The electronic spectra of protonated PANH molecules

International audienceAims. This study was designed to examine the viability of protonated nitrogen-substituted polycyclic aromatic hydrocarbons (H + PANHs) as candidates for the carriers of the diffuse interstellar bands (DIBs). Methods. We obtained the electronic spectra of two protonated PANH cations, protonated acridine and phenanthridine, using parent ion photo-fragment spectroscopy and generated theoretical electronic spectra using ab initio calculations. Results. We show that the spectra of the two species studied here do not correspond to known DIBs. However, based on the general properties derived from the spectra of these small protonated nitrogen-substituted PAHs, we propose that larger H + PANH cations represent good candidates for DIB carriers due to the expected positions of their electronic transitions in the UV-visible and their narrow spectral bands

UV Photoinduced Dynamics of Conformer-Resolved Aromatic Peptides

International audienceA detailed understanding of radiative and nonradiative processes in peptides containing an aromatic chromophore requires the knowledge of the nature and energy level of low-lying excited states that could be coupled to the bright 1 * excited state. Isolated aromatic amino acids and short peptides provide benchmark cases to study, at the molecular level, the photoinduced processes that govern their excited state dynamics. Recent advances in gas phase laser spectroscopy of conformer-selected peptides have paved the way to a better, yet not fully complete, understanding of the influence of intramolecular interactions on the properties of aromatic chromophores. This review aims at providing an overview of the photophysics and photochemistry at play in neutral and charged aromatic chromophore containing peptides, with a particular emphasis on the charge (electron, proton) and energy transfer processes. A significant impact is exerted by the experimental progress in energy-and time-resolved spectroscopy of protonated species, which leads to a growing demand for theoretical supports to accurately describe their excited state properties

Photoinduced dynamics in protonated aromatic amino acid

UV photoinduced fragmentation of protonated aromatics amino acids have emerged the last few years, coming from a situation where nothing was known to what we think a good understanding of the optical properties. We will mainly focus this review on the tryptophan case. Three groups have mostly done spectroscopic studies and one has mainly been involved in dynamics studies of the excited states in the femtosecond/picosecond range and also in the fragmentation kinetics from nanosecond to millisecond. All these data, along with high level ab initio calculations, have shed light on the role of the different electronic states of the protonated molecules upon the fragmentation mechanisms

What is the meaning of lifetime measurement?

The lifetime measurement of molecular excited state has been the subject of many papers and experiments. Very often the experimental data are fitted by single or bi exponential decays which in many case is the best fit that can be done owing the signal to noise ratio. The times constants obtained from these fit are often discussed in term of one species associated with one lifetime: depending on the studied system, the species can be one type of molecule, one isomer from a given molecule or local environment. How justified is this assumption

Excited state of protonated benzene and toluene

International audienceWe present photo-fragmentation electronic spectra of the simplest protonated aromatic molecules, protonated benzene and toluene, recorded under medium resolution conditions and compared with the photo-fragmentation spectrum of protonated pyridine. Despite the resolution and cold temperature achieved in the experiment, the electronic spectra of protonated benzene and toluene are structure-less, thus intrinsically broadened. This is in agreement with the large geometrical changes and the fast dynamic toward internal conversion predicted by ab-initio calculations for protonated benzene (M. F. Rode, A. L. Sobolewski, C. Dedonder, C. Jouvet, and O. Dopfer, J. Phys. Chem. A 113, 5865–5873 (2009)

UV spectroscopy of cold ions as a probe of the protonation site

International audienceThe best determination of the most stable protonation site in aromatic molecule relies 5 nowadays on the IRspectroscopy and ab initio calculations. It appears that these methods are not necessarily unambiguousand cannot always be safely employed. We present in this paper an example showing that electronicspectroscopy of cold ions complemented with ab initio calculations gives clear results on the protonationsite. In the example given on the aminophenol isomers (in ortho, meta and para positions), the protonation10 site is assigned from the electronic spectroscopy and in particular we show that for the meta isomer theproton is not on the amino group as observed for the other isomers. It shows also that the protonation siteis not conserved in the electrospray evaporation/ ionization process

