The Fourier transform microwave spectrum of methyl neopentyl ketone, CH3C(O)CH2C(CH3)3 has been measured under molecular beam conditions and a conformer with an effective ground state Cs symmetry was assigned. According to quantum chemical calculations carried out at the B3LYP/6-311++G(d,p) and the MP2/6-311++G(d,p) level of theory no symmetry plane is present in the equilibrium structure of the same conformer. This apparent disagreement is discussed. The barrier to internal rotation of the acetyl methyl group was found to be 173.539(36) cm−1 using an effective internal rotation Hamiltonian. Two different methods and programs (the BELGI code and the XIAM code) were used and the results are compared.Graphical abstractHighlights► The rotational spectrum of methyl neopentyl ketone was measured and analyzed. ► The barrier to internal rotation of the methyl group was determined. ► A different equilibrium and effective ground state geometry was found.

The rotational spectra of two conformers of ethyl pivalate, (CH3)3C–COO–C2H5 have been recorded by molecular beam FT microwave spectroscopy. The analysis yielded a set of three rotational constants and five quartic centrifugal distortion constants for each conformer. The conformers were identified by comparing the experimental rotational constants with those obtained by ab initio calculations at MP2/6-311++G∗∗ level. One conformer has Cs symmetry, the other one forms a pair of enantiomers with C1 symmetry. Additionally, the torsional potentials of the tert-butyl group and of the methyl groups were obtained by ab initio methods.

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.