Anharmonic force fields of cis- and trans-S₁ C₂H₂

Abstract

We calculate second-order vibrational perturbation theory (VPT2) anharmonic force fields for the cis and trans conformers of S₁ C₂H₂, and compare the results to experiment. The vibrational assignments of recently observed levels belonging to the cis well are of particular interest. A refined estimate of the cis origin position (44,870 ± 10 cm⁻¹) is proposed, and preliminary low-energy fits to the global J = K = 0 trans level structure are also described. The performance of perturbation theory in this isomerizing system is examined, and both surprising successes and failures are encountered. We examine these and their causes, and offer practical suggestions for avoiding the pitfalls of applying perturbation theory to systems with large amplitude motions.

Keywords:

• acetylene
• spectroscopy
• cis-trans isomerization
• vibrational perturbation theory
• anharmonic force fields

Acknowledgements

This work was supported at MIT by DOE Grant No. DE-FG0287ER13671. AJM thanks Academia Sinica, Taiwan, for financial support and the University of British Columbia for some travel funding. JFS also wishes to acknowledge support from the US National Science Foundation (Grant CHE-1012743).