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

Non-Radiative Deactivation in Isolated Quinoline

The photophysics of the S2 1(ππ*) state of the polycyclic aromatic nitrogen-containing hydrocarbon (PANH) quinoline is investigated in a free jet using a picosecond laser system. A [1 + 1] multiphoton ionization spectrum yields the S2 origin at around 32 200 cm-1 and reveals several vibronic bands. In time-resolved experiments, quinoline is then excited between 312.2 and 279.7 nm. Probe wavelengths of 351 and 263.5 nm are employed. The dynamics is monitored by time-resolved photoelectron imaging. The images reveal a short-lived band at high electron kinetic energies with a ps lifetime and a band at lower electron kinetic energies that shows an offset at long delay times. In comparison with previous work, the offset is assigned to ionization from the T1 state. Lifetimes decrease from 45 ps at the S2 origin to 11 ps at +3550 cm-1. Most likely, the S2 1(ππ*) state deactivates by internal conversion to the S1 1(nπ*) state, followed by intersystem crossing to the triplet manifold.

Infrared spectra of isoquinolinium (iso-C9H7NH+) and isoquinolinyl radicals (iso-C9H7NH and 1-, 3-, 4-, 5-, 6-, 7- and 8-iso-HC9H7N) isolated in solid para-hydrogen

Protonated polycyclic aromatic nitrogen heterocycles (H⁺PANH) are prospective candidates that may contribute to interstellar unidentified infrared (UIR) emission bands because protonation enhances the relative intensities of the bands near 6.2, 7.7 and 8.6 μm, and the presence of the N atom induces a blue shift of the ring-stretching modes so that the spectra of H⁺PANH match better with the 6.2 μm feature in class-A UIR spectra. We report the infrared (IR) spectra of protonated isoquinoline (the 2-isoquinolinium cation, iso-C₉H₇NH⁺), its neutral counterpart (the 2-isoquinolinyl radical, iso-C₉H₇NH), and another mono-hydrogenated product (the 6-isoquinolinyl radical, 6-iso-HC₉H₇N), produced on the electron-bombardment of a mixture of isoquinoline (iso-C₉H₇N) with excess para-hydrogen (p-H₂) during matrix deposition at 3.2 K. To generate additional isomers of hydrogenated isoquinoline, we irradiated iso-C₉H₇N/Cl₂/p-H₂ matrices at 365 nm to generate Cl atoms, followed by IR irradiation to generate H atoms via Cl + H₂ (v = 1) → HCl + H; the H atoms thus generated reacted with iso-C₉H₇N. In addition to iso-C₉H₇NH and 6-iso-HC₉H₇N observed in the electron-bombardment experiments, we identified six additional hydrogenated isoquinoline species, 1-, 3-, 4-, 5-, 7- and 8-iso-HC₉H₇N, via their IR spectra; hydrogenation on the N atom and all available carbon atoms except for the two sharing carbon atoms on the fused ring was observed. Spectral groupings were achieved according to their behaviors after maintenance of the matrix in darkness and on secondary photolysis at various wavelengths. The assignments were supported via comparison of the experimental results with the vibrational wavenumbers and IR intensities of possible isomers predicted using the B3LYP/6-311++G(d,p) method. The implications in the identification of the UIR band are discussed.

A, Bouwman J, Avaldi L, Bolognesi P, Richter R. Comprehensive survey of dissociative photoionization of quinoline by PEPICO experiments

Dissociative photoionization of quinoline induced by vacuum ultraviolet radiation is investigated using photoelectron–photoion coincidence spectroscopy. Branching ratios of all the detectable fragment ions are measured as a function of internal energy ranging from 2 to 30 eV. A specific generation hierarchy is observed in the breakdown curves of a set of dissociation channels. Moreover, a careful comparison of the breakdown curves of fragments among the successive generations allowed to establish a decay sequence in the fragmentation of quinoline cation. This enabled us to revisit and refine the understanding of the first generation decay and reassign the origin of a few of the higher generation decay products of quinoline cation. With the help of the accompanying computational work (reported concurrently), we have demonstrated the dominance of two different HCN elimination pathways over previously interpreted mechanisms. For the first time, a specific pathway for acetylene elimination is identified in quinoline+ and the role of isomerization in both acetylene as well as hydrogen cyanide loss is also demonstrated. The experiment also established that the acetylene elimination exclusively occurs from the non-nitrogen containing rings of quinoline cation. The formation of a few astronomically important species is also discussed.

