Microwave rotational spectrum of methyl azidoformate

The R branch, a dipole microwave rotational spectra of four isotopic species of methyl azidoformate have been assigned over the range 18–40 GHz. The assignment includes vibrational satellite series as well as the vibrational ground state transitions. The methyl barrier to internal rotation as determined from the E–A doubling data for the first excited state of the methyl torsion is V3=400±10 cm−1. The methyl barrier for the first excited state of another vibrational mode, methyl ground state, is determined to be V3=370±10 cm−1. Only one conformation, that with the azido group and the methyl group cis to the C=O group, was found to be measurably populated at room temperature. A partial vibration–rotation anaylsis has been performed with extremely limited success.