The vibrational energy levels in acetylene 12C2H2: Towards a regular pattern at higher energies

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Abstract
We have gathered 122 known experimental data concerning the vibrational energy levels of acetylene, C2H2, up to 12 000 cm−1. We have fitted the data simultaneously to a model based on the clustering of the levels induced by a set of eight anharmonic resonances. That set is carefully built from the literature information on various isotopomers of acetylene. The model takes full advantage of the anharmonic pattern and systematically block diagonalizes the entire vibrational energy matrix. The 122 input energies are reproduced within a standard deviation of 0.74 cm−1 with 35 fitted parameters. Decisive support to the model is brought by using the resulting vibrational parameters and matrix eigenvectors to reproduce energies, principal rotational parameters, and relative intensity features. The cluster picture is discussed, in terms of the 7 normal modes of vibration in acetylene, according to the theoretical model developed by Kellman [M. E. Kellman and G. Chen, J. Chem. Phys. 95, 8671 (1991)].