Influence of fluorine substitution in nitrosoethylene and their vibrational spectra, calculated with Møller-Plesset perturbation theory of second order

The conformational and structural stability of monofluoro-nitrosoethylene CH₂CF-NO, and difluoro-nitrosoethylene CF₂CH-NO were investigated by ab initio Møller-Plesset Perturbation Theory of second order (MP2) calculations using the 6-311+G** basis set to include electron correlation. From the calculations the two molecules were predicted to exist predominantly in the planar trans structure with high trans-cis rotational barriers of about 10-11 kcal/mol as a result of pronounced conjugation between CC and NO bonds. The differences in the rotational barriers when going from the parent molecule, nitrosoethylene, to its fluorinated derivatives are explained on the basis of electronegativity. The vibrational frequencies were computed for the two molecules at the MP2 level. Normal coordinate analyses were carried out and the potential energy distributions, PED, among the symmetry coordinates of the normal modes of the molecules were computed. Complete vibrational assignments were made on the basis of normal coordinate analyses for the molecules. As expected we find there low Raman and appreciable infrared intensities in all modes which contain a high content of fluorine movements because vibrations of C-F bonds lead to a small change in polarizability, but to a large change in dipole moment. However, also modes involving double bonds have quite large intensities in both spectra. An appreciable number of modes in these molecules are composed of relatively large numbers of symmetry coordinates, i.e. they are highly mixed.