Microwave Spectrum of Methylhydrazine; Rotational Isomerism, Internal Motions, Dipole Moments, and Quadrupole Coupling Constants

The microwave spectrum of methylhydrazine has been investigated. Two rotamers have been observed, and their spectra have been assigned and analyzed. The ground‐state rotational constants of the inner skew rotamer are $\text{A\hspace{0.17em}=\hspace{0.17em}36 704.37}\mathrm{MHz}\text{, B\hspace{0.17em}=\hspace{0.17em}9689.70}\mathrm{MHz}$ , and $\text{C\hspace{0.17em}=\hspace{0.17em}8531.95}\mathrm{MHz}$ , while the values obtained for the outer skew rotamer are $\text{A\hspace{0.17em}−\hspace{0.17em}C\hspace{0.17em}=\hspace{0.17em}29 597.20}\mathrm{MHz}$ and $\text{κ\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}0.925849}$ . Strong evidence for the conformational identifications was provided by the measured dipole moment components, which were ${\mu }_a\text{\hspace{0.17em}=\hspace{0.17em}1.04}\mathrm{D}{\mathrm{,\; \mu }}_b\text{\hspace{0.17em}=\hspace{0.17em}1.21}\mathrm{D}{\mathrm{,\; \mu }}_c\text{\hspace{0.17em}=\hspace{0.17em}0.46}\mathrm{D}$ for the inner rotamer and ${\mu }_a\text{\hspace{0.17em}=\hspace{0.17em}0.26}\mathrm{D}{\mathrm{,\; \mu }}_b\text{\hspace{0.17em}=\hspace{0.17em}0.60}\mathrm{D}$ , and ${\mu }_c\text{\hspace{0.17em}=\hspace{0.17em}1.70}\mathrm{D}$ for the outer rotamer. Relative intensity measurements indicate that the inner rotamer is more stable than the outer rotamer by 293 ± 23 cm⁻¹. Spectral splittings produced by inversion and methyl tunneling have been observed. Analysis of the latter effect leads to a barrier of $V_3\text{\hspace{0.17em}=\hspace{0.17em}1329}{\mathrm{cm}}^{\text{−1}}$ for the inner rotamer. Hyperfine splittings of the inner rotamer spectrum were described by the single set of ¹⁴N quadrupole coupling constants, ${\chi }_{\mathrm{aa}}\text{\hspace{0.17em}=\hspace{0.17em}4.09}\mathrm{MHz}{\mathrm{,\; \chi }}_{\mathrm{bb}}\text{\hspace{0.17em}=\hspace{0.17em}0.69}\mathrm{MHz}$ , and ${\chi }_{\mathrm{cc}}\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}4.78}\mathrm{MHz}$ .