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Reduced dimension variational calculations have been performed for the rovibrational level structure of the S[subscript 1] state of acetylene. The state exhibits an unusually complicated level structure, for various reasons. First, the potential energy surface has two accessible conformers, trans and cis. The cis conformer lies about 2700 cm[superscript −1] above the trans, and the barrier to cis-trans isomerization lies about 5000 cm[superscript −1] above the trans minimum. The trans vibrations ν[subscript 4] (torsion) and ν[subscript 6] (asym. bend) interact very strongly by Darling-Dennison and Coriolis resonances, such that their combination levels and overtones form polyads with unexpected structures. Both conformers exhibit very large x [subscript 36] cross-anharmonicity since the pathway to isomerization is a combination of ν[subscript 6] and ν[subscript 3] (sym. bend). Near the isomerization barrier, the vibrational levels show an even-odd K-staggering of their rotational levels as a result of quantum mechanical tunneling through the barrier. The present calculations address all of these complications, and reproduce the observed K-structures of the bending and C–C stretching levels with good qualitative accuracy. It is expected that they will assist with the assignment of the irregular patterns near the isomerization barrier.United States. Dept. of Energy (Grant DE-FG0287ER13671)Massachusetts Institute of Technology. Undergraduate Research Opportunities Progra

Reduced dimension rovibrational variational calculations of the S1 state of C2H2. II. The S1 rovibrational manifold and the effects of isomerization