Sensing the Molecular Structures of Hexan-2-one by Internal Rotation and Microwave Spectroscopy

Abstract

Using two molecular jet Fourier transform spectrometers, the microwave spectrum of hexan-2-one, also called methyl n-butyl ketone, was recorded in the frequency range from 2 to 40 GHz. Three conformers were assigned and fine splittings caused by the internal rotations of the two terminal methyl groups were analyzed. For the acetyl methyl group CH₃ COC₃ H₆ CH₃ , the torsional barrier is 186.9198(50) cm^-1 , 233.5913(97) cm^-1 , and 182.2481(25) cm^-1 for the three observed conformers, respectively. The value of this parameter could be linked to the structure of the individual conformer, which enabled us to create a rule for predicting the barrier height of the acetyl methyl torsion in ketones. The very small splittings arising from the internal rotation of the butyl methyl group CH₃ COC₃ H₆ CH₃ could be resolved as well, yielding the respective torsional barriers of 979.99(88) cm^-1 , 1016.30(77) cm^-1 , and 961.9(32) cm^-1 .

Keywords: ab initio calculations; ketones; large amplitude motion; pheromones; rotational spectroscopy.