Steroisomeric Distinction By the Kinetic Method: 2,3-Butanediol

Abstract

The kinetic method is used to differentiate the diastereomers and enantiomers of 2,3-butanediol. By using a number of reference compounds of known gas phase basicity (GB), the GBs of (2R,3R- and meso-butanediol are determined as 191.5 and 191.2 kcal/mol, respectively, with an estimated uncertainty of ±0.4 kcal/mol and an uncertainty in the 0.3 kcal/mol difference of 0.12 kcal/mol. This result is consistent with a literature GB value for a mixture of isomers of 192.3 kcal/mol (C. Gruenat et al, Helv. Chim, Acta, 68, 1647 (1985). The smaller GB of the meso form is ascribed to destabilization associated with the two eclipsed methyl groups in the eclipsed conformation which is energetically favored by simultaneous bonding (chelation) of the two oxygen atoms to the proton. Differentiation between the enantiomers is achieved by collision-induced dissociation of the diasteromeric proton-bound dimers formed from each isomer with a chiral reference compound; this experiment is successful for both positively- and negatively-charged cluster ions. The observed small difference in fragment ion abundance ratios in the dissociation of these two dimers is ascribed to a difference in free energy barriers to enantiomeric product formation from the two diastereomeric intermediates. The present study demonstrates that the kinetic method can reveal small differences in free energies resulting from differences in chirality and, in this case at least, it provides a simple method to differentiate steroisomers. © 1997 John Wiley & Sons, Ltd.