Photoinduced water splitting in pyridine water clusters

International audienceAb-initio calculations predict that pyridine (Py) can act as a photo-catalyst to split water by absorption of a UV photon following the reaction Py-H 2 O + hν → PyH● + OH●. To test this prediction, we performed two types of experiment: in the first, we characterize the electronic spectroscopy of the PyH● radical in the gas phase. In the second, we evidence the reaction through UV excitation of molecular Py-(H2O) n clusters obtained in a supersonic expansion and monitoring the PyH● reaction product. The results show unambigu-ously that PyH● is produced, thus that water is split using pyridine as photo-catalyst. In this paper we show that pyridine can act as a photo-catalytic molecular system, which can dissociate the water covalent bond with UV C light. Water molecule is a ubiquitous system to produce H2 , however VUV light is needed to photo-dissociate the H-OH covalent bond (186 nm, 6.66 eV). Since most of the sunlight reaching the earth is in the visible spectral region, we cannot dissociate the H-OH bond under normal sunlight conditions and it is necessary to design a system that could break the H-OH bond with visible light

Non-destructive detection of large molecules without mass limitation

The problem for molecular identification knows many solutions which include mass spectrometers whose mass sensitivity depends on the performance of the detector involved. The purpose of this article is to show by means of molecular dynamics simulations, how a laser-cooled ion cloud, confined in a linear radio-frequency trap, can reach the ultimate sensitivity providing the detection of individual charged heavy molecular ions. In our simulations, we model the laser-cooled Ca + ions as two-level atoms, confined thanks to a set of constant and time oscillating electrical fields. A singly-charged molecular ion with a mass of 10 6 amu is propelled through the ion cloud. The induced change in the fluorescence rate of the lather is used as the detection signal. We show that this signal is due to a significant temperature variation triggered by the Coulombian repulsion and amplified by the radio-frequency heating induced by the trap itself. We identify the optimum initial energy for the molecular ion to be detected and furthermore, we characterize the performance of the detector for a large range of confinement voltages

Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation

Reprinted (adapted) with permission from Journal of Physical Chemistry A Copyright (2015) American Chemical SocietyInternational audienceThe electronic spectroscopy and the electronic excited state properties of cold protonated phenylalanine and protonated tyrosine have been revisited on a large spectral domain and interpreted by comparison with ab initio calculations. The protonated species are stored in a cryogenically cooled Paul trap, maintained at ~ 10K, and the parent and all the photo-fragment ions are mass-analyzed in a time-of-flight mass spectrometer, which allows detecting the ionic species with an improved mass resolution compared to what is routinely achieved with a quadrupole mass spectrometer. These new results emphasize the competition around the band origin between two proton transfer reactions from the ammonium group toward either the aromatic chromophore or the carboxylic acid group. These reactions are initiated by the coupling of the locally excited ππ* state with higher charge transfer states, the positions and coupling of which depend on the conformation of the protonated molecules. Each of these reaction processes gives rise to specific fragmentation channels that supports the conformer selectivity observed in the photofragmentation spectra of protonated Tyrosine and Phenylalanine

Communication: UV photoionization of cytosine catalyzed by Ag+

The photo-induced damages of DNA in interaction with metal cations, which are found in various environments, still remain to be characterized. In this paper, we show how the complexation of a DNA base (cytosine (Cyt)) with a metal cation (Ag+) changes its electronic properties. By means of UV photofragment spectroscopy of cold ions, it was found that the photoexcitation of the CytAg+ complex at low energy (315-282) nm efficiently leads to ionized cytosine (Cyt+) as the single product. This occurs through a charge transfer state in which an electron from the p orbital of Cyt is promoted to Ag+, as confirmed by ab initio calculations at the TD-DFT/B3LYP and RI-ADC(2) theory level using the SV(P) basis set. The low ionization energy of Cyt in the presence of Ag+ could have important implications as point mutation of DNA upon sunlight exposition.Fil: Taccone, Martín Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Féraud, Geraldine. Aix Marseille Université. Physique des Interactions Ioniques et Moléculaires; FranciaFil: Berdakin, Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Dedonder Lardeux, Claude. Aix Marseille Université. Physique des Interactions Ioniques et Moléculaires; FranciaFil: Jouvet, Christophe. Physique des Interactions Ioniques et Moléculaires; FranciaFil: Pino, Gustavo Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentin