Internal rotation, spin–orbit coupling, and low-frequency vibrations in the ground state of CH₃–CC–CH⁺₃ and CD₃–CC–CD⁺₃

Abstract

Pulsed-field-ionisation zero-kinetic-energy (PFI-ZEKE) photoelectron spectra of 2-butyne (CH₃–CC–CH₃) and its fully deuterated isotopomer have been recorded in the region of the origin band of the ionising transition. The spectral congestion originating from the combined effects of the internal rotation of the methyl groups, the spin–orbit coupling, and the Jahn–Teller effect prevented the full resolution of the rotational structure of the photoelectron spectra. A tentative analysis of the rotational branch structure of the photoelectron spectra using rovibronic photoionisation selection rules derived in the permutation–inversion spin double group G₃₆(M²) suggests a splitting of ∼10.5 cm⁻¹ between the two spin–orbit components E_3/2 and E_1/2 of the ² E₁ ground state and an almost free internal rotation of the methyl groups in the cations. Assignments are proposed for several low-lying vibrational levels of the cations.

Keywords:

• PFI-ZEKE photoelectron spectroscopy
• 2-butyne
• Jahn–Teller effect
• spin–orbit coupling
• internal rotation
• dimethylacetylene radical cation

Acknowledgements

We thank Prof. M. Reiher and Dr S. Knecht (both ETH Zurich) for providing the results of their ab initio calculations and allowing us to quote them in this article. We also thank them for helpful discussions. We thank Dr J. Michaud for her initial work on the photoelectron spectrum of 2-butyne and G. Grassi for synthesising the sample of d₆-2-butyne. This work is supported financially by the Swiss National Science Foundation under project Nr. 200020-146759.

Disclosure statement

No potential conflict of interest was reported by the authors.