STEREOISOMERIC DISTINCTION BY THE KINETIC METHOD - 2,3-BUTANEDIOL

The kinetic method is used to differentiate the diastereomers and enan tiomers of 2,3-butanediol. By using a number of reference compounds of known gas phase basicity (GB), the GBs of (2R,3R)- and meso-butanedio l are determined as 191.5 and 191.2 kcal/mol, respectively with an est imated uncertainty of +/- 0.4 kcal/mol and an uncertainty in the 0.3 k cal/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. Gru enat et at, Helv. Chim, Acta, 68, 1647 (1985)), The smaller GB of the meso form is ascribed to destabilization associated with the two eclip sed 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 co llision-induced dissocation of the diasteromeric proton-bound diners f ormed from each isomer with a chiral reference compound; this experime nt is successful for both positively- and negatively-charged cluster i ons, The observed small difference In fragment ion abundance ratios in the dissociation of these two dimers is ascribed to a difference in f ree energy barriers to enantiomeric product formation from the two dia stereomeric intermediates, The present study demonstrates that the kin etic method can reveal small differences in free energies resulting fr om differences in chirality and, in this case at least, it provides a simple method to differentiate stereoisomers. (C) 1997 by John Wiley & Sons, Ltd.