ELECTRONIC AND VIBRATIONAL CIRCULAR-DICHROISM OF MODEL BETA-LACTAMS -3-METHYLAZETIDIN-2-ONE AND 4-METHYLAZETIDIN-2-ONE

The chiroptical properties of the simplest chiral beta-lactams, 3- and 4-methylazetidin-2-one, 1 and 2, respectively, were investigated. The experimental vibrational circular dichroism (VCD) and electronic circ ular dichroism (ECD) spectra were measured and compared with ab initio predictions. Both compounds were found to form dimers with calculated binding enthalpies and free energies of about -51 kJ/mol and -6 to -8 kJ/mol, respectively. The experimentally measured IR and VCD spectra a were measured in concentrated nonpolar (CCl4) solution and are in ag reement with the predicted IR and VCD spectra of the dimeric forms, 1( 2) and 2(2), but not the monomers. The most intense dimer VCD bands or iginate from in-plane N-H wags, which perturb the H-bonded cyclic arra y. At the more dilute concentrations employed for the ECD spectra, the experimental ECD spectra in heptane were interpreted satisfactorily a s arising from a mixture consisting predominantly of monomers. In prot ic solvent (H2O, MeOH), the ECD spectra are consistent with H-bonded m onomers. Simple modeling suggests that the rotational strengths of the first electronic transition pain most of their intensity from the non planarity of the amide chromophore. the contributions of which follow a spiral rule previously enunciated.