Internal Rotation of Molecules with Two Inequivalent Methyl Groups: The Microwave Spectrum of N-Methylethylidenimine

The microwave spectra of two isotopic species of trans‐N‐methylethylidenimine $({\mathrm{CH}}_3\mathrm{CH}={\mathrm{NCH}}_3)$ were recorded between 8 and 40 GHz. The rotational transitions clearly demonstrate the coupling with the internal motions of the two methyl groups, which are inequivalent in this molecule. The theory for the over‐all and internal rotation was extended to molecules with two inequivalent symmetric internal rotors. The internal axis method (IAM) was used to calculate the internal rotors' splittings of the rotational transitions. They were split into five components according to the different irreducible representations of the symmetry group. In addition to this, the spectra were further complicated by the nuclear quadrupole multiplets of the nitrogen. A least squares analysis of the measured transition frequencies yielded the following rotational and quadrupole coupling constants for ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{NCH}}_3\text{:\hspace{0.17em}A=38258.627± 0.048\hspace{0.17em}}\mathrm{MHz}\text{,\hspace{0.17em}B=4078.1354± 0.0058\hspace{0.17em}}\mathrm{MHz}\text{,\hspace{0.17em}C=3862.7391± 0.0072\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\hspace{0.17em}\chi }}_{\mathrm{aa}}\text{=1.578± 0.039\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\hspace{0.17em}\chi }}_{\mathrm{bb}}\text{=−4.601± 0.037\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\hspace{0.17em}\chi }}_{\mathrm{cc}}\text{=3.023± 0.041\hspace{0.17em}}\mathrm{MHz}$ ; and for ${\mathrm{CH}}_3\mathrm{CD}={\mathrm{NCH}}_3\text{:\hspace{0.17em}A=32982.409± 0.050\hspace{0.17em}}\mathrm{MHz}\text{,\hspace{0.17em}B=4076.6142± 0.0066\hspace{0.17em}}\mathrm{MHz}\text{,\hspace{0.17em}C=3799.5528± 0.0079\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\hspace{0.17em}\chi }}_{\mathrm{aa}}\text{=1.551± 0.038\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\hspace{0.17em}\chi }}_{\mathrm{bb}}\text{=−4.595± 0.023\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\hspace{0.17em}\chi }}_{\mathrm{cc}}\text{=3.044± 0.036\hspace{0.17em}}\mathrm{MHz}$ . The mean values for the barriers of the two methyl groups were found to be $\text{1642± 19\hspace{0.17em}}\mathrm{cal/mole}$ for the ${\mathrm{C–CH}}_3$ top and $\text{2109± 84\hspace{0.17em}}\mathrm{cal/mole}$ for the ${\mathrm{N–CH}}_3$ top. A dipole moment of $\text{1.499± 0.007\hspace{0.17em}}\mathrm{D}$ was determined from the measured Stark splittings.