A fitting program for molecules with two inequivalent methyl tops and a plane of symmetry at equilibrium: Application to new microwave and millimeter-wave measurements of methyl acetate

► Spectral fitting program written for Cs molecules with two inequivalent tops. ► Measurement of 800 pure rotational transitions in methyl acetate. ► Even sampling of G18 permutation-inversion group symmetry species A, E1, E2, E3, E4. ► Spectral fit to experimental error: 4 kHz for microwave, 40 kHz for sub-mm lines. ► Internal rotation barriers: V3 = 102 cm−1 for the acetyl top, 422 cm−1 for the ester top.