Emission spectra of seven fluorobenzene cations

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.