Les pel·lícules del mes de febrer

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Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices

Structure and reactivity of the eight enolic forms (one chelated and seven non-chelated) of malonaldehyde and acetylacetone are compared through theoretical and experimental data. Ground-state geometries, energies, and vibrational frequencies are calculated with the B3LYP/6–311++G(2d,2p) model chemistry. The electronic delocalisation as well as the cis/trans rotamer properties are analysed. The hydrogen bond strength of the chelated forms can be estimated by the energy difference between chelated and non-chelated forms, and its enhancement due to methyl-induced electron release is estimated at 1.7 kcal·mol⁻¹. UV- and IR-induced reactivity of molecules isolated in nitrogen matrices is studied by means of FT–IR spectrometry. Interconversion between rotamers is the main process observed for both molecules, only some among the seven non-chelated forms being created

Infrared Photoisomerization of 1-Propanol CD3 and OD Trapped in Four Cryogenic Matrices: Ne, N-2, Ar, and Xe

International audienceThe conformational equilibria and isomerization processes of 1-propanol -OD and -CD3 have been studied by vibrational spectroscopy at low temperatures in four cryogenic matrices to investigate the effect of deuteration on their photochemistry. These isotopic species were selectively irradiated in the nu(OH) and nu(OD) domains, resulting in the identification of several conformers that are able to interconvert upon selective IR irradiation. The experimental results were compared with theoretical geometries obtained at the B3LYP/6-311+G(d) level of theory. Alkyl chain isomerization can be induced in rare gas and nitrogen cryogenic matrices by suitable selective irradiation. Selective excitation of the OH and OD stretches of two Gauche isomers transfers the alkyl chain from the gauche to the trans form. The competition between intramolecular vibrational energy relaxation and the matrix-dopant interaction determines the torsional subspace dynamics of the vibrationally excited propanol molecules

IRFEL Selective Irradiation of Amorphous Solid Water: from Dangling to Bulk Modes

Amorphous solid water (ASW) is one of the most widely studied solid phase systems. A better understanding of the nature of inter- and intramolecular forces in ASW is, however, still required to correctly interpret the catalytic role of ASW in the formation and preservation of molecular species in environments such as the icy surfaces of Solar System objects, on interstellar icy dust grains and potentially even in the upper layers of the Earth's atmosphere. In this work, we have systematically exposed porous ASW (pASW) to mid-infrared radiation generated by a free-electron laser at the HFML-FELIX facility in the Netherlands to study the effect of vibrational energy injection into the surface and bulk modes of pASW. During multiple sequential irradiations on the same ice spot, we observed selective effects both at the surface and in the bulk of the ice. Although the density of states in pASW should allow for a fast vibrational relaxation through the H-bonded network, part of the injected energy is converted into structural ice changes as illustrated by the observation of spectral modifications when performing Fourier transform infrared spectroscopy in reflection-absorption mode. Future studies will include the quantification of such effects by systematically investigating ice thickness, ice morphology, and ice composition.Comment: Published in The Journal of Physical Chemistry

Infrared Resonant Vibrationally Induced Restructuring of Amorphous Solid Water

Amorphous solid water (ASW) is abundantly present in the interstellar medium, where it forms a mantle on interstellar dust particles and it is the precursor for cometary ices. In space, ASW acts as substrate for interstellar surface chemistry leading to complex molecules and it is postulated to play a critical role in proton-transfer reactions. Although ASW is widely studied and is generally well characterized by different techniques, energetically-induced structural changes, such as ion, electron and photon irradiation, in these materials are less well understood. Selective pumping of specific infrared (IR) vibrational modes can aid in understanding the role of vibrations in restructuring of hydrogen bonding networks. Here we present the first experimental results on hydrogen bonding changes in ASW induced by the intense, nearly monochromatic mid-IR free-electron laser (FEL) radiation of the FELIX-2 beamline at the FELIX Laboratory at the Radboud University in Nijmegen, the Netherlands. The changes are monitored by reflection-absorption infrared spectroscopy. Upon resonant irradiation, a modification in IR absorption band profile of ASW is observed in agreement with a growing crystalline-like contribution and a decreasing amorphous contribution. This phenomenon saturates within a few minutes of FEL irradiation, modifying upwards of 94~\% of the irradiated ice. The effect is further analysed in terms of hydrogen bonding donors and acceptors and the experiments are complimented with Molecular Dynamics simulations to constrain the effect at the molecular level.Comment: Accepted for publication in J. Phys. Chem.

Energy Transfer and Restructuring in Amorphous Solid Water upon Consecutive Irradiation

34 page, 9 figuresInternational audienceInterstellar and cometary ices play an important role in the formation of planetary systems around young stars. Their main constituent is amorphous solid water (ASW). Although ASW is widely studied, vibrational energy dissipation and structural changes due to vibrational excitation are less well understood. The hydrogen-bonding network is likely a crucial component in this. Here we present experimental results on hydrogen-bonding changes in ASW induced by the intense, nearly monochromatic mid-IR free-electron laser (FEL) radiation of the FELIX-2 beamline at the HFML-FELIX facility at the Radboud University in Nijmegen, the Netherlands. Structural changes in ASW are monitored by reflection-absorption infrared spectroscopy and depend on the irradiation history of the ice. The experiments show that FEL irradiation can induce changes in the local neighborhood of the excited molecules due to energy transfer. Molecular Dynamics simulations confirm this picture: vibrationally excited molecules can reorient for a more optimal tetrahedral surrounding without breaking existing hydrogen bonds. The vibrational energy can transfer through the hydrogen-bonding network to water molecules that have the same vibrational frequency. We hence expect a reduced energy dissipation in amorphous material with respect to crystalline material due to the inhomogeneity in vibrational frequencies as well as the presence of specific hydrogen-bonding defect sites which can also hamper the energy transfer