Molecular Structures and Rotational Potential Energy Surfaces of E and Z Geometrical Isomers of Propionaldehyde Oxime:  ab Initio and DFT Studies

Abstract

Molecular structure and conformational stability of E and Z geometrical isomers of propionaldehyde oxime (C(1)H3C(2)H2C(3)HNOH) have been studied by using the ab initio and DFT methods. The molecular geometries were optimized by employing the atomic basis sets 6-31G* at the HF-SCF and MP2 levels of theory in the ab initio method. The basis sets 6-31G and 6-31G* are used in the BLYP method of DFT to optimize the molecule. The optimized structural parameters of the above methods are discussed in the light of the electron diffraction results of the molecule. The variations in CN bond length, C1C2C3 and C2C3N angles of the sp and ac conformers of E isomer and of the ap conformer of Z isomer have been discussed in terms of nonbonding interactions of CH3 and NOH groups. The rotational potential energy surfaces of E and Z isomers were obtained for the C2−C3 rotational angle of propionaldehyde oxime at HF/6-31G*, MP2/6-31G*, and BLYP/6-31G* levels of theory. The global minimum occurs at φ(CCCN) = 120° and φ = 0° for the ac and sp conformations of the E isomer and φ = 180° for the Z isomer. The ac form is found to be more stable than the sp form by 0.16 kcal/mol in HF/6-31G* level of theory; this difference agrees very well with the experimental value of 0.15 kcal/mol. The rotational potential curve of Z form shows that it has large-amplitude motion. The chemical hardness values obtained for the different conformers of the two isomers are in disagreement with the statement that the higher stable conformation has higher chemical hardness, but the trend obeys the general trend of oxime molecules. The Fourier decompositions of the rotational potential of the propionaldehyde oxime are analyzed.

Molecular structure, conformational analysis and charge distribution of pralidoxime: Ab initio and DFT studies

“…One distinct disagreement between the three methods occurs regarding the results for the bond lengths of the CANOO moiety. This has been attributed to the presence of lone pair electrons and has also been found in other studies on simpler oximes [30]. In the current case, the presence of conjugation may have added to the problem.…”

The Microwave Spectrum ofn-Butyraldehyde Oxime

“…Furthermore, the partial vapor pressure of (Z)-isomers might be too low to detect their spectra with the vapor evaporated from neat liquid. Table 5 shows the molecular parameters of the (E)-1 conformer optimized by ab initio calculations, along with those of propionaldehyde oxime (6). The molecular parameters of (E)-1 which may be ascribed to the analyzed spectrum are consistently compared with the values of propionaldehyde oxime ((E)-anticlinal conformer) obtained from ab initio calculation at the MP2/6-31G* level of theory, as well as with those determined by gas-phase electron diffraction-microwave spectrum joint analysis (4,5).…”

“…The molecular structures of two rotational conformers of (E)-isomer and one conformer of the (Z)-isomer in the gas phase have been reported in studies of microwave spectroscopy (2, 3), gas-phase electron diffraction (4,5), and ab initio MO calculations (6). n-Butyraldehyde oxime (CH 3 OCH 2 CH 2 CHANOH) has been reported to exist in two geometrical forms with respect to the CAN bond, named (E)-and (Z)-isomers, whose population ratio was E:Z ϭ 54:46, based on nuclear magnetic resonance spectroscopy in the neat liquid state (7).…”

Isomerization study of C5H5NO molecules

“…Even though the energetically least stable isomer has a small value of chemical hardness, the order of stability of C 5 H 5 NO isomers arrived from the chemical hardness has been varied. In the earlier study of C 2 H 3 NO and propionaldehyde oxime molecule [24,25] the higher chemical hardness value could not predict the most stable isomer. Very recently we [4] have studied the isomerization of C 3 H 3 NO isomers, the higher chemical hardness value could not predict the most stable isomer and also the order of stability of isomers arrived at from the relative energies.…”