Millimeter Wave Spectrum and Molecular Force Field of Tetratomic C2υ Molecules: NO2F

The millimeter rotational spectrum of NO₂F has been measured in the frequency range 90–200 kMc/sec. By centrifugal distortion analysis, based on the Kivelson and Wilson first‐order perturbation treatment, the following values of the ground‐state parameters were obtained: $$\text{A\hspace{0.17em}=\hspace{0.17em}13 201.31 B\hspace{0.17em}=\hspace{0.17em}11 446.25 C\hspace{0.17em}=\hspace{0.17em}6118.99}$$ $${\tau }_{\mathrm{aaaa}}{\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}0.039814 τ}}_{\mathrm{bbbb}}\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}0.078638}$$ $${\tau }_{\mathrm{aabb}}{\text{\hspace{0.17em}=\hspace{0.17em}0.012452 τ}}_{\mathrm{abab}}\text{\hspace{0.17em}=\hspace{0.17em}−0.048868}$$ . Two different methods were used in order to determine the general quadratic force field, by combination of infrared and microwave data. The first method is based on the complete consistency between force constants and vibration frequencies of the normal species and on the best fit of the rotational spectrum. The second method uses the least‐square determination of the corrections on some starting set of force constants. All the isotopic‐ and normal‐species vibration frequencies measured by Bernitt, Miller, and Hisatsune and all the centrifugal distortion contributions on millimetric rotational transitions were used as experimental data, taken with appropriate weights. The two methods yield very similar results. The agreement between observed and calculated rotational and vibration frequencies is of the order of magnitude of the experimental errors. Two different sets of force constants were obtained, corresponding to an exchange of ${\nu }_2$ and ${\nu }_3$ vibration frequencies in the diagonalization of the GF matrix.