Conformational analysis of tert-butyl acetate using a combination of microwave spectroscopy and quantum chemical calculations

Abstract

tert-Butyl acetate was investigated using a combination of quantum chemical calculations and molecular beam Fourier transform microwave spectroscopy. The microwave spectrum was recorded in the frequency range from 8.00 to 15.75GHz. Due to its rather rigid frame, the molecule possesses only two conformers: one of Cs symmetry and one of C1 symmetry that appears as a pair of enantiomers. The Cs conformer is the most abundant in the supersonic jet and according to ab initio calculations at the MP2/6-311++G(d,p) level of theory it is 46kJ/mol lower in energy than the C1 conformer.Here, we report on the structure and dynamics of the most abundant conformer of tert-butyl acetate, for which a set of rotational and centrifugal distortion constants, as well as the barrier to internal rotation of the acetyl methyl group were determined with high accuracy. Splittings due to the internal rotation of the methyl group of up to 1.3GHz were observed in the spectrum. Using the programs XIAM and BELGI-Cs, we were able to determine a barrier height of about 113cm−1 and subsequently compare the molecular parameters obtained from these two codes.