Jean-Pierre Aycard

Quantum study of the active sites of the γ-alumina surface: chemisorption and adsorption of water, hydrogen sulfide and carbon monoxide on aluminum and oxygen sites

Abstract In this work, we have performed quantum calculations, using cluster models, in order to study the reactivity of the aluminum and oxygen sites of the γ-alumina surface. Water, hydrogen sulfide and carbon monoxide molecules are employed as the test molecules for our cluster models. It is found that the tricoordinated aluminum sites of the surface are the dissociative sites for the water and the hydrogen sulfide molecules. The tetracoordinated aluminum sites are the non-dissociative adsorption sites for the water molecule, whereas they are the dissociative sites for the hydrogen sulfide molecule. The pentacoordinated aluminum sites are the non-dissociative sites for the water and the hydrogen sulfide molecules. On the aluminum sites, carbon monoxide is preferentially adsorbed via its carbon atom. In contrast to the aluminum sites, the oxygen sites of the alumina surface are not adsorption sites for water, hydrogen sulfide and carbon monoxide molecules.

Thermal reactivity of HNCO with water ice: an infrared and theoretical study

Abstract The structure and energy of the 1:1 complexes between isocyanic acid (HNCO) and H 2 O are investigated using FTIR matrix isolation spectroscopy and quantum calculations at the MP2/6-31G(d,p) level. Calculations yield two stable complexes. The first and most stable one (Δ E =23.3 kJ/mol) corresponds a form which involves a hydrogen bond between the acid hydrogen of HNCO and the oxygen of water. The second form involves a hydrogen bond between the terminal oxygen of HNCO and hydrogen of water. In an argon matrix at 10 K, only the first form is observed. Adsorption on amorphous ice water at 10 K shows the formation of only one adsorption site between HNCO and ice. It is comparable to the complex observed in matrix and involves an interaction with the dangling oxygen site of ice. Modeling using computer code indicates the formation of polymeric structure on ice surface. Warming of HNCO, adsorbed on H 2 O ice film or co-deposited with H 2 O samples above 110 K, induces the formation of isocyanate ion (OCN − ) characterized by its ν as NCO infrared absorption band near 2170 cm −1 . OCN − can be produced by purely solvation-induced HNCO dissociative ionization. The transition state of this process is calculated 42 kJ/mol above the initial state, using the ONIOM model in B3LYP/6-31g(d,p).

Reactivity of HNCO with NH3 at low temperature monitored by FTIR spectroscopy: formation of NH4+OCN−

Abstract The reactivity of isocyanic acid (HNCO) with solid ammonia (NH3) was first studied at 10 K, using FTIR spectroscopy. The ammonium isocyanate (NH4+OCN−) is formed from a reaction between HNCO and NH3. Vibrational band assignments for NH4+OCN− have been given. On the other hand, when HNCO is adsorbed on amorphous NH3 film, the reaction does not occur. Warming up of this sample at 90 K induces the NH4+OCN− formation. Quantum calculations showed that the solvation of NH3 directly bonded to HNCO by at least three NH3 molecules plays a major role in the NH4+OCN− formation process and confirmed the spontaneous character of this reaction.

UV photoisomerisation of cyano and dicyanoacetylene: the first identification of CCNCH and CCCNCN isomers – matrix isolation, infrared and ab initio study

Abstract Infrared spectra of UV photolysed cyanoacetylene and dicyanoacetylene trapped in argon matrices at 10 K have been performed. From cyanoacetylene photolysed at λ >120 nm, we obtained three different isomers. The formation of HCCNC, HNCCC and the exotic one CCNCH is observed. In the case of photodissociation of dicyanoacetylene at λ >235 nm, three others isomers were isolated: NCCCNC, CNCCNC and the higher energy one CCCNCN. The identification of isomers is based on B3LYP/6-31g** density functional study and deuterium experiment for cyanoacetylene.

