Nathalie Piétri

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.

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.

Photolysis of Astrophysically Relevant Acrylonitrile: A Matrix Experimental Study

This report documents the photochemical study of H2C ═ C(H)CN (acrylonitrile) trapped in low-temperature argon matrices and irradiated with a microwave-discharge hydrogen-flow lamp (λ > 120 nm). We succeeded in identifying H2C ═ C(H)NC (isoacrylonitrile) as a photoproduct. HC3N (cyanoacetylene), C2H2:HCN (acetylene:hydrogen cyanide), and C2H2:HNC (acetylene:hydrogen isocyanide) complexes, which are molecules detected in molecular clouds or in the Titan atmosphere, were also identified. No imine product was observed, but other compounds coming from the HC3N photolysis have been found. Fourier transform infrared measurements and (2)H substitution experiments coupled with density functional theory calculations (B3LYP/6-31G**) were performed to confirm the spectral assignments of the photochemical products and intermediate species.

Electronic absorption and phosphorescence of cyanodiacetylene.

Electronic absorption and emission spectra have been investigated for cyanodiacetylene, HC(5)N, an astrophysically relevant molecule. The analysis of gas-phase absorption was assisted with the parallel rare gas matrix isolation experiments and with density functional theory (DFT) predictions concerning the excited electronic states. Mid-UV systems B (1)Delta<--X (1)Sigma(+) (origin at 282.5 nm) and A (1)Sigma(-)<--X (1)Sigma(+) (306.8 nm) were observed. Vibronic assignments have been facilitated by the discovery of the visible phosphorescence a (3)Sigma(+)<--X (1)Sigma(+) in solid Ar, Kr, and Xe. Phosphorescence excitation spectra, as well as UV absorption measurements in rare gas matrices, revealed the enhancement of A<--X transitions. The vibronic structure of dispersed phosphorescence spectra supplied new data concerning the ground state bending fundamentals of matrix-isolated HC(5)N. The experimental singlet-triplet splitting, 2.92 eV in Ar, closely matches the value of 3.0 eV predicted by DFT.

Synthesis and spectroscopy of cyanotriacetylene (HC7N) in solid argon.

UV laser irradiations of cryogenic solid argon matrices doped with a mixture of acetylene and cyanodiacetylene (HC5N) resulted in the formation of a longer carbon-nitrogen chain, cyanotriacetylene (HC7N). The identification of this species was accomplished based on IR vibrational spectroscopy (including the study of isotopically labeled compounds), on electronic luminescence spectroscopy, and on theoretical predictions. Additionally, IR absorption bands recognized as due to HC7N were detected in photolysed Ar matrices doped with a cyanoacetylene/diacetylene mixture; this assignment was confirmed with the mass spectrometry of gases released upon the warm-up of the sample.

Low-temperature phosphorescence of dicyanoacetylene in rare gas solids

A strong visible a3Σu+ – X1Σg+ luminescence was observed upon UV excitation of cryogenic rare gas (argon, krypton, and xenon) matrices doped with dicyanoacetylene (NC4N). Spectra and lifetimes of this phosphorescence have been measured. A detailed analysis of resolved vibronic bands is presented.

Experimental and Theoretical Investigation of HC5N Adsorption on Amorphous Ice Surface : Simulation of the Interstellar Chemistry

HC 5N adsorbed on amorphous water ice at 10 K presents an interaction with the ice surface and induces the restructuring of the ice amorphous bulk. Warming up the sample induces the HC 5N desorption from the H 2O ice film, between 120 and 160 K, and the associated desorption energy is 90 kJ/mol. This value is in good agreement with that calculated E d (80 kJ/mol) and gives evidence that the amorphous ice surface is essentially dynamic. From theoretical calculations, it is shown that the HC 5N moiety presents a curvature and is no more linear and stabilized by two strong N...H bonds (2.09 and 2.29 A) and one H...O bond (1.84 A).

Water/cyanobutadiyne complexes: an infrared matrix isolation and theoretical study

The structures and energies of the 1:1 HC5N:H2O complexes in solid argon matrices have been investigated using FTIR spectroscopy and ab initio calculations, at the B3LYP/6-31G** and MP2/6-31G** levels of theory. Two types of 1:1 complexes are observed. The first one corresponds to the NH structure characterized by a hydrogen bond between H2O and the nitrogen of HC5N. The second corresponds to the OH form that involves a van der Waals interaction between the hydrogen of HC5N and the oxygen of water. HC5N can thus act either as an electrophile or as a nucleophile in complexes with water.

UV-laser photoisomerization of fumaryl chloride: The first identification of maleoyl chloride: matrix isolation infrared and ab initio studies

Fourier transform infrared spectra of fumaryl chloride 1 isolated in an argon matrix at 10 K have been analyzed. The comparison between the ab initio HF/6-31G calculated infrared spectra with the experimental ones reveals the existence of three planar conformers, the cis cis 1a, the cis trans 1b and the trans trans 1c. Laser UV irradiation of 1 at lambda = 340 nm yields maleoyl chloride 2 by a carbon carbon double bond photoisomerization process. The first identification of this compound was performed by comparison of the experimental infrared spectra with the calculated ones at the MP2/6-1G** level. AM1 semiempirical and ab initio calculations were used to calculate the structure and the relative stability of the three non planar maleoyl chloride conformers.