The microwave spectrum of isopropenyl acetate – An asymmetric molecule with two internal rotors

Abstract

The Fourier transform microwave spectrum of isopropenyl acetate (CH 3 COO C(CH 3) CH 2) has been measured under molecular beam conditions. The experimental data as well as quantum chemical calculations have shown that this molecule exists as only one conformer of C 1 symmetry, in which the vinyl group is tilted by an angle of approximately 70° against the plane containing the ester group. Due to internal rotation of the acetyl methyl group, we found large A– E splittings of all lines (from a few MHz up to 1 GHz or more). We also were able to resolve the splitting due to the internal rotation of the second isopropenyl methyl group. The A species lines split into doublets and the E species lines into triplets. These splittings vary from 10 kHz up to 1 MHz, much smaller than the splittings due to the acetyl methyl group. By analyzing the spectrum with the program Xiam, a torsional barriers of 135.3498(38) and 711.7(73) cm −1 for the acetyl methyl group and the isopropenyl methyl group were observed, respectively. All lines in the spectrum were also fitted with the program Erham to a standard deviation of only 2.3 kHz.