J. Pourcin

Induced conformational isomerization of 2-fluoro ethanol by selective infrared irradiation dm argon matrix

Abstract The light induced isomerization of the 2-fluoro ethanol G g conformer is studied using either a broad band IR source, or an HF laser to excite the O-H stretching vibration spectral range. The products obtained following irradiation can be assigned to other conformers, especially the t t one the increase of its concentration coincides with the decrease of that of the G g conformer The isomerization involves intramolecular energy transfer after a single photon absorption.

Site-selective infrared photoisomerizations of 2-fluoroethanol in low-temperature matrices

Abstract The Gg′ conformer of 2-fluoroethanol has been isolated in low-temperature noble gas matrices. In high-resolution infrared spectrometry, most vibrational modes exhibit fine structures which are accounted for in terms of multiple trapping sites. The relative intensities of these bands depend on the deposition temperature. Reversible spectral changes with the matrix temperature are observed. Gg′ ⇄ Tt photoisomerizations have been induced from an infrared broad band source irradiation with a rate which depends on the site. The assignments are supported by valence force field calculations on the assumption of a pseudosymmetry for the FCH 2 CH 2 O fragment. In xenon matrices, the Tt → Gg′ thermal isomerization is observed, for the two main sites, at different temperatures, 25 and 40 K. This result and the difference between the photoisomerization rates suggest that one of the sites is larger than the other. An expected first-order behavior of the photoisomerization process is observed except for a certain site, for which a frequency shift occurs as the photoisomerization proceeds. We propose that very different rate constants govern the molecules embedded in a specific site; reorientations of molecules in their cage may allow the less reactive ones to be photoisomerized, the overall process being limited only by the Tt → Gg′ reverse photoreaction.

Determination experimentale et theorique des vibrations fondamentales des halogeno-2 ethanols: Partie 2. Conformères trans du fluoro-2-ethanol

Abstract Nineteen fundamental vibrational frequencies of the trans conformer of 2-fluoroethanol were identified from the Raman spectra before and after crystallisation disappearance of the trans conformer and from the FT IR spectra of the molecules isolated in an argon matrix and photoisomerized by IR irradiation. The experimental and theoretical (GVFF calculations) results are shown to be consistent with a C 2h pseudosymmetry of the trans conformer.

Determination experimentale et theorique des vibrations fondamentales des halogeno-2 ethanols: Partie 1. Conformères gauches

Abstract From the Raman spectral study of XCH 2 CH 2 OH and XCH 2 CH 2 OD, XF, Cl (liquid, solution, crystal) and from their IR spectra (4000 to 50 cm −1 ) at different concentrations in CCl 4 , CS 2 and C 6 H 12 solutions, a coherent assignment of the fundamental vibrations was established for the gauche conformers of these molecules as auto-associated or monomeric species. The experimental and theoretical (GVFF calculations) results are shown to be consistent with a C 2 pseudosymmetry for the gauche conformer of 2-fluoroethanol.

Ultraviolet photoisomerizations and FT-IR investigations of matrix isolated oxalyl halide conformers

Abstract UV photoexcited trans -oxalyl chloride (COCl) 2 trapped in an argon matrix isomerizes to a new cis conformer not previously observed. Subsequent warm up experiments at 28 and 31.5 K lead mainly to cis ⇌ gauche isomerization. Further UV irradiation leads back to the cis conformer. Each stereoisomer is embedded in several sites, all of them detected by FT-IR spectroscopy; the annealing procedures have been kinetically investigated. The high energy conformers are identified from a valence force field set up in the preceding paper for the trans isomer; the transfer to the other stereoisomers only requires the CO/CO interaction force constant to be slightly reduced.

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.

Site selective thermal stereoisomerization of matrix isolated molecules: simulation by molecular mechanics

Abstract The MM@ molecular mechanics program is used to perform energy minimizations on molecules embedded in rare gas matrices, i.e. in trapping cages resulting from the removal of one or two rare gas atoms; the atoms of the first shell are allowed to move. These simulations have been checked on the CO molecule isolated in an argon matrix and the results compared with those previously obtained by Mirsky. Calculations have been performed to investigate the 2-fluoroethanol conformers isolated in a xenon matrix and in particular to make a structural identification of the two sites we have experimentally identified (J. Mol. Spectrosc., 109 (1985) 186). Embedded in a one atom substitutional site (site 1), the Gg′ conformer presents a potential energy surface with only one potential well corresponding to the CC bond oriented along the (1,1,1) direction of the lattice. Therefore, the second site must be a two atom substitutional site (site 2), the investigation of which leads to an optimal position of the Gg′ conformer with the CC bond along the axis of the two removed xenon atoms. To investigate the coupling interaction between the molecule and the matrix and its influence on internal rotation, we have calculated the energies of the five conformers and that of the eight transition states in each site; the potential wells of Gt, Gg and Tg conformers are not sufficiently deepened to expect their isolation at T ⩾ 10 K, but some barriers which allow the Tt → Gg′ thermal isomerization are increased in site 1 and decreased in site 2; the calculated activation energies are 4.3 and 1.8 kcal mol −1 , respectively. By comparison with the experimental values of 3.0 and 1.7 kcal mol −1 for the two observed sites, these sites are structurally identified.

Deuterated ammonia in krypton and nitrogen matrices: High-resolution spectroscopy of the ν2 mode

Abstract FTIR spectra of ND 3 in krypton and nitrogen matrices are recorded at low temperatures (7.5 K). The analysis of the ν 2 region shows that in krypton the rotation is slightly perturbed, with an inversion splitting of the ν 2 state equal to 2 cm −1 . In nitrogen, the inversion splitting has been searched for without success under a resolution of 0.06 cm −1 ; in this matrix, a low-resolution measurement yields a librational frequency of 133 cm −1 . The present experimental results are compared with previous ones on NH 3 and with a recent theoretical model.

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.

Vibrational analysis of 3-diazo-2-butanone. Infrared and raman spectra: calculated normal modes of CH3COCN2CH3 and CD3COCN2CD3

Abstract The infrared (gas, liquid, solution, Kr matrix) and Raman (liquid) spectra of s-E CH3COCN2CH3 and CD3COCN2CD3 are studied between 3200 and 100 cm−1. A vibrational assignment is given, which is consistent with both the IR rotation—vibration band shapes and Raman depolarization data as with matrix trapping site effects. The assignment is supported by a theoretical prediction of the fundamental frequencies calculated with a zero order valence force field transferred from s-Z diazo-2-propanone for which a preliminary GVFF normal modes calculation is performed from previously assigned fundamental frequencies. After refinement of the force constants, the new force field is used to predict the vibrational spectrum of a folded conformer of 3-diazo-2-butanone s-Z.