P. Roubin

FT-IR study of UV-induced isomerization of intramolecularly hydrogen-bonded carbonyl compounds isolated in xenon matrices

Strongly intramolecularly hydrogen-bonded dicarbonyl compounds (malonaldehyde and acetylacetone) isolated in xenon matrices have been studied by high resolution FT-IR spectroscopy. Broad-band UV irradiations have been performed to promote their (π, π*) excited states: experiment analysis leads to the conclusion of a decay mechanism involving the non-hydrogen-bonded conformers formation.

Volumetric and infrared co-measurements of CH4 and CO isotherms on microporous ice

Abstract CH 4 and CO adsorption isotherms on microporous ice surfaces have been performed simultaneously by pressure measurements and infrared spectrometry in the 50–80 K temperature range in order to compare the two techniques: volumetric isotherms allow evaluation of the surface area of the ice and the net heat of adsorption by the BET model, while infrared spectrometry gives complementary information on the nature of the chemical or physical bonding interaction. Our results show that the two methods are well correlated and that their comparison provides valuable information on the different adsorption sites.

Spectroscopic and volumetric characterization of a non-microporous amorphous ice

Abstract The aim of this Letter is to re-investigate the characterization of ice porosity. N 2 , CH 4 and Ar adsorption on amorphous ice has been compared to that on crystalline ice at low temperatures, using adsorption isotherm volumetry and infrared spectroscopy simultaneously. Here we show that amorphous ice can present a large specific surface area and nevertheless be non-microporous; this provides new ways for the understanding of interstellar reactivity.

Adsorption of small molecules on amorphous ice: volumetric and FT-IR isotherm co-measurements Part I. Different probe molecules

Ar, Kr, CH 4 , N 2 , CO and CF 4 adsorption on amorphous ice has been studied by using a combination of volumetric isotherm measurements and FT-IR spectroscopy. We have been able to characterize the modifications in the three vibrational surface modes of ice throughout the adlayer formation and to observe different types of wetting. For these gases, we have measured the net heat of adsorption within a range of 1-2.6 kJ mol -1 , showing the weakness of surface interaction and conversely, we have measured the shift of the dangling OH bond within a range of 7-60 cm -1 , showing its high sensitivity to the adsorption process. We have succeeded in correlating both techniques by measuring the evolution of vibrational surface modes as a function of the relative pressure, from submonolayer to bulk.

Adsorption of small molecules on amorphous ice: volumetric and FT-IR isotherm co-measurements: Part II. The case of CO

Abstract The aim of this paper is to deepen the investigation of vibrational properties of CO adsorbed on amorphous ice, by using the combination of volumetric and spectroscopic isothermal measurements as presented in part I. CO is found to be one of the more interactive molecules that can be physisorbed on ice, as proved by the sharp increase in the adsorbed amount at the monolayer formation. The analysis of the evolution of the infrared signals as a function of equilibrium pressure, for both ice and CO, allows us to elucidate the origin of the peak at 2139 cm −1 as due to CO–CO interactions and to characterize the evolution of the three ice surface signals. Pre-adsorbing CF 4 or CH 4 enables site selectivity to be evidenced, CO being preferentially associated to dangling O–H bonds.

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

The 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 delocalization as well as the cis/trans rotamer properties are analyzed. 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 as 1.7kcal∙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.

UV and IR photoisomerizations of an intramolecularly H-bonded molecule: acetylacetone trapped in nitrogen matrix

Abstract UV- and IR-induced photoisomerizations of acetylacetone trapped in a nitrogen matrix at 10 K have been carried out using a tunable (UV and IR) laser, or a mercury lamp, and have been studied by UV and FTIR spectrometries. After deposition the main form of acetylacetone is the chelated form (enol). Upon UV irradiation the intramolecular H-bond is broken, leading to non-chelated forms. These forms have been then irradiated, by a resonant Π → Π ∗ UV irradiation, or by resonant ν OH IR irradiations. Interconversions between the non-chelated forms have been observed, providing a great amount of data for identifying acetylacetone isomers and thus for exploring intramolecular reactivity paths.

EXAFS studies of the trapping site structure for molecules isolated in cryogenic matrices

We present here results concerning the first attempt of determining the trapping site structure of molecules isolated in inert matrices at low temperature by the EXAFS (Extended X-ray Absorption Fine Structure) method. The experiments have been performed at the K edge of argon, silicon, sulfur, and chlorine for pure solid argon, and for SiH4, OCS, and HCl isolated in different cryogenic matrices. The EXAFS technique is sensitive to the local environment around the absorbing atom, and the spectral features induced by the matrix material (Ar, Xe, N2, and CH4) are clearly evidenced here. The data allow a characterization of the double substitutional site for OCS in argon and xenon, while no structure can be determined for the accommodation of SiH4 in argon. A discussion of the best choice for the guest/host system to obtain a good EXAFS signal is included.

IR photoinduced isomerization of CH2DCH2D isolated in a xenon matrix

CO2 laser and IR sources have been used to irradiate gauche CH2DCH2D isolated in a xenon matrix and the isomerization process monitored by FTIR spectroscopy. Conformational interconversion has been observed, showing the unselective coupling of vibrational modes with internal rotation. The existence of back-isomerization at 10 K provides evidence for a tunnelling exchange process.