Alkyl nitrates as achiral and chiral solute probes in gas chromatography -Novel properties of a beta-cyclodextrin derivative and characterization ofits enantioselective forces

Achiral and chiral interactions of alkyl nitrates (R-O-NO2) with heptakis-( 3-O-acetyl-2,6-di-O-pentyl)-beta-cyclodextrin (LIPODEX-D) in the gas phase were investigated chromatographically. Two major outcomes can be summarized . First, LIPODEX-D shows very fast temperature-dependent variations of the selectivity up to changes in the order of elution for C-1-C-5 alkyl nitrate s. These changes in selectivity reveal that LIPODEX-D possesses different s hape selectivities for small alkyl nitrates at different temperatures (40-8 0 degrees C), i.e. with increasing isothermal separation temperature extend ed (chainlike) alkyl nitrates have increased retention relative to bulky al kyl nitrates. The observations are highly reproducible and might indicate c onformational changes of the cyclodextrin, however, an ultimate proof would require further spectroscopic investigation. Secondly, the chiral separati ons of systematically varied sets of C-4-C-11 alkyl nitrates allowed the th ermodynamic characterization of enantiodiscriminating interactions. Quantit ative evidence is provided showing that the presence of an ethyl group at t he asymmetric carbon atom of an n-alkyl nitrate gives a strong enthalpic co ntribution to the resulting enantioselectivity, The Gibbs free energy diffe rences -Delta R-R,R-S(Delta G) decrease systematically three to six times i f the ethyl group is either shortened or enlarged by only a -CH2-increment. The results are based on two separate thermodynamic approaches, i.e. the d etermination of thermodynamic quantities (-Delta(R,S)(Delta G), -Delta(R,S) (Delta H), -Delta(R,S)(Delta S), T-iso) and a theoretical concept of enthal py-entropy compensation. The data from our laboratory experiments also indi cate that van der Waals interactions are responsible for chiral discriminat ion. (C) 1999 Elsevier Science B.V. All rights reserved.