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

Abstract

Reduced dimension variational calculations have been performed for the rovibrational level structure of the S1 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−1 above the trans, and the barrier to cis-trans isomerization lies about 5000 cm−1 above the trans minimum. The trans vibrations ν4 (torsion) and ν6 (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 x36 cross-anharmonicity since the pathway to isomerization is a combination of ν6 and ν3 (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.

Probing cis-trans isomerization in the S1 state of C2H2 via H-atom action and hot band-pumped IR-UV double resonance spectroscopies

“…At the saddle point, the local bending vibrational intervals must go through a minimum, in similar fashion to the "Dixon dip" 27 of the large amplitude bending intervals in triatomic molecules. Recent calculations of the level structure near the barrier 25,28,29 support this conclusion.…”

“…Indeed, reduced dimension ab initio calculations predict an increasing staggering magnitude along the 3 4 6 0−2 progression. 28 This discrepancy is not understood. Misassignment is a possibility, though in the present case it seems unlikely.…”

“…If isomerization through the torsional vibration is important, as in H 2 O 2 , there will be a further staggering of the K = 4n levels versus the K = 4n + 2 levels. 30 Preliminary results suggest that this further staggering is not significant, 28 but it is clear that full analyses of the K-structure require data from K values up to 3 or possibly 4.…”

“…In addition, interactions occur between cis and trans levels at similar energies. 18,28,30 These are the mechanisms by which the cis levels obtain their intensity by mixing with nearby trans levels, though the selection rules and level shifts are different for even and odd K-values. Combining these two effects, the resulting K-staggering varies in a seemingly random fashion from level to level.…”

Full dimensional Franck-Condon factors for the acetylene $\tilde{\mathbf {A}}$Ã 1 A u—$\mathbf {\tilde{X}}$X̃ $\mathbf {^1\Sigma _g^+}$Σg+1 transition. II. Vibrational overlap factors for levels involving excitation in ungerade modes

“…[1][2][3] Of particular interest to the chemical community are the acetylene↔vinylidene 27,[38][39][40][41] and cis↔trans 37,[42][43][44][45] isomerizations that take place on theX-state andÃ-state surfaces, respectively. Because so much detailed work has been done on these electronic states, acetylene is a prototype for studying the spectroscopic signatures of isomerizing systems and the vibrational dynamics that occur at chemically interesting regions of the potential surface.…”