Infrared Spectra of 1-Quinolinium (C9H7NH+) Cation and Quinolinyl Radicals (C9H7NH and 3-, 4-, 7-, and 8-HC9H7N) Isolated in Solid para-Hydrogen

Large protonated polycyclic aromatic hydrocarbons (H⁺PAH) and the corresponding nitrogen heterocycles (H⁺PANH) have been proposed as possible carriers of unidentified infrared (UIR) emission bands from galactic objects. The nitrogen atom in H⁺PANH is expected to induce a blue shift of the band associated with the CC-stretching mode of H⁺PAH near 6.3 μm so that their emission bands might agree better with the UIR band near 6.2 μm. We report the IR spectrum of protonated quinoline (1-quinolinium cation, C₉H₇NH⁺) and its neutral species (1-quinolinyl radical, C₉H₇NH) measured upon electron bombardment during the deposition of a mixture of quinoline (C₉H₇N) and para-hydrogen (p-H₂) at 3.2 K, indicating that the protonation and hydrogenation occur mainly at the N atom site. Additional experiments on the irradiation of C₉H₇N/Cl₂/p-H₂ matrices at 365 nm to generate Cl atoms, followed by irradiation with IR light to generate H atoms via Cl + H₂ (v = 1), were performed to induce the reaction H + C₉H₇N. This method proved to be efficient for hydrogenation reactions in solid p-H₂; we identified, in addition to C₉H₇NH observed in electron-bombardment experiments, four radicals with hydrogenation at the C-atom site─3-, 4-, 7-, and 8-HC₉H₇N. Spectral assignments were achieved according to the behavior upon secondary photolysis and a comparison of experimental results with vibrational wavenumbers and IR intensities predicted with the B3LYP/6-311++G(d,p) method. The observed lines at 1641.4, 1598.4, and 1562.0 cm^-1 associated with the CC-stretching mode of C₉H₇NH⁺ are blue-shifted from those at 1618.7, 1580.8, 1556.7, and 1510.0 cm^-1 of the corresponding protonated naphthalene (C₁₀H₉⁺).

Laser Photodissocation, Action Spectroscopy and Mass Spectrometry Unite to Detect and Separate Isomers

The separation and detection of isomers remains a challenge for many areas of mass spectrometry. This article highlights laser photodissociation and ion mobility strategies that have been deployed to tackle...

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

While action spectroscopy of cold molecular ions is a well-established technique to provide vibrationally resolved absorption features, fluorescence experiments are still challenging. Here we report the fluorescence spectra of pyronin-Y and resorufin ions at 100 K using a newly constructed setup. Spectra narrow upon cooling, and the emission maxima blueshift. Temperature effects are attributed to the population of vibrational excited levels in S₁, and that frequencies are lower in S₁ than in S₀. This picture is supported by calculated spectra based on a Franck-Condon model that not only predicts the observed change in maximum, but also assigns Franck-Condon active vibrations. In-plane vibrational modes that preserve the mirror plane present in both S₀ and S₁ of resorufin and pyronin Y account for most of the observed vibrational bands. Finally, at low temperatures, it is important to pick an excitation wavelength as far to the red as possible to not reheat the ions.

