The microwave spectrum of the trans conformer of ethyl acetate

The Fourier transform microwave spectrum of ethyl acetate has been measured under molecular beam conditions. The trans conformer, where all heavy atoms are located within a mirror plane, was identified after analyzing the spectrum by comparison with theoretical calculations. The barrier to internal rotation of the acetate methyl group was found to be only 99.57(11) cm−1 whereas for methyl torsion in the ethyl group a barrier of 1112.3(37) cm−1 was determined. A comparison between two theoretical approaches treating the internal rotation, the so-called RAM (Rho Axis Method) and CAM (Combined Axis Method), is also performed.