Ab initio MO Study of Vibrational Frequencies, Force Field and Normal Coordinate Analysis of the cis and trans Isomers of Methyl Nitrite

The force constant matries of the cis and trans isomers of methyl nitrite (CH₃ONO) have been analytically calculated based on SCF ab initio method with 3-21G basis set. Further, the vibrational frequencies and normal modes of vibrations of both isomers of the title molecule have also been theoretically determined. A comparison of theoretical frequencies with the IR experimental data is made. It is difficult to get the complete force field by experimental data made. It is dilficult to get the complete force field by experiments. The complete theoretical force field affords much interesting information about the title molecule, which can't be gotten based an experimental data. For the force constants associated with the NCO and ONO angles and their interaction force constant, the values of cis conformer are obviously bigger than those of trans conformer. It can be considerd as the reason of the fact that the cis conformer of CH₃ONO is more stable, than trans conformer. On the other hand, our calculated results take out many mistakes from the interaction force constants obtained by experiments.