Excited state of protonated benzene and toluene

Microhydration of small protonated polyaromatic hydrocarbons: a first principles study

Using first principles methodology, we investigate the microsolvation of protonated benzene (BzH+), protonated coronene (CorH+) and protonated dodecabenzocoronene (DbcH+). Gas phase complexes of these small protonated polyaromatic hydrocarbons (H+PAHs) with mono-, di-, and tri-hydrated water molecules are considered. Their most stable forms are presented, where we discuss their structural, energetic aromaticity and IR and UV spectral features. In particular, we focus on the analysis of the bonding and various non-bonded interactions between these protonated aromatics and water clusters. The strength of non-bonded interactions is quantified and correlated with their electron density profiles. Furthermore, insights into the interfacial interactions and stability of these complexes were obtained through non-covalent index and symmetry-adapted perturbation theory (SAPT0) analyses. We also discuss the effects of the extension of the π aromatic cloud on the water solvation of these protonated aromatics. In particular, we extended our predictions for the S0 → S1 and S0 → T1 wavelength transitions of micro hydrated H+PAHs to deduce those of these species solvated in aqueous solution. The present findings should be useful for understanding, at the microscopic level, the effects of water interacting with H+PAHs, which are relevant for organic chemistry, astrochemistry, atmospheric chemistry, combustion and materials science.

Vibrational and electronic spectra of protonated vanillin: exploring protonation sites and isomerisation

Photofragmentation spectra of protonated vanillin produced under electrospray ionisation (ESI) conditions have been recorded in the 3000-3700 cm-1 (vibrational) and 225-460 nm (electronic) ranges, using room temperature IRMPD (infrared multiphoton dissociation) and cryogenic UVPD (ultraviolet photodissociation) spectroscopies, respectively. The cold (∼50 K) electronic UVPD spectrum exhibits very well resolved vibrational structure for the S1 ← S0 and S3 ← S0 transitions, suggesting long excited state dynamics, similar to its simplest analogue, protonated benzaldehyde. The experimental data were combined with theoretical calculations to determine the protonation site and configurational isomer observed in the experiments.

Protomers of the green and cyan fluorescent protein chromophores investigated using action spectroscopy

The photophysics of biochromophore ions often depends on the isomeric or protomeric distribution, yet this distribution, and the individual isomer contributions to an action spectrum, can be difficult to quantify. Here, we use two separate photodissociation action spectroscopy instruments to record electronic spectra for protonated forms of the green (pHBDI⁺) and cyan (Cyan⁺) fluorescent protein chromophores. One instrument allows for cryogenic (T = 40 ± 10 K) cooling of the ions, while the other offers the ability to perform protomer-selective photodissociation spectroscopy. We show that both chromophores are generated as two protomers when using electrospray ionisation, and that the protomers have partially overlapping absorption profiles associated with the S₁ ← S₀ transition. The action spectra for both species span the 340-460 nm range, although the spectral onset for the pHBDI⁺ protomer with the proton residing on the carbonyl oxygen is red-shifted by ≈40 nm relative to the lower-energy imine protomer. Similarly, the imine and carbonyl protomers are the lowest energy forms of Cyan⁺, with the main band for the carbonyl protomer red-shifted by ≈60 nm relative to the lower-energy imine protomer. The present strategy for investigating protomers can be applied to a wide range of other biochromophore ions.

TOLUENIUM AND OTHER GASEOUS METHYLBENZENIUM IONS: COMPLEX INTERPLAY OF PROTONATED ARENES AND CYCLO-OLEFINS

The development of the current knowledge of the gas-phase chemistry of protonated methylbenzenes, such as toluenium, xylenium and mesitylenium ions, their higher congeners as well as of their mostly cyclo-olefinic isomers by mass spectrometric methodology is presented. Starting from the observation of the characteristic expulsion of dihydrogen from metastable C₇ H₉ ⁺ ions, which is associated with the release of large amounts of kinetic energy, and the composite C- and H-scrambling prior to the loss of methane, in particular, insights into the isomerization scenario of various isomeric C₇ H₉ ⁺ , C₈ H₁₁ ⁺ , and C₉ H₁₃ ⁺ ions, based on a large variety of independent techniques, are discussed. Besides isotope labeling and metastable ion methodology, these include flowing afterglow mass spectrometry, gas-phase titration and infrared spectroscopy of mass-selected ions. The particularly complex energy hypersurface of isomerizing and fragmenting toluenium ions, which has been elaborated in various reports over the years, is presented in a combined way to assess the role of protonated cycloheptatriene, norbornadiene, and 6-methylfulvene as well as a number of further C₇ H₉ ⁺ isomers. The formation and nature of C₇ H₉ ⁺ ions generated by fragmentation of various hydrocarbon precursors, such as monoterpenes and adamantane, is also addressed. The contribution of infrared multiphoton dissociation spectroscopy (IRMPD) and tagged-ion infrared photodissociation (IRPD) of the gaseous C₇ H₉ ⁺ ions as compared to the wealth of previous understanding of their chemistry is commented on as well. Finally, remarkable parallels of the gas-phase chemistry of methylbenzenium ions and the role of such species within the cavities of acidic zeolite catalysts in the course of the industrially important methanol-to-hydrocarbon reaction are discussed. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Loss of CO2 from Monodeprotonated Phthalic Acid upon Photodissociation and Dissociative Electron Detachment

The decarboxylation (CO₂ loss) mechanism of cold monodeprotonated phthalic acid was studied in a photodissociation action spectrometer by quantifying mass-selected product anions and neutral particles as a function of the excitation energy. The analysis proceeded by interpreting the translational energy distribution of the generated uncharged products, and with the help of quantum calculations. In particular, this study reveals different fragmentation pathways in the deprotonated anion and in the radical generated upon electron photodetachment. Unlike the behavior found in other deprotonated aryl carboxylic acids, which do not fragment in the anion excited state, a double loss of CO₂ molecules takes place in the phthalic monoanion. Moreover, at higher excitation energies the phthalic monoanion experiences decarboxylative photodetachment with a statistical distribution of product translational energies, which contrasts with the impulsive dissociation reactions characteristic of other aryl carboxylic anions.

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.

Dissociative photodetachment vs. photodissociation of aromatic carboxylates: the benzoate and naphthoate anions

Photodetachment leads to a stable radical and to dissociation. Both processes are characterized by the kinetic energy release of the neutral particles.

Tautomerism and electronic spectroscopy of protonated 1- and 2-aminonaphthalene

Protonation sites can be controlled by the electrospray source as written in the figure.

Vibronic spectra of protonated hydroxypyridines: contributions of prefulvenic and planar structures

The UV photofragmentation spectra of cold protonated hydroxypyridines display well resolved vibrational structures, interpreted with calculations at the CC2 level.