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International audience; Gaseous iodomethane are naturally emitted in the atmosphere over oceans through the algae and phytoplankton activities. The fate of naturally emitted iodomethane is of great interest because of the oxidizing properties of iodine in the atmosphere and its impact on the catalytic destruction of the ozone layer. Additionally, iodomethane is one of the gaseous species that can be emitted in the case of severe nuclear accident. The radiological impact of gaseous iodomethane is of concerns and requires knowledge about its behavior in the atmosphere. Water is one of the major species in the atmosphere which is responsible for atmospheric aerosol nucleation and thus, for cloud condensation nuclei (CCN). The fundamental knowledge concerning the interaction between methyl iodine and water at the molecular scale contributes to the better understanding of the fate of such species into the atmosphere and their role in CCN formation. Here the microhydration of iodomethane was investigated using cryogenic matrix experiments which were supported by theoretical DFT calculations. A large excess of water regarding CH3I was used in order to mimic atmospheric conditions. Dimers and trimers of CH3I are observed despite the high water amount in the initial mixture together with hetero aggregates between CH3I and water clusters. This may be explained by the low affinity of CH3I with water. Considering the concentration of iodomethane used in our experiments, the aggregates are rather formed in gas phase and not in the matrix cage. The interaction between CH3I and H2O molecules studied experimentally and supported by DFT calculation highlights that, in the atmosphere, gaseous iodomethane and water will likely form association between water and iodomethane aggregates instead of (CH3I)n-(H2O)m hetero complexes. Our results have important consequences for the understanding of the alkyl halide solvation in primary processes and contribute to the understanding of reactive halogen species in tropospheric chemistry. In the context of a nuclear severe accident, our work is contributing to better understand the fate of nuclear species in the atmosphere and thus, the radionuclide dispersion.

International audience; UV photodecomposition of azidomethyl methyl sulfide (AMMS) yields a transient S-methylthiaziridine which rapidly evolves to S-methyl-N-sulfenylmethanimine at 10 K. This species was detected by infrared matrix isolation spectroscopy. The mechanism of the photoreaction of AMMS has been investigated by a combined approach, using low-temperature matrix isolation FTIR spectroscopy in conjunction with two theoretical methods, namely, complete active space self-consistent field and multiconfigurational second-order perturbation. The key step of the reaction is governed by a S2/S1 conical intersection localized in the neighborhood of the singlet nitrene minimum which is formed in the first reaction step of the photolysis, that is, N2 elimination from AMMS. Full assignment of the observed infrared spectra of AMMS has been carried out based on comparison with density functional theory and second-order perturbation Møller–Plesset methods.

International audience; The photochemistry of Fe:H2O adducts is of interest in fields as diverse as catalysis and astrochemistry. Industrially, iron can be used as a catalyst to convert H2O to H2, whereas in the interstellar medium it may be an important component of dust grains, influencing the chemistry on their icy surfaces. This study consisted of the deposition and spectral characterization of binary systems of atomic iron with H2O in cryogenic argon matrixes. In this way, we were able to obtain information about the interaction of the two species; we observed the formation of adducts of iron monomers and dimers with water molecules in the mid-IR and UV–visible spectral domains. Upon irradiation with a UV radiation source, the iron species were inserted into the water molecules to form HFeOH and HFe2OH, leading in some cases to the formation of FeO possibly accompanied by the production of H2. DFT and correlated multireference wave function calculations confirmed our attributions. This combination of IR and UV–visible spectroscopy with theoretical calculations allowed us to determine, for the first time, the spectral characteristics of iron adducts and their photoproducts in the UV–visible and in the OH stretching region of the mid-IR domain.

In aromatic systems that contain an amino group, there is competition between protonation on a carbon atom of the skeleton and protonation on the amino group. Herein, we studied the photofragmentation of protonated 1-aminopyrene in a cold ion trap and mainly observed the protonated amino tautomer, which led to fragmentation pathways through the loss of H or NH3 groups. Several excited states were assigned, among which the fourth excited state showed broadened bands, thus indicating a fast decay that was attributed to the presence of a πσ* charge-transfer state by comparison of the experimental results with ab initio calculations. We deduced the πσ* transition energies in protonated aromatic amino compounds of increasing size directly from the ionization potentials of the neutral aromatic unsubstituted molecules. Tautomers that were protonated on a carbon atom of the pyrene skeleton were also weakly observed, and we showed that two tautomers that were protonated on a carbon atom of the aromatic ring could be distinguished by using electronic spectroscopy.

Photochemistry of a polyaromatic hydrocarbon, pyrene C(16)H(10), with water has been investigated at cryogenic temperatures. Photoprocessing of this species, performed at λ > 235 nm, in argon matrices, adsorbed onto amorphous water surfaces, and trapped in solid water, led to the formation of ketonic isomers, C(16)H(10)O, and possibly quinones. These species have been identified for the first time by infrared spectroscopy with the support of isotopic substitution experiments and DFT calculations. These oxidized pyrene-like species, of atmospherical and astrochemical interest, most likely arise from a tautomeric rearrangement of their analogous hydroxylated molecules, these latter being formed by reaction of water with pyrene cations.

