Spectroscopy and Photodissociation of the Perfluorooctanoate Anion

International audiencePerfluorocarbons, a class of fully fluorinated compounds, are highly persistent and toxic pollutants that are receiving increasing attention due to their widespread environmental distribution. In this study, attention was focused on one compound in particular, namely, perfluorooctanoic acid (PFOA). The first investigation of the UV/VUV photochemistry of the PFOA anion in the gas phase by action spectroscopy of selected ions is reported. Two main relaxation channels were identified: photodissociation and photodetachment. Absolute cross sections for the individual observed processes were measured. DFT calculations and natural transition orbital analysis were carried out to help in the interpretation of the experimental results

ESI formation of a Meisenheimer complex from tetryl and its unusual dissociation

The reactivity of the explosive tetryl (N-methyl-N,2,4,6-tetranitroaniline; Mw = 287 u) was studied using electrospray ionization in negative mode. The main species detected in the spectrum corresponds to the ion observed at m/z 318 (previously assumed to be the odd-electron ion [tetryl + HNO]-•, C7H6O9N6). In this study, we show using D-labeling combined with high-resolution mass spectrometry that this species corresponds to an even-electron anion (i.e. C8H8O9N5), resulting from the formation of a Meisenheimer complex between tetryl and the methanol used as the solvent. Fragmentation of this complex under CID conditions revealed an unexpected fragment: the formation of a 2,4,6-trinitrophenoxide anion at m/z 228. 18O-labeling combined with quantum chemical calculations helped us better understand the reaction pathways and mechanisms involved in the formation of this product ion. This occurs via a transition state leading to a SN2-type reaction, consequently evolving toward an ion-dipole complex. The latter finally dissociates into deprotonated picric acid

Comprehensive VUV photoionization study of the CF 3 • trifluoromethyl radical using synchrotron radiation

International audienceThe trifluoromethyl radical, CF 3 • , is studied for the first time by means of threshold photoelectron spectroscopy (TPES). The radical is produced in the gas phase using the flash-pyrolysis technique from hexafluoroethane as a precursor. CF 3 + total ion yield and mass-selected TPES of the radical are recorded using a spectrometer based upon velocity map imaging and Wiley-McLaren time-of-flight coupled to the synchrotron radiation. The high resolution of the instrument and of the photons allows the observation of rich vibrational progressions in the TPES of CF 3 •. By using Franck-Condon factors computed by Bowman and coworkers, we have been able to simulate the TPES. The initial vibrational temperature of the radical beam has been evaluated at ca. 350 K. The structures have been identified as transitions between (n 1 ,n 2) and (n 1 + ,n 2 +) vibrational levels of CF 3 and CF 3 + with small excitation of the breathing mode,  1 + , and large excitation (n 2 + = 10-26) of the umbrella mode,  2 + , in the cation. From the energy separation between the two resolved peaks of each band, a value of 994 ± 16 cm-1 has been derived for the  1 + breathing frequency of CF 3 +. For the high-lying n 2 + levels, th

Identifying and Understanding Strong Vibronic Interaction Effects Observed in the Asymmetry of Chiral Molecule Photoelectron Angular Distributions

International audienceno abstrac

Extended kinetic method and RRKM modeling to reinvestigate proline’s proton affinity and approach the meaning of effective temperature

International audienc

Exploring Phosphine Electronic Effects on Molybdenum Complexes: A Combined Photoelectron Spectroscopy and Energy Decomposition Analysis Study

International audienceIn organometallic chemistry, and especially in the catalysis area, accessing the finest tuning of a catalytic reaction pathway requires a detailed knowledge of the steric and electronic influence of the ligands bound to the metal center. Usually, the M-L bond between a ligand and a metal is depicted by the Dewar-Chatt-Duncanson model involving two opposite interactions, a σ-donor and a π-acceptor effect of the ligand. The experimental evaluation of these effects is essential and complementary to in-depth theoretical approaches that are able to provide a detailed description of the M-L bond. In this work, we present a study of LMo(CO)5 complexes with L being various tertiary phosphines ligands by means of mass-selected high-resolution photoelectron spectroscopy (PES) performed with synchrotron radiation, DFT and energy decomposition analyses (EDA) combined with the natural orbitals for chemical valence (NOCV) analysis. These methods enable a separated access of the σ-donor and π-acceptor effects of ligands by probing either the electronic configuration of the complex (PES) or the interaction of the ligand with the metal (EDA). Three series of PX3 ligands with various electronic influence are investigated: the strong donating alkyl substituents (PMe3, PEt3 and PiPr3), the intermediate PPhxMe(3-x) (x = 0-3) set and the PPhxPyrl(3-x) set (x = 0-3 with Pyrl being the strong electron withdrawing pyrrolyl group C4H4N). For each complex, their adiabatic and vertical ionization energies (IEs) could be determined with a 0.03 eV precision. Experiment and theory show an excellent agreement, either for the IEs determination or for the electronic effect analysis. The ability to interpret the spectra is shown to depend on the character of the ligand. “Innocent” ligands provide the spectra the most straightforward to analyze whereas the “non-innocent” ligands (which are ionized prior to the metal center) render the analysis more difficult due to an increased number of molecular orbitals in the energy range considered. A very good linear correlation is finally found between the measured adiabatic ionization energies and the interaction energy term obtained by EDA for each of these two types of ligands which opens interesting perspective for the prediction of ligand characters

