Ultraviolet photodissociation action spectroscopy of gas-phase protonated quinoline and isoquinoline cations

Abstract

The gas-phase photodissociation action spectroscopy of protonated quinoline and isoquinoline cations (quinolineH⁺ and isoquinolineH⁺) is investigated at ambient temperature. Both isomers exhibit vibronic detail and wavelength-dependent photoproduct partitioning across two broad bands in the ultraviolet. Photodissociation action spectra are reported spanning 370–285 nm and 250–220 nm and analysed with the aid of electronic structure calculations: TD-DFT (CAM-B3LYP/aug-cc-pVDZ) is used for spectra simulations and CBS-QB3 for dissociation enthalpies. It is shown that the action spectra are afforded predominantly by two-photon excitation. The first band is attributed to both the S₁ ← S₀ and S₂ ← S₀ electronic transitions in quinolineH⁺, with a S₁ ← S₀ electronic origin assigned at 27 900 cm⁻¹. For isoquinolineH⁺ the S₁ ← S₀ transition is observed with an assigned electronic origin at 27 500 cm⁻¹. A separate higher energy band is observed for both species, corresponding to the S₃ ← S₀ transition, with origins assigned at 42 100 cm⁻¹ and 42 500 cm⁻¹ for quinolineH⁺ and isoquinolineH⁺, respectively. Franck–Condon absorption simulations provide an explanation for some vibrational structure observed in both bands allowing several normal mode assignments. The nature of the electronic transitions is discussed and it is shown that the excited states active in the reported spectra should be of ππ* character with some degree of charge transfer from the homocycle to the heterocycle.