Rotational spectrum of acetone, CH 3 COCH 3 , in the first torsional excited state

The rotational spectrum of acetone in its first torsional excited state (80 cm −1 above the ground state) has been identified and assigned. The vibrational levels of acetone are split into four torsional substates because of the interaction between overall rotation and the internal rotation motions of the two methyl groups, which results in torsional splittings of the rotational transitions. These splittings are significantly larger in the torsional excited state than in the ground vibrational state, making assignment difficult. Nevertheless, initial assignments were made between 260 and 344 GHz using Loomis–Wood diagrams. The assignment was then extended both into the 72–122 and 176–253 GHz regions and into the microwave spectral region. Eventually, 40 parameters of an effective rotational Hamiltonian for a molecule with two periodic internal motions were fit to frequencies of 571 transitions and 50 blends to near experimental precision. Preliminary results have already been used to help radio-astronomers identify vibrationally excited acetone in interstellar hot cores.