Infrared Reflection−Absorption Spectroscopy of Racemic and Enantioenriched Methyl 17,18-Dihydroxyoctadecanoate at the Air−Water Interface

Abstract

Surface pressure−area isotherms (Π−A) of racemic and enantioenriched methyl 17,18-dihydroxyoctadecanoate (rac- and (17R)-17,18-DHO) at the air−water interface are very similar, while these esters show significant differences in infrared reflection−absorption spectroscopy (IRRAS). Substantial shifts of the νas(CH2) from 2927 to 2917 cm-1 and of the ν(CO) bands from 1722 to 1742 cm-1 are observed upon monolayer compression from 0.8 to 0.2 nm2 per molecule. This shift occurs gradually for rac-17,18-DHO and sharply within a small compression interval for (17R)-17,18-DHO. This suggests first, a change from a weak heterochiral to a weak homochiral preference, and second a removal of the two hydroxyl groups from the air−water interface, while the ester function immerses into the uppermost water layer in the compressed monolayer. This arrangement appears to be stabilized by intermolecular hydrogen bonds.