L(ΔH), ΔD,L(ΔS) and Tisoenant were obtained. It was found that minute changes in the structure of the spiroketals as well as miniscule differences of the nature of the chiral selector present in the stationary phase (chemically bonded versus physically dissolved) led to profound differences in enantioselectivity. The observation of a low isoenantioselective temperature Tisoenant represents are rare phenomenon in enantioselective gas chromatography which both complicates the study of chiral recognition mechanisms and the correlation of the absolute configuration and retention due to the temperature-dependent reversal of the elution order.