Etude par spectroscopie infrarouge des complexes C2H2-HCl et C2H4-HCl

Intermolecular interactions paly a crucial role in numerous processes in the fields of physics and chemistry as well. The study of small systems bonded by such interactions (molecular complexes) can provide accurate informations in the processes resulting from the interaction (predissociation mecanism, structural modification in the components of the complex ... ). In order to perform experimental investigation of the molecular complexes, we have set an experimental setup which couples a pulsed slit jet molecular beam and a diode laser spectrometer. This setup allows us to bserve the high resolution infrared absorption spectra of complexes at low Temperature (around 10 Kelvin) with a high sensitivity due to the lock-in detection scheme we use. We have observed the high resoltution infrared spectra of the C2H2-HCl and C2H4-HCl complexes. The experimental results provided by the analysis of these intramolecular rovibrational spectra (vibrational shifts, rotational constants variations) gave rise to information on the interactions involved in the complexes (influence of the vibrational excitation on the interaction, binding energy estimation from a pseudo-diatomic model). In order to have a comprehensive view of the studied systems, we have completed our experimental results by quantum chemistry studies overcoming the usual approximations of ab initio calculations in view of obtaining the anharmonic intermolecular potential energy surface calculating by means of a two dimensional grid calculation. Thus we have been able to obtain information on the effects of anharmonicity, of