The cyclopropene radical cation: Rovibrational level structure at low energies from high-resolution photoelectron spectra

The cyclopropene radical cation (c-C3\documentclass[12pt]{minimal}\begin{document}${\rm H}_4^+$\end{document}H4+) is an important but poorly characterized three-membered-ring hydrocarbon. We report on a measurement of the high-resolution photoelectron and photoionization spectra of cyclopropene and several deuterated isotopomers, from which we have determined the rovibrational energy level structure of the \documentclass[12pt]{minimal}\begin{document}$\widetilde{\rm X}^+$\end{document}X̃+ 2B2 ground electronic state of c-C3\documentclass[12pt]{minimal}\begin{document}${\rm H}_4^+$\end{document}H4+ at low energies for the first time. The synthesis of the partially deuterated isotopomers always resulted in mixtures of several isotopomers, differing in their number of D atoms and in the location of these atoms, so that the photoelectron spectra of deuterated samples are superpositions of the spectra of several isotopomers. The rotationally resolved spectra indicate a C2v-symmetric R0 structure for the ground electronic state of c-C3\documentclass[12pt]{minimal}\begin{document}${\rm H}_4^+$\end{document}H4+. Two vibrational modes of c-C3\documentclass[12pt]{minimal}\begin{document}${\rm H}_4^+$\end{document}H4+ are found to have vibrational wave numbers below 300 cm−1, which is surprising for such a small cyclic hydrocarbon. The analysis of the isotopic shifts of the vibrational levels enabled the assignment of the lowest-frequency mode (fundamental wave number of ≈110 cm−1 in c-C3\documentclass[12pt]{minimal}\begin{document}${\rm H}_4^+$\end{document}H4+) to the CH2 torsional mode (\documentclass[12pt]{minimal}\begin{document}$\nu _8^+$\end{document}ν8+, A2 symmetry) and of the second-lowest-frequency mode (≈210 cm−1 in c-C3\documentclass[12pt]{minimal}\begin{document}${\rm H}_4^+$\end{document}H4+) to a mode combining a CH out-of-plane with a CH2 rocking motion (\documentclass[12pt]{minimal}\begin{document}$\nu _{15}^+$\end{document}ν15+, B2 symmetry). The potential energy along the CH2 torsional coordinate is flat near the equilibrium structure and leads to a pronounced anharmonicity.