Solid (PCy3)2Ni(CO2) (1) reacts with dioxygen to afford the peroxocarbonate complex (PCy3)2Ni(CO4) (3). The use of labelled 13CO2, C18O2, 18O2 coupled with a FTIR study of both the gas-phase in equilibrium with the solid and the solid resulting complex, allows to propose the reaction mechanism that implies CO2 deco-ordination, O2 co-ordination, and CO2 insertion into the OO bond of the newly formed, reactive (PCy3)2Ni(O2) complex 2. A normal mode analysis substantiates the band assignment and the proposed mechanism. Peroxocarbonate (3) exhibits a 13C resonance at 166.6 ppm and a single 31P signal at 43.1 ppm below 200 K. The reactivity of the peroxo-group as one-oxygen transfer agent prevents the spectroscopic characterisation of 3 in solution at room temperature. An out-of-sphere phosphine can be easily oxidised. A gaseous olefin, mono-ene (ethylene) or diene (allene), added to 3, makes easier the deco-ordination process of one phosphine ligand, that is then oxidised to phosphine oxide. The gaseous olefin itself is not oxidised until free phosphine is present in the medium. In solution, styrene is oxidised through a two- or one-oxygen transfer pathway, according to the reaction condition.

Even though carbon dioxide is used in a great number of industrial applications, it remains a molecule with low reactivity. Its activation by coordination to metal complexes is used in both stoichiometric and catalytic reactions. This article describes the different kinds of metal carbon dioxide complexes observed in the gas phase, in cryogenic solids, and in solution, and briefly presents the reactivity of coordinated CO2 toward electrophiles and nucleophiles. Keywords: carbon dioxide; transition-metal complexes; activation of small molecules; coordination chemistry; organometallic chemistry

Infrared and Raman spectroscopies have been used to monitor the [2+2] photodimerisation reactions of α-trans-cinnamic acid and of a number of its derivatives. The principal changes observed in the spectra upon dimerisation are decay of a band around 1637 cm−1, which is assigned to ν(CC) of the ethene bond of the monomer, and growth of bands just above 3000 cm−1, which result from ν(CH) of saturated carbon atoms of the dimer. The use of microscope attachments has allowed us to follow the reactions of single crystals and we conclude that the reactions are topotactic in nature. We have carried out preliminary kinetic experiments in which spectra of one single crystal are recorded after sequential periods of photolysis. We find that the rates of dimerisation of differently substituted cinnamic acids are similar, with physical effects, such as the thickness of an individual crystal, outweighing any observed electronic effects (inductive or mesomeric).

IR and electronic spectroscopic evidence, including a study of 13 CO enrichment pathways, is presented to show that photolysis of tris (dimethylpyrazolyl)-boratorhodium dicarbonyl and bis(dimethylpyrazolyl)-boratorhodium dicarbonyl complexes in argon and methane matrices at ca. 12 K produces CO loss and ligand dechelation products. No evidence was found for photochemical CH bond activation in methane matrices but the facile tridentate α bidentate interconversion for tris(dimethylpyrazolyl)-boratorhodium dicarbonyl provides support for the proposal that such a process could be the key step in the highly efficient CH photoactivation reaction of this complex at 298 K. In dinitrogen matrices the new CO loss products reacted to form monodinitrogen complexes. In an attempt to investigate whether additional thermal energy is needed for the photoactivation processes, the tris(dimethylpyrazolyl)-boratorhodium dicarbonyl and bis(dimethylpyrazolyl)-boratorhodium dicarbonyl complexes were photolysed in perfluorokerosine and Nujol mulls which could be warmed from ca. 12 K to 298 K. No CH bond photoactivation was observed in Nujol mulls at 12 K or 77 K but at 298 K photoactivation of Nujol by tris(dimethylpyrazolyl)-boratorhodium dicarbonyl was observed, i.e. there appears to be a thermal contribution to the photochemical CH activation process for this particular complex. The results of the low temperature studies are related to photochemical reactions observed in solutions at 298 K.

The reaction mechanism of the Cu atom with OCS and CO2 has been studied by means of density functional method (B3LYP). The overall energetics has been refined at the CCSD(T) level. In the case of the Cu + OCS reaction, the CS insertion route is found much more favorable than the CO insertion one. This later reaction is direct and involves an activation energy of 83.3 kcal/mol and is endothermic by 50.0 kcal/mol at the CCSD(T) level. The insertion into the CS bond proceeds through the eta1s and eta2cs coordination species as intermediates and is found exothermic by about 20 kcal/mol. The highest transition structure along this route is only 11.5 kcal/mol higher in energy than the reactant's ground states. In the case of the Cu + CO2 reaction, the insertion route into the CO bond is also found direct but with a lower endothermicity (30.6 kcal/mol) and smaller activation energy (61.1 kcal/mol) than that into the CO bond of OCS. In all cases, the insertion mechanism proceeds simultaneously with electron transfer from the Cu atom to OCS (or CO2) molecule.

Publisher Summary Transition-metal peroxocarbonates of formula L n M(CO 4 )X m are known since a long time. If the peroxocarbonate complexes are prepared by reaction of dioxygen complexes of transition metals L n M(O 2 )X m (M =Pd, Pt, Rh, Ir) with carbon dioxide, [1a] in principle, two ways are possible, that imply the formal insertion of carbon dioxide into the O-O or M-O bond.