ABSTRACT:The conventional strain energies for all four isomers of oxadiazetidine are determined within the isodesmic, homodesmotic, and hyperhomodesmotic models. Optimum equilibrium geometries, harmonic vibrational frequencies, and corresponding electronic energies are computed for all pertinent molecular systems using self-consistent field theory, second-order perturbation theory, and density functional theory (DFT) and employing two basis sets of triple-zeta valence quality: 6-311G(d,p) and 6-311ϩG(2df,2pd). The DFT functional employed is Becke's three-parameter hybrid functional using the Lee, Yang, and Paar correlation functional. Single-point fourth-order perturbation theory and coupled-cluster theory restricted to single and double excitations [CCSD(T)] calculations employing the larger basis set also are computed, at both the second-order Møller-Plesset (MP2)/6-311G(d,p) and the MP2/6-311ϩG(2df,2pd) optimized geometries, to determine the effect of higher-order correlation effects on strain energy computation and to gauge the effect of geometry on these effects. Using the same models and methods, the conventional strain energies for both isomers of oxazetidine also are computed, to determine the effect on the strain energy of replacing a nitrogen with a carbon in the ring.