Microwave spectrum of formic acetic anhydride

Rotational transitions of formic acetic anhydride (HCO-O-COCH3) have been measured with angular momenta J = 2 to 6 between 11 and 26 GHz. All transitions showed splittings of up to 60 MHz due to the hindered internal rotation of the methyl group. The frequencies of observed transitions have been fitted to those calculated numerically for the simultaneous overall and internal rotation of formic acetic anhydride. Three moments of inertia, the barrier to internal rotation and the angle between the internal rotation axis and the principal axis a have been determined, but the moment of inertia of the methyl group had to be assumed. The moments of inertia are consistent with all nuclei lying in a symmetry plane except of two hydrogen nuclei in the methyl group.