Investigation of mandelic acid bonding on Pirkle type chromatographic stationary phases by Raman spectroscopy

The bonding of mandelic acid enantiomers has been studied on benzene-leucin e, dinitrobenzene-leucine and dinitrobenzene-phenylalanine type chiral stat ionary phases connected to zeolite A supports. The pi-donor, pi-acceptor an d H-bonding interactions responsible for diastereomer pair formations can b e studied under quasi in situ chromatographic conditions by Fourier transfo rm Raman and surface enhanced Raman spectroscopic techniques. Structural di fferences between diastereomer pairs result in observable spectral differen ces at a phase load of approx. 50%. It was shown that the decreasing pi-acc eptor character of the phase is associated with its increasing capability o f H-bond formation. Correlating spectral data to chromatographic results it can be concluded that, in addition to H-bonding as well as to pi-donor-pi- acceptor interactions, steric hindrances due to bulky moieties of either th e stationary phase or the analyte molecules are of importance in successful separations. (C) 2000 Elsevier Science B.V. All rights reserved.