Microwave Spectra, Barriers to Internal Rotation, Quadrupole Coupling Constants, and Dipole Moments of cis- and trans-Acetaldoxime

The microwave spectra of cis‐and trans‐acetaldoxime (CH₃CHNOH) have been investigated in the region 8–37 GHz. Several a‐type transitions have been identified for both species and the rotational constants in MHz are as follows: cis‐acetaldoxime, $\text{A\hspace{0.17em}=\hspace{0.17em}17364}$ , $\text{B\hspace{0.17em}=\hspace{0.17em}6658.92}$ , $\text{C\hspace{0.17em}=\hspace{0.17em}4921.04}$ ; trans‐acetaldoxime, $\text{A\hspace{0.17em}=\hspace{0.17em}44613}$ , $\text{B\hspace{0.17em}=\hspace{0.17em}4237.73}$ , $\text{C\hspace{0.17em}=\hspace{0.17em}3973.81}$ . The transitions of cis‐ and trans‐acetaldoxime are split by tunneling of the methyl group through the potential barrier hindering the internal rotation. From these separations the height of the barrier has been determined to be 375 cal/mole for cis and 1835 cal/mole for trans‐acetaldoxime. The quadrupole coupling constants are as follows: cis acetaldoxime, ${\chi }_{\mathrm{aa}}\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}2.8}\mathrm{MHz}$ , ${\chi }_{\mathrm{bb}}{\mathrm{\hspace{0.17em}-\hspace{0.17em}\chi }}_{\mathrm{cc}}\text{\hspace{0.17em}=\hspace{0.17em}0.6}\mathrm{MHz}$ ; trans‐acetaldoxime, ${\chi }_{\mathrm{aa}}\text{\hspace{0.17em}=\hspace{0.17em}4.07}\mathrm{MHz}$ , ${\chi }_{\mathrm{bb}}{\mathrm{\hspace{0.17em}-\hspace{0.17em}\chi }}_{\mathrm{cc}}\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}5.0}\mathrm{MHz}$ . The dipole moments for the cis and trans species are 0.850 and 0.938 D, respectively.