Ab initio and density functional calculations of the structures and vibrational spectra of formaldoxime

Molecular structure and vibrational frequencies of formaldoxime, CH ₂NOH, have been investigated with ab initio and density functional theory (DFT) methods. The molecular geometries for all the possible conformers of the molecule were optimized using DFT-B3LYP, DFT-BLYP and MP2 applying the standard 6-311+G** basis set. From the calculations, the molecule was predicted to exit predominantly in trans conformation with the trans-cis rotational barrier of about 9.2-9.7 kcal/mol depending on the level of theory applied. The vibrational frequencies and the corresponding vibrational assignments of formaldoxime in C_s symmetry were examined theoretically and the calculated infrared and Raman spectra of the molecule in the trans conformation were plotted. Observed frequencies for normal modes were compared with those calculated from normal mode coordinate analysis carried out on the basis of ab initio and DFT force fields using the standard 6-311+G** basis set of the theoretical optimized geometry. Theoretical IR intensities and Raman activities are reported.