Abstract
The chiroptieal properties of the simplest chiral β-lactams, 3- and 4-methylazetidin-2-one, 1 and 2, respectively, were investigated. The experimental vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectra were measured and compared with <i>ab initio</i> predictions. Both compounds were found to form dimers with calculated binding enthalpies and free energies of about −51 k J/tool and −6 to −8 kJ/mol, respectively. The experimentally measured IR and VCD spectra were measured in concentrated non-polar (CCI<sub>4</sub>) solution and are in agreement with the predicted IR and VCD spectra of the dimeric forms, 1<sub>2</sub> and 2<sub>2</sub>, but not the monomers. The most intense dimer VCD bands originate from in-plane N–H wags, which perturb the H-bonded cyclic array. At the more dilute concentrations employed for the ECD spectra, the experimental ECD spectra in heptane were interpreted satisfactorily as arising from a mixture consisting predominantly of monomers. In protic solvent (H<sub>2</sub>O, MeOH), the ECD spectra are consistent with H-bonded monomers. Simple modeling suggests that the rotational strengths of the first electronic transition gain most of their intensity from the nonplanarity of the amide chromophore, the contributions of which follow a spiral rule previously enunciated.
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