Competition between Loss of H2 versus H+H in the Fragmentation of the Fluorene Cation.

The fluorene cation is a frequently studied molecule in the context of fragmentation experiments. This is because of its potential role in interstellar chemistry, notably as a precursor of PAH cages. In this paper, we analyze H, H+ , H2 and H 2 + ${{\rm{H}}_2^ + }$ losses from the fluorene cation using the SMF (Statistical Molecular Fragmentation) model. We calculate the probabilities of all the 534 possible fragmentation channels as a function of the excitation energy, up to the loss of three hydrogens. Four different types of hydrogen atom pairings (from the same carbon, from the same ring, from different rings and across-the-bay) have been tested in order to determine which types contribute to the actual production of hydrogen molecules. The simulated breakdown curves are in very good agreement with different experimental results when same ring pairing is taken into account. It was possible to deduce from the model the locations of the emitted H, H+ , H2 and H 2 + ${{\rm{H}}_2^ + }$ species.

Multiple dehydrogenation of fluorene cation and neutral fluorene using the statistical molecular fragmentation model.

The statistical molecular fragmentation (SMF) model was used to analyze the 306 fragmentation channels (containing 611 different species) that result from the fluorene (C13H10+) cation losing up to three hydrogen atoms (neutral radicals and/or a proton). Breakdown curves from such analysis permit one to extract experimentally inaccessible information about the fragmentation of the fluorene cation, such as the locations of the lost hydrogen atoms (or proton), yields of the neutral fragments, electronic states of the residues, and quantification of very low probability channels that would be difficult to detect. Charge localization during the fragmentation pathways was studied to provide a qualitative understanding of the fragmentation process. Breakdown curves for both the fluorene cation and neutral fluorene were compared. The SMF results match the rise and fall of the one hydrogen loss yield experimentally measured by imaging photoelectron-photoion coincidence spectroscopy using a VUV synchrotron.

Statistical molecular fragmentation: which parameters influence the branching ratios?

The fragmentation of molecules under conditions that result in yields of products that are thermodynamically controlled can be readily studied with statistical models. We explore which parameters influence the branching ratios using our recently developed Statistical Molecular Fragmentation model (SMF) and apply it to the decomposition of propane. We find that the fragmentation process has low sensitivity to the differences between the molecular descriptions given by commonly used ab initio methods (B3LYP, CCSD(T) and composite methods with different atom-centered basis sets). However, the branching ratios are most influenced by the vibrational frequencies of the molecules and radicals present in the decomposition pathways.

The statistical molecular fragmentation model compared to experimental plasma induced hydrocarbon decays.

We compare the predictions of our recently developed statistical molecular fragmentation (SMF) model with experimental results from plasma induced hydrocarbon decay. The SMF model is an exactly solvable statistical model, able to calculate the probabilities for all possible fragmentation channels as a function of the deposited excitation energy. The weights of the channels are calculated from the corresponding volume of the accessible phase space of the system, taking into account all relevant degeneracies, symmetries and density functions. An experiment designed to study the abatement of propene in N2 using a photo-triggered discharge producing a homogeneous plasma at sub-atmospheric pressure was also performed. Using a 0D model that simulates the complex chemical kinetics in the plasma, it was possible to assess the percentages of the original parent hydrocarbon's fragmentation channels based on the detected species. These results were compared to those obtained from the SMF model. Previous plasma induced hydrocarbon fragmentation experiments for ethene, ethane and propane, were also compared to the predictions of the SMF model. For energies below that of metastable dinitrogen (i.e. below 6.17 eV and 8.4 eV), the SMF model and the experimental fragmentation channels coincide. This study allows one to draw conclusions both on the range of excitation energies transferred to the parent hydrocarbon molecules during plasma discharge and on the probability of the dynamical coupling of two H atoms from neighbouring carbon atoms to form H2 molecules.

A Statistical Model and DFT Study of the Fragmentation Mechanisms of Metronidazole by Advanced Oxidation Processes.

The degradation pathway of the antibiotic metronidazole (MNZ) in wastewater was investigated computationally with a physical statistical method and a quantum chemical approach. In both cases, density functional theory (DFT) at the M06-2X/6-311+G(d,p) level was used to calculate the structures and property parameters of all molecules. On one hand, decay of the isolated MNZ molecule excited at a given excitation energy was studied using the statistical molecular fragmentation (SMF) model. On the other hand, the reaction mechanisms of MNZ oxidized by hydroxyl radicals (•OH) in advanced oxidation processes (AOPs) were analyzed. Both studies show that the main reaction sites in MNZ are, by decreasing importance, -NO2, -CH2OH, and -CH2CH2OH. The main degradation reactions are (i) alcohol group oxidation including the abstraction of hydrogen on C in the -CH2OH group and oxidation of the hydroxyl group to the aldehyde and further to the carboxylic acid and (ii) addition-elimination reactions happening on the imidazole ring which finally replace the nitro by hydroxyl radicals. The results gained are in a good agreement with the available experimental data on MNZ degradation by AOPs. The structures of intermediates, transition states, and free energy surfaces are helpful in elucidating the details of the elimination mechanism, supplementing current experimental knowledge.

Photofragmentation of the fluorene cation: I. New experimental procedure using sequential multiphoton absorption.

The hydrogen-loss channel, induced by sequential multiphoton absorption, of the vapor-phase fluorene cation was investigated using a supersonic molecular beam and a time-of-flight mass spectrometer. The fluorene cation was prepared by resonantly enhanced multiphoton ionization. The ultimate goal of this experiment is the determination of the evolution of the dissociation rate constant in a wide energy range. In this paper, we give a description of the original experimental procedure, show that the absorption process is non-Poissonian, and determine the absolute photon absorption cross section.

Photofragmentation of the fluorene cation: II. Determination of the H-loss energy-dependent rate constant.

The hydrogen-loss channel, induced by sequential multiphoton absorption, of the vapor-phase fluorene cation was investigated using a pulsed supersonic molecular beam, a time-of-flight mass spectrometer, and pulsed nanosecond lasers. Our new method leads to the determination of the absolute absorption cross section. Its attenuation with the number of absorbed photons has been approximated by means of statistical models. A model-free determination of the evolution of the dissociation rate constant in a relatively large energy range was obtained by solving the set of coupled differential kinetic equations numerically. Particular attention was focused on the data analysis techniques. The free fit of these rate constants is close to the photothermodissociation statistical model, but shows a discrepancy with the Rice and Ramsperger and Kassel model mainly at high energy. The resulting activation energy is in agreement with both that deduced from the ab initio calculations and that from the tight-binding energy potential surface model.