Photodissociation Electronic Spectra of Cold Protonated Quinoline and Isoquinoline in the Gas Phase

Abstract

Abstract:Photofragmentation electronic spectra of isolated single-isomeric N-protonated quinoline (quinolinium) and isoquinoline (isoquinolinium) ions have been measured at a temperature of 40 K using a mass-selective, 10 cm 1 spectral resolution, photodissociation spectrometer.Additionally, ab-initio adiabatic transition energies calculated using the RI-ADC(2) method have been employed to assist in the assignment of the spectra. Three electronic transitions having * character were clearly evidenced for both protonated ions within the UV and deep-UV spectral ranges. The corresponding spectra at room temperature were previously reported by Hansen et al., 24 together with TD-DFT calculations and a careful analysis of the possible fragmentation mechanisms. This information will be complemented in the present study by appending better resolved spectra, characteristic of cold ions, in which well-defined vibrational progressions associated to the S 1 S 0 and S 3 S 0 transitions exhibit clear 00 bands at h 00 = 27868 and 42230 cm 1 , for protonated quinoline, and at h 00 = 28043 and 41988 cm 1 , for protonated isoquinoline. Active vibrations in the spectra were assigned with the help of calculated normal modes, looking very similar to those of the structurally related protonated naphthalene. Finally, we have observed that the bandwidths associated with the deep-UV S 3 S 0 transition denote a lifetime for the S 3 excited state in the subpicosecond time scale, in contrast with that of S 1 .

Infrared Spectroscopy of Protonated Phenol–Water Clusters

“…The presence of protonated aromatics in interstellar space has been suggested. , Moreover, protonated aromatics are expected to play important roles in biochemical processes since the proton is one of the major carriers of positive charge in such systems . Therefore, characterization of protonated aromatic molecules and their microsolvated clusters in the gas phase has been extensively performed by spectroscopic, mass spectrometric, and theoretical approaches. − Not only the structure determination, e.g ., identification of the protonated site and their microsolvation structures, but also (excited state) proton transfer mechanisms have been studied.…”

Discrimination between Protonation Isomers of Quinazoline by Ion Mobility and UV-Photodissociation Action Spectroscopy

“…Quinazoline is a fundamental aromatic chromophore with two nondegenerate protonatable nitrogens, with very similar stabilities, and these systems are small enough to allow detailed treatment with quantum chemical calculations for assignment of vibronic spectra and, in turn, provide valuable benchmarking data. To our knowledge there are no reported spectra of protonated diazanaphthalenes in the literature, while spectra of protonated azanaphthalenes (quinoline and isoquinoline) ,, as well as protonated diazines (pyrimidine, pyrazine and pyridazine) have been reported.…”

Effect of Freezing out Vibrational Modes on Gas-Phase Fluorescence Spectra of Small Ionic Dyes

“…The coupling with in-plane vibrational modes also dominates electronic photodissociation spectra of cold polycyclic aromatic hydrocarbons and nitrogen-substituted ones ( e . g ., protonated acridine, a three-ringed molecule) that contain a planar ring-system in S 0 and S 1 as well as spectra of flavin ions (tricyclic isoalloxazine heterocycle). − …”