Ionic liquid chiral resolution: methyl 2-ammonium chloride propanoate on Al(854)S surface

Abstract

The adsorption of the chiral methyl 2-ammonium chloride propanoate ionic liquid on the chiral Al(854)(S) surface has been investigated by density functional calculations on periodic slab models. The results show that the molecule features an enantioselective dissociative adsorption at the surface chiral center. The low coordination of Al atoms at kinked steps and the strong attractive forces toward molecular O atoms are the main causes of the dissociation. At the surface, 2-ammonium propanal, methoxy groups, and Cl atoms are generated, attached at different sites depending on the precursor enantiomer. The adsorption strengths reveal that the bonding of the R-enantiomer is more favorable than S-enantiomer by 0.20 eV, enough for a chiral resolution process with an enantiomeric excess of >99%, whereas adsorption on achiral Al(111) surface reveals no enantiomeric discrimination with a weak molecular adsorption and no dissociation. Enantiomeric discrimination on chiral Al(854)(S) surface is possible due to different semicore molecular levels binding energies and to distinct infrared vibrational fingerprints. The present results open the possibility for a rather simple way to separate these enantiomer