Microwave Spectrum of cis-Glyoxal

Abstract

The rotational spectrum of glyoxal has been investigated in the region 18.0–40.0 GHz. Only B‐type transitions were observed. Both Q‐branch and R‐branch assignments have been made for the ground state, and the Q‐branch assignment has been made for the first excited state of the internal rotational mode. From relative intensity measurements of vibrational excited states, the torsional vibration is found to have a frequency of $\text{114± 8\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ .. From a consideration of the rotational constants, it is concluded that the isomer giving rise to the microwave spectrum is the planar cis form and not the gauche isomer. The dipole moment was determined to be $\text{4.8 ± 0.2\hspace{0.17em}}\mathrm{D}.$