VCD technique in determining intermolecular H-bond geometry: a DFT study

Model chiral beta-lactam molecules, (3S)-and (4R)-fluoro-2-azetidinone, were calculated at the B3PW91/aug-cc-pVTZ level, to be hydrogen bonded with the achiral HX molecule (X = F, Cl, Br). Two stable structures of the complex are possible: a cyclic or a bent H-bond, in which the HX molecule interacts with the CO group and is either close to NH or CH2 (CHF) moiety, respectively. The VCD effect of these two forms differs in several respects, however, the main difference is the sign of the VCD rotational strength of the nu(HX) stretching vibrations revealing the geometry of the hydrogen bond complex. (C) 2003 Elsevier B.V. All rights reserved.