Peculiarities of supramolecular chiral recognition upon adsorption on the surface of ortho-toluic acid crystals
Abstract
Classical chiral recognition assumes the presence of a chiral selector, a molecule with an asymmetric carbon atom. However, the potential for expanding the capabilities of this type of recognition is almost exhausted today. Therefore, there is a need to search for new chiral selectors that operate on different principles. Therefore, it is interesting to study systems based on supramolecular chirality. This type of chirality is of particular importance due to the fact that supramolecular chirality underlies the formation of life on Earth. One manifestation of supramolecular chirality is enantiomorphic crystals.
For such crystals, the mechanism of supramolecular chiral recognition was previously studied in the case of adsorption of optically active substances on them. However, a detailed study of this mechanism required investigation of a large number of chiral crystals with different properties. In this study, we investigated the adsorption isotherms of enantiomers on the surface of enantiomorphic crystals of o-toluic acid.
The Viedma ripening method was used to obtain homochiral crystals. The crystals obtained in this way were deposited on the surface of ASKG silica gel. Adsorption on pure crystals was studied under reversed gas chromatography conditions using limonenes as adsorbates. Silica gel modified with o-toluic acid crystals was used to study the adsorption of menthols from solutions in n-heptane.
The analysis of the adsorption isotherms of limonene enantiomers showed that the isotherms differ at temperatures of 50 and 60°C. The enantioselectivity coefficient α, calculated as the ratio of higher adsorption to lower adsorption, is 1.21-1.23). At 70°C, the difference in the adsorption of enantiomers visually practically disappeared. Above 70°C enantioselectivity was not observed. However, the use of the t-criterion for assessment of the differences in adsorption isotherms showed a statistically significant difference in the equilibrium adsorption starting from a partial pressure of 4.17 kPa and higher. The adsorption isotherms were assigned to type III according to the BET classification and were approximated by the Freundlich equation.
In the case of adsorption of menthols from solutions, even the shape of the isotherm was different. Thus, the adsorption isotherm of D-menthol can be classified as type I according to the BET classification, and is close to linear for the majority of the studied enantiomer concentrations. At the same time, the shape of the L-menthol adsorption isotherm is close to type II. Thus, the mechanism of adsorption on silica gel modified with o-toluic acid is different. The enantioselectivity coefficient was 1.45. Thus, the achieved enantioselectivity coefficients for o-toluic acid turned out to be among the best coefficients for previously studied enantiomorphic crystals.
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