Rotational spectrum of acetone, CH3COCH3, in the ν17 torsional excited state

The rotational spectrum of acetone in its second torsional excited state (ν17, 125 cm−1 above the ground state) has been identified and assigned. The interaction between overall rotation and the internal rotation motions causes rotational transitions to split into four components, one for each torsional substate. The splittings in this torsional excited state are significantly larger than any splittings assigned yet for molecules with two methyl internal rotors, making assignments rather difficult. Over 700 frequencies between 72 and 372 GHz in the sub-millimeter wave spectrum have been assigned to bR-transitions in all four torsional substates. Eventually, 32 parameters of an effective rotational Hamiltonian for a molecule with two periodic internal motions were fit to observed frequencies of 612 transitions from all torsional substates with a standard deviation of 0.485 MHz.