Extraordinary chiral discrimination in inclusion gas chromatography. thermodynamics of enantioselectivity between a racemic perfluorodiether and a modified gamma-cyclodextrin.
The enantiomers of the perfluorodiether "compound B" [2-(fluoromethoxy)-3-methoxy-1,1,1,3,3-pentafluoropropane], a decomposition product of the inhalational anesthetic sevoflurane [2-(fluoromethoxy)-1,1,1,3,3,3-hexafluoropropane], were separated by gas chromatography on octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin (Lipodex E), dissolved in polysiloxane PS 255 (30% w/w), with an unexpectedly high separation factor of alpha = 10.6 at 26 degrees C. Using the concept of the retention increment R', non-enantioselective and enantioselective contributions to retention were separated and thus reliable thermodynamic parameters of enantioselectivity, i.e. - deltaS,R(deltaG) = 5.7 (0.05) kJ/mol at 303 K, - deltaS,R(deltaH) = 20.1 (0.64) kJ/mol, deltaS,R(deltaS) = -47.4 (2.0) J/K mol and T(isoenant) = 424 (30) K or approximately 150 degrees C, were determined by temperature-dependent measurements. The enantiomeric bias represents the largest values ever measured in enantioselective gas chromatography. An equation is presented which allows calculation of the non-enantioselective contributions to retention from measurements at two arbitrary concentrations of Lipodex E in polysiloxane. Surprisingly, the enantioselectivity is greatly reduced when employing the beta-cyclodextrin analogue and breaks down completely with the alpha-cyclodextrin analogue of Lipodex E.
Schurig
Volker
V
Institute of Organic Chemistry, University of Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany. volker.schurig@uni-tuebingen.de
Schmidt
Roswitha
R
D002849:Chromatography, Gas / Q000379:methods*; D003505:Cyclodextrins / Q000737:chemistry*; D004987:Ethers / Q000737:chemistry*; D013237:Stereoisomerism; D013816:Thermodynamics; D047408:gamma-Cyclodextrins
D003505:Cyclodextrins; D004987:Ethers; D047408:gamma-Cyclodextrins; C023792:gamma-cyclodextrin