15min:

YUNG-CHING CHOU AND I-CHIA CHEN, Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30013, Republic of China; JON T. HOUGEN, Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441.

The G₆ group-theoretical high-barrier formalism developed previously for internally rotating and inverting CH₃NHD is used to interpret the abnormal torsional splittings in the S₁ state of acetaldehyde for levels 14^0-15⁰, 14^0-15¹ and14^0-15², in which 14^0- denotes the asymmetric inversion tunneling component of the aldehyde hydrogen and 15 denotes vibrational mode for the methyl torsional motion. This formalism, derived using an extended permutation-inversion group G₆^m, treats simultaneously methyl torsion tunneling, aldehyde hydrogen inversion tunneling and overall rotation. Fits to the rotational states of the four pairs of inversion-torsion vibrational levels (14⁰⁺15⁰, 14^0-15⁰), (14⁰⁺15¹, 14^0-15¹), (14⁰⁺15², 14^0-15²), and (14⁰⁺15³, 14^0-15³) are performed, giving rms deviations of 0.003, 0.003, 0.004 and 0.004 cm^-1, respectively, in which the deviations are comparable with the experimental uncertainty 0.003 cm^-1. For torsional levels lying near the summit of the torsional barrier, this theoretical model including high-order terms provides satisfactory fits to the experimental data. The K structures deviated from a pure torsional rotor are fitted using this formalism and are proved to be from coupling to aldehyde hydrogen inversion.