The beta-lactam molecules of 2-azetidinone and 3-methyl-2-azetidinone were studied by ab initio methods. The calculated spectra of both molecules were compared with experimental vibrational frequencies of cyclobutane, 2-azetidinone, and 3-methyl-4-benzoyl-4-phenyl-2-azetidinone. The vibrational frequencies and intensities of the normal modes, especially from the four-membered ring, were identified. Several frequencies were proposed as analytically useful to characterize the beta-lactam ring. The relative utility of these ring frequencies was discussed. The optimized parameters, bond lengths and bond and torsional angles of all molecular structures, were computed. Also calculated were the total atomic charges, electronic density, and several thermochemical parameters : ZPE, thermal energy, rotational constants, and entropies.