Picosecond excited-state lifetimes of protonated indazole and benzimidazole: The role of the N-N bond

Certain chemical groups give rise to characteristic excited-state deactivation mechanisms. Here, we target the role of a protonated N-N chemical group in the excited-state deactivation of protonated indazole by comparison to its isomer that lacks this group, protonated benzimidazole. Gas-phase protonated indazole and protonated benzimidazole ions are investigated at room temperature using picosecond laser pump-probe photodissociation experiments in a linear ion-trap. Excited state lifetimes are measured across a range of pump energies (4.0-5.4 eV). The ¹ππ* lifetimes of protonated indazole range from 390 ± 70 ps using 4.0 eV pump energy to ≤18 ps using 4.6 eV pump energy. The ¹ππ* lifetimes of protonated benzimidazole are systematically longer, ranging from 3700 ± 1100 ps at 4.6 eV pump energy to 400 ± 200 ps at 5.4 eV. Based on these experimental results and accompanying quantum chemical calculations and potential energy surfaces, the shorter lifetimes of protonated indazole are attributed to πσ* state mediated elongation of the protonated N-N bond.

Influence of the N atom position on the excited state photodynamics of protonated azaindole

We present a study of the photofragmentation of three protonated azaindole molecules - 7-azaindole, 6-azaindole, and 5-azaindole - consisting of fused pyrrole-pyridine bicyclic aromatic systems, in which the pyridinic (protonated) nitrogen heteroatom is located at the 7, 6, and 5 positions, respectively. Photofragmentation electronic spectra of the isolated aforementioned azaindolinium cations reveal that their photodynamics extends over timescales covering nine orders of magnitude and provide evidence about the resultant fragmentation pathways. Moreover, we show how the position of the heteroatom in the aromatic skeleton influences the excited state energetics, fragmentation pathways, and fragmentation timescales. Computed ab initio adiabatic transition energies are used to assist the assignation of the spectra, while geometry optimisation in the excited electronic states as well as ab initio calculations along the potential surfaces demonstrate the role of ππ*/πσ* coupling and/or large geometry changes in the dynamics of these species. Evidence supporting the formation of Dewar valence isomers as intermediates involved in sub-picosecond relaxation processes is discussed.

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

The influence of oriented electric fields on chemical reactivity and photochemistry is an area of increasing interest. Within a molecule, different protonation sites offer the opportunity to control the location of charge and thus orientation of electric fields. New techniques are thus needed to discriminate between protonation isomers in order to understand this effect. This investigation reports the UV-photodissociation action spectroscopy of two protonation isomers (protomers) of 1,3-diazanaphthalene (quinazoline) arising from protonation of a nitrogen at either the 1- or 3-position. It is shown that these protomers are separable by field-asymmetric ion mobility spectrometry (FAIMS) with confirmation provided by UV-photodissociation (PD) action spectroscopy. Vibronic features in the UVPD action spectra and computational input allow assignment of the origin transitions to the S₁ and S₅ states of both protomers. These experiments also provide vital benchmarks for protomer-specific calculations and examination of isomer-resolved reaction kinetics and thermodynamics.

Measuring Electronic Spectra of Differential Mobility-Selected Ions in the Gas Phase

We describe the modification of a commercially available tandem differential mobility mass spectrometer (DMS) that has been retrofitted to facilitate photodissociation (PD) of differential mobility-separated, mass-selected molecular ions. We first show that a mixture of protonated quinoline/isoquinoline (QH⁺/iQH⁺) can be separated using differential mobility spectrometry. Efficient separation is facilitated by addition of methanol to the DMS environment and increased residence time within the DMS. In action spectroscopy experiments, we gate each isomer using appropriate DMS settings, trap the ions in the third quadrupole of a triple quadrupole mass spectrometer, and irradiate them with tunable light from an optical parametric oscillator (OPO). The resulting mass spectra are recorded as the OPO wavelength is scanned, giving PD action spectra. We compare our PD spectra with previously recorded spectra for the same species and show that our instrument reproduces previous works faithfully.

Spectroscopy of prospective interstellar ions and radicals isolated in para-hydrogen matrices

The p-H₂ matrix-isolation technique coupled with photolysis in situ or electron bombardment produces protonated or hydrogenated species important in astrochemistry.

Experiment and theory confirm that UV laser photodissociation spectroscopy can distinguish protomers formed via electrospray

Low-resolution UV spectroscopy within a laser-interfaced commercial mass spectrometer can be used to identify electrosprayed protomers of para-aminobenzoic acid (PABA).