A theoretical study on the stability and spectra of cycloaddition products: Methylene-β-lactam isomers

Abstract

The theoretical thermodynamic stability was calculated for products of cycloaddition of vinylimine to ketene and isocyanic acid to allene by using the MP2, DFT(B3PW91), and HF methods involving the 6-311++G** basis set. These cycloaddition reactions could yield fourteen different molecules; practically however, free Gibbs energies calculated for the considered molecules suggest that only 4- and 3-methylene-β-lactams can be formed in observable amounts. For these particular isomers the heats of formation, geometry, rotational constants, harmonic vibrational frequencies, infrared intensities, Raman activities, depolarisation ratios, and ¹H, ¹³C, ¹⁷O and ¹⁵N NMR spectra were evaluated and discussed. The PED analysis carried out for theoretical IR spectra allowed us to assign vibrations of the two isomers. The IR and NMR assignments form a basis for the elucidation of future experimental data.