Could linear solvation energy relationships give insights into chiral recognition mechanisms? 1. Pi-pi and charge interaction in the reversed versus the normal phase mode.

ABSTRACT

Linear solvation energy relationships (LSER) have been used for years in liquid chromatography to access the factors that lead to retention and, more recently, to selectivity. In chiral separations, two enantiomers will receive exactly the same descriptors correctly predicting that they will not be separated by any non-chiral stationary phase. However, LSER studies could be used considering that each enantiomer sees a chiral stationary phase (CSP) differently. Working with the enantioselectivity factor, k'2lk'1 could give interesting information on the chiral recognition mechanisms. The b and v system parameters always predominantly contribute to a solute's retention in reversed phase liquid chromatography (RPLC) compared to the a, e and s parameters. However, these minor parameters for retention may become the essential ones for enantiomeric separations where a minimum of three simultaneous interactions is needed for an enantioseparation to occur. In this non-chiral study, six different stationary phases, a classical C(18), a diphenyl bonded stationary phase (DP), a polystyrene-divinylbenzene (DVB), a polar embedded new stationary phase, an anion exchanger (SAX) and a teicoplanin aglycone (TAG) CSP were studied with achiral test solutes. Significantly higher e terms were obtained for the SAX and TAG columns. It seems that the ability of stationary surface charges to induce dipoles in polarizable molecules is encoded mainly in the e term. Since the DP and DVB columns did not produce e parameters significantly higher than the C(18) column, it seems that pi-pi interactions are (a) extremely weak in RPLC or (b) they are not simply encoded in this single e system parameter but spread in at least three parameters. The TAG CSP produced logically very different parameters when used in the reversed phase mode compared to the normal phase mode showing the critical role of the mobile phase.