a b s t r a c tThe 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 À1 whereas for methyl torsion in the ethyl group a barrier of 1112.3(37) cm À1 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.

No abstract

We observed the microwave spectrum of ethyl isovalerate by molecular beam Fourier transform microwave spectroscopy. The rotational and centrifugal distortion constants of the most abundant conformer were determined. Its structure was investigated by comparison of the experimental rotational constants with those obtained by ab initio methods. In a first step, the rotational constants of various conformers were calculated at the MP2/6-311++G** level of theory. Surprisingly, no agreement with the experimental results was found. Therefore, we concluded that in the case of ethyl isovalerate more advanced quantum chemical methods are required to obtain a reliable molecular geometry. Ab initio calculations carried out at MP3/6-311++G**, MP4/6-311++G**, and CCSD/6-311++G** levels and also density functional theory calculations using the B3LYP/6-311++G** method gave similar results for the rotational constants, but they were clearly distinct from those obtained at the MP2/6-311++G** level. With use of these more advanced methods, the rotational constants of the lowest energy conformer were in good agreement with those obtained from the microwave spectrum.

No abstract

High-resolution spectroscopy techniques play a pivotal role to validate and efficiently benchmark available methods from quantum chemistry. In this work, we analyzed the microwave spectrum of ethyl butyrate within the...