An ab initio quantum chemical study of the five possible isomers, trans- and cis-conformations of nitrosoethylene, 4H-1,2-oxazete, 2H-1 azirine 1-oxide and acetonitrile oxide, of C 2H 3NO is reported. Molecular geometries and energies have been obtained at the Hartree-Fock and second-order Møller-Plesset levels of theory. It is important to note that in both the levels of theory, acetonitrile oxide is found to be the most stable isomer among all the five structures. Atomic charges, through Mulliken and natural population analysis schemes, dipole moments, entropies and rotational constants have been calculated using the above-mentioned levels. Various basis sets, 3–21G, 6–31G, 6–31G ∗∗ and 6–311G ∗, have been used at the HF level of calculation and 6–31G ∗ basis set has been utilized at the MP2 level of calculation. Finally, the vibrational spectra of all the five structures have been obtained using the HF/3–21G, HF/6–31G ∗∗ and HF/6–31 1G ∗ levels. An imaginary frequency has been found for the cis conformer of the nitrosoethylene molecule at both the HF/6–31G ∗∗ and HF/6–311G ∗ levels.