Infrared matrix spectra of the N2O ···O2 complex in solid nitrogen. The N2O ··· O2+O thermal diffusion limited reaction

Abstract The vibrational spectrum of N 2 O diluted in a nitrogen matrix is compared to the spectrum of N 2 O produced after the photolysis of ozone diluted in solid nitrogen. The small shift observed between the two spectra is due to the formation of a 1−1 complex between nitrous oxide and molecular oxygen. Upon increasing the temperature, the complex formed after the photolysis of ozone in nitrogen reacts with an oxygen atom leading to O 3 and N 2 O. Analysis of the results brings new information on oxygen atom diffusion in cryogenic solids. As predicted by theoretical investigations, the long-range diffusion of atomic oxygen in a nearly perfect matrix is negligible between 20 and 30 K. However, a fast motion of O( 3 P) at short range, independent of the matrix morphology, is experimentally suggested between 15 and 20 K.

Spectroscopy of cyanodiacetylene in solid argon and the photochemical generation of isocyanodiacetylene

Following the measurements of UV and mid-IR spectra of cyanodiacetylene, H-(CC)2-CN, isolated in low temperature Ar matrices, the first photochemical study on this compound and on its 2H isotopomer was carried out with the laser light tuned to 267 nm and with far-UV discharge lamps. Evidence for the formation of isocyanodiacetylene, H-(CC)2-NC, was found in infrared absorption spectra interpreted with the aid of available theoretical predictions.

C5N− anion and new carbenic isomers of cyanodiacetylene: A matrix isolation IR study

Products of the vacuum-UV photolysis of cyanodiacetylene (HC(5)N) in solid argon -- the anion C(5)N(-), imine HNC(5), and the branched carbene C(4)(H)CN -- have been identified by IR absorption spectroscopy, in addition to the already discovered isonitrile HC(4)NC. Spectral assignments were assisted by deuterium substitution experiments, by BD(T) calculations, and by the results of a recent density functional theory study.

Quantum study of the active sites of the γ alumina surface (II): QM/MM (LSCF) approach to water, hydrogen disulfide and carbon monoxide adsorption

Abstract The mixed quantum mechanics/molecular mechanics (QM/MM) local self consistent field (LSCF) method is applied to study the adsorption of water, hydrogen sulfide, and carbon monoxide molecules on γ alumina surfaces. The effect of the long-range contributions included in the LSCF adsorption/dissociation energies are compared to cluster results. For the carbon monoxide, the long-range contributions do not change the adsorption energies in comparison with the cluster approach. In opposition, the long-range contributions lower the adsorption and dissociation energies of water and hydrogen disulfide. Cautions to be taken on the application of the LSCF method to γ alumina are also discussed.

FT-IR identification, characterization and ab initio vibrational analysis of phosgene, oxalyl chloride and 1,2-dichlorocyclobutene-3,4-dione trapped in argon cryogenic matrices

Abstract Optimized equilibrium geometries, vibrational frequencies and valence force field in the complete set of internal coordinates, including redundancies, are calculated ab initio , in the 4-31G∗ basis set, for phosgene, oxalyl chloride and 1,2-dichlorocyclobutene-3,4-dione. The reliability of the scaled valence force fields is checked out by the computation of isotopic frequency shift. The cis high-energy conformer of oxalyl chloride is clearly identified from its theoretically predicted spectrum.

Electrochemical behaviour and redox reactivity of some 4-R-1,2,4-triazolin-3,5-diones

Abstract Cyclic voltammetry with stationary and rotating disk electrodes results in two 4-R-1,2,4-triazolin-3,5-diones (RTAD) and the corresponding urazoles in dimethylsulphoxide and acetonitrile are reported and discussed. The results show a first reversible reduction step, corresponding to fairly stable anion-radicals, characterised by well resolved EPR spectra. From the electrochemical and spectral data, a reduction mechanism for 4-R-triazolindiones is proposed, leading finally to the corresponding urazoles. Anodic oxidation of 4-methyl urazole gives the corresponding dione, accounting for the reversibility of the reduction process and furnishing further experimental support for the proposed mechanism. Possible intermediates in these redox processes indicated by the electrochemical data are the anion-radical ( A − ), the dianion ( A −2 ), the urazolyl radical ( AH ) and the urazolide anion ( AH − ). MO calculations at the semiempirical AM1 level of the electronic structure of these species account reasonably for the differences in the redox reactivity between the compounds investigated.