Ligand effects in gold-carbonyl complexes: evaluation of the bond dissociation energies using blackbody infrared radiative dissociation

International audienceIn a previous work, the ability of gold-carbonyl bond dissociation energies (BDEs) to describe ligand effects in gold(I) complexes was probed using collision-induced dissociation measurements. Despite the large experimental error of the technique, results agreed with theoretical description of these effects. We propose here another experimental approach to evaluate the BDEs of [LAu-CO]+ complexes, by using Blackbody Infrared Radiative Dissociation (BIRD) method which is able to provide absolute energetic measurements. To this aim, the dissociation of 8 gold-carbonyl complexes was studied with this approach to obtain their Arrhenius activation energies. Because these complexes do not reach the rapid energy exchange (REX) limit conditions, critical energies of these dissociations were then determined using the kinetic data and an equilibrium truncated thermal (ETT) internal energy distribution model implemented in the RRKM-QET MassKinetics software. To validate our approach, the known critical energies of proton-bound amino acid dimers were first reinvestigated using this model. The results are in excellent agreement with those reported in the literature. This indicates that the assumptions of this simple model are reasonable and allow the determination of accurate dissociation energies. For [LAu-CO]+ complexes, even though a shift (from 0.18 to 0.26 eV) is observed between binding energies obtained and calculated values, a linear tendency between these two sets of data is still obtained. These results validate the use of BIRD in combination with equilibrium truncated thermal internal energy distributions model to obtain dissociation energies for relatively small ions. It is also shown herein that the measure of the gold-carbonyl bond dissociation energies is a good descriptor of ligand effects in gold(I) complexes that may be reliably evaluated using the BIRD technique

Clarification of the 30 Da releases from the [M-H] − and M −• ions of trinitrotoluene by electrospray high resolution mass spectrometry

International audienceAlthough some nitroaromatic compounds can naturally occur in the environment, the vast majority of them come from anthropogenic sources. Indeed, nitroaromatic compounds such as 2,4,6-trinitrotoluene (TNT) and related-compounds are widely used as chemicals or synthetic intermediates in industrial manufacturing of explosives, dyes, pharmaceuticals, polyurethane foams and pesticides. [1] Considering the proven toxicity of nitroaromatic compounds on living organisms [2] , and their significance in the forensic sciences, much attention has been given to these compounds [3]. Thus, nitroaromatic compounds have been extensively studied by mass spectrometry (MS) coupled with different ionization sources. At first, classical vacuum ionization techniques such as Electron Ionization (EI) [4, 5] and Chemical Ionization (CI) [5-7] were widely used to examine nitroaromatic compounds. Upon the development of atmospheric pressure ionization (API) techniques, atmospheric pressure chemical ionization (APCI) [8] and electrospray ionization (ESI) [9] became established as preferred techniques to analyze nitroaromatic compounds [10]. In particular, ESI of TNT in the negative ion mode can produce competitive processes : (i) deprotonation [M-H]-and (ii) electrochemical reduction M •-. [11, 12] Afterwards, Collision-Induced-Dissociation (CID) has been extensively used for structural as well as quantitative information [13] TNT samples are commonly analyzed at low resolution by tandem mass spectrometry. Under CID conditions, these negatively charged molecular species dissociate by competitive losses of either OH • (implicating the "ortho effect", Scheme 1), or by loss of NO • (after NO 2 /ONO isomerization). In addition, NO 2  release was also observed. Scheme 1. Stepwise OH  release implicating the "ortho effect" promoted by the radical anion [M] •-(m/z 227) of TNT. In this study, we have used a standard solution (1 mg/mL in MeOH:ACN (1:1)) of 2,4,6-trinitrotoluene (TNT), obtained from AccuStandard Europe (Niederbipp, Switzerland). TNT was prepared by dilution at 1 µg mL-1 in H 2 O/MeOH (1:1), then infused at a flow rate of 5 µL min-1 into an LTQ-Orbitrap XL mass spectrometer (Thermo Fisher Scientific, Courtaboeuf, France) and ionized by ESI in the negative ion mode. The employed spray voltage was-2.5 kV giving mainly the deprotonated molecule [M-H]-at m/z 226 and th