Vibrational spectroscopy of a cetane improver for biodiesel

International audienceBiofuels represent one of the serious ways to succeed in the ecological transition. Diesel and biodiesel contains additives to facilitate the combustion reaction: theses additives improve the engine performance and reduce the emissions of unburned hydrocarbons, particulates, carbon monoxide and nitrogen oxides. These constituents are called cetane improvers and the most common is 2-ethyl hexylnitrate (2-EHN) [1].Since this additive is produced in very large quantities [2], it is important to be able to qualitatively and quantitatively identify its accidental release. The IR spectrum is needed for atmospheric detection.We have accurately measured the room temperature absorption cross section of 2-EHN, between 850 and 3000 cm-1, using a FT-IR spectrometer. The main vibrational bands have been attributed, thanks to DFT calculations. Geometry optimizations also show the complexity of this flexible molecule, leading to almost iso-energetic conformers with slightly different vibrational spectra. The co-existence of all these isomers experimentally leads to particularly broad and complex absorption bands with shoulders. As an application, we also used these cross sections to probe the adsorption/desorption processes that can occur on the cell walls. All those aspects will be presented in this talk

Vibrational spectroscopy of a cetane improver for biodiesel

International audienceBiofuels represent one of the serious ways to succeed in the ecological transition. Diesel and biodiesel contains additives to facilitate the combustion reaction: theses additives improve the engine performance and reduce the emissions of unburned hydrocarbons, particulates, carbon monoxide and nitrogen oxides. These constituents are called cetane improvers and the most common is 2-ethyl hexylnitrate (2-EHN) [1].Since this additive is produced in very large quantities [2], it is important to be able to qualitatively and quantitatively identify its accidental release. The IR spectrum is needed for atmospheric detection.We have accurately measured the room temperature absorption cross section of 2-EHN, between 850 and 3000 cm-1, using a FT-IR spectrometer. The main vibrational bands have been attributed, thanks to DFT calculations. Geometry optimizations also show the complexity of this flexible molecule, leading to almost iso-energetic conformers with slightly different vibrational spectra. The co-existence of all these isomers experimentally leads to particularly broad and complex absorption bands with shoulders. As an application, we also used these cross sections to probe the adsorption/desorption processes that can occur on the cell walls. All those aspects will be presented in this talk

Dehydrocoupling of dimethylamine borane by titanocene: Elucidation of ten years of inconsistency between theoretical and experimental descriptions

International audienceMore than ten years ago, Manners and coworkers published the first experimental study on the efficiency of titanocene to catalyze the dehydrocoupling of dimethylamine borane (DMAB, JACS 2006, 128, 9582). Several experimental investigations have shown that a two-step mechanism leads to the formation of a cyclic diborazane (Me 2 N-BH 2) 2 via the linear diborazane (HNMe 2-BH 2-NMe 2-BH 3). This finding stood in contradiction with following theoretical investigations of the reaction pathway. Herein, using dispersion-corrected density functional theory (DFT-D), we propose an energetically favored reaction mechanism in perfect agreement with the experimental findings. It is shown that van der Waals interactions play a prominent role in the reaction pathway. The formation of 3-center 2-electron interactions, classical dihydrogen bonds, as well as non-classical dihydrogen bonds, was identified with the help of topological and localized orbital approaches

Activation of C–H and B–H bonds through agostic bonding: an ELF/QTAIM insight

International audienceAgostic bonding is of paramount importance in C–H bond activation processes. The reactivity of the σ C–H bond thus activated will depend on the nature of the metallic center, the nature of the ligand involved in the interaction and co-ligands, as well as on geometric parameters. Because of their importance in organometallic chemistry, a qualitative classification of agostic bonding could be very much helpful. Herein we propose descriptors of the agostic character of bonding based on the electron localization function (ELF) and Quantum Theory of Atoms in Molecules (QTAIM) topological analysis. A set of 31 metallic complexes taken, or derived, from the literature was chosen to illustrate our methodology. First, some criteria should prove that an interaction between a metallic center and a σ X–H bond can indeed be described as “agostic” bonding. Then, the contribution of the metallic center in the protonated agostic basin, in the ELF topological description, may be used to evaluate the agostic character of bonding. A σ X–H bond is in agostic interaction with a metal center when the protonated X–H basin is a trisynaptic basin with a metal contribution strictly larger than the numerical uncertainty, i.e. 0.01 e. In addition, it was shown that the weakening of the electron density at the X–Hagostic bond critical point with respect to that of X–Hfree well correlates with the lengthening of the agostic X–H bond distance as well as with the shift of the vibrational frequency associated with the νX–H stretching mode. Furthermore, the use of a normalized parameter that takes into account the total population of the protonated basin, allows the comparison of the agostic character of bonding involved in different complexes

