Emission spectra of seven fluorobenzene cations

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Abstract
The emission spectra of C6F6 +, C6F5H+, the three C6F4H2 + and two C6F3H3 + (1, 2, 4 and 1, 3, 5) cations were obtained in the gas phase, in a special discharge tube. Symmetry and mode correlation of the fundamental frequencies of the parent neutral molecules enabled the isodynamic molecule method to be used for achieving vibrational analysis of the ion spectra. The vibronic structure in each emission spectrum is interpreted. Dynamic Jahn-Teller effects are shown to occur in the degenerate ground state of C6F6 + and of 1, 3, 5-C6F3H3 +, in particular irregular ground-state progressions in mode 6 are observed. Extensive Jahn-Teller calculations were made for 1, 3, 5-C6F3H3 +; frequencies and intensities in its progressions are in good agreement with the results of calculations based on the linear coupling model of Longuet-Higgins et al., using a deperturbed frequency ω = 477–484 cm-1 and a dimensionless interaction parameter D = 0·13-0·10. A review of the vibrational data of photoelectron spectra and Rydberg far-ultra-violet absorption of C6F6 and 1, 3, 5-C6F3H3 shows that some reassignments are to be made in the light of the present work.