Enantiomeric Separations of Chiral Sulfonic and Phosphoric
Acids with Barium-Doped Cyclofructan Selectors via an Ion Interaction
Mechanism
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Abstract
New
cyclofructan-6 (CF6)-based chiral stationary phases (CSPs)
bind barium cations. As a result, the barium-complexed CSPs exhibit
enantioselectivity toward 16 chiral phosphoric and sulfonic acids
in the polar organic mode (e.g., methanol or ethanol mobile phase
containing a barium salt additive). Retention is predominantly governed
by a strong ionic interaction between the analyte and the complexed
barium cation as well as hydrogen bonding with the cyclofructan macrocycle.
The log <i>k</i> versus log [X], where [X] = the concentration
of the barium counteranion, plots for LARIHC–CF6-P were linear
with negative slopes demonstrating typical anion exchange behavior.
The nature of the barium counteranion also was investigated (acetate,
methanesulfonate, trifluoroacetate, and perchlorate), and the apparent
elution strength was found to be acetate > methanesulfonate >
trifluoroacetate
> perchlorate. A theory based upon a double layer model was proposed
wherein kosmotropic anions are selectively adsorbed to the cyclofructan
macrocycle and attenuate the effect of the barium cation. van’t
Hoff studies for two analytes were conducted on the LARIHC–CF6-P
for three of the barium salts (acetate, trifluoroacetate, and perchlorate),
and the thermodynamic parameters governing retention and enantioselectivity
are discussed. Interestingly, for the entropically driven separations,
enantiomeric selectivity can increase at higher temperatures, even
with decreasing retention