Toward a quantitative evaluation of the strength of Cp2M···η 2–borate interactions

International audienceAmine boranes might be a crucial material toward a successful energy transition. A precise description of metal-ligand interactions involved in the homogeneous catalysis of the dehydrogenation of amine boranes would represent a major step toward a global understanding of the reaction process. η 2 interactions between borates and organometallic compounds were identified as key intermediates in the reaction pathways. Herein are proposed reduced topological descriptors to measure the efficiency of several metallocenes to activate B-H bonds in prototypical Cp 2 M ···η 2-borates complexes. The combined use of QTAIM and ELF partitions allowed us to ascertain the 3C/2e interactions between borates and metallocenes. The strength of the B-H bond is affected by two different parameters: the nature of the ligands bonded to the borates and the effect of the interaction with the metallocene. The use of reduced descriptors allows to evaluate the activation of the B-H bond due to the sole effect of the interaction between the borates and the metallocenes. Herein, we suggest the concomitant use of the reduced electron density and reduced distance of the B-H bonds to classify the {borates + metallocenes} systems as a function of the strength of the B-H interaction with the metallocene. We further suggest to complement this evaluation based on the QTAIM analysis by a quantization of the contribution of the metallic center on the protonated basin defined within the ELF framework

What is the hydrophobic interaction contribution to the stabilisation of micro-hydrated complexes of trimethylamine oxide (TMAO)? A joint DFT-D, QTAIM and MESP study

International audienceMicro-hydrated trimethylamine oxide (TMAO) has been investigated using a range-separated-hybrid functional including empirical dispersion correction. Electrophilic and nucleophilic sites on TMAO and water clusters have been identified using the molecular electrostatic potential (MESP). The nature of the chemical bonding in the different isomers of the micro-hydrated complexes has been investigated with the topological analysis of the electron density (QTAIM) method. For complexes containing one to four water molecules, the strongest intermolecular interactions consist in hydrogen bonding between the oxygen atom of the TMAO and hydrogen atoms of water molecules. From five water molecules, interactions between water molecules become the main source of stabilization of the most stable isomer. From four stationary points corresponding to the 1:1 (TMAO:H 2 O) complex, we determined the minimum distances between water molecules and central TMAO allowing the latter molecule to be encapsulated within a water clathrate-type cage. Optimization of TMAO encapsulated within two water cages (5 12 and 5 12 6 2) suggests that only in the case of the 5 12 6 2 water cage the insertion of TMAO, the preservation of the hydrogen bonding between water molecules is energetically favorable. The interaction energy between one inserted TMAO and the 5 12 6 2 water cage was calculated to be around 150 kJ/mol with respect to the ground state of two partners. This result suggest that a thorough investigation of mono-hydrated complexes may be particularly relevant to identify the most suitable water cage for encapsulating a given solute

Reactivity Between Non-Energetic Hydroxyl (OH) Radicals and Methane (CH 4 )

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Study of the Reactivity of CH3COOH+• and COOH+ Ions with CH3NH2: Evidence of the Formation of New Peptide-like C(O)−N Bonds

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Investigations of the fragmentation pathways of benzylpyridinium ions under ESI/MS conditions.

International audienceBenzylpyridinium ions are often used as 'thermometer ions' in order to evaluate the internal energy distribution of the ions formed in sources of mass spectrometers. However, the detailed fragmentation pathways of these parent ions were not well established. In particular, fragmentation involving a rearrangement (RR) process may be influencing the simulated distribution curves. In a previous study, we suggested that such RR actually occurred under electrospray ionization/mass spectrometry (ESI/MS) and fast atom bombardment/mass spectrometry (FAB/MS) experiments. Here, we present a systematic study of different substituted benzylpyridinium ions. Theoretical calculations showed that RR fragmentation leading to substituted tropylium ions could occur under 'soft ionization' conditions, such as ESI or FAB. Experimental results obtained under gas-phase reactivity conditions showed that some substituted benzylpiridinium compounds actually undergo RR fragmentations under ESI/MS conditions. Mass-analyzed kinetic experiments were also carried out to gain information on the reaction pathways that actually occur, and these experimental results are in agreement with the reaction pathways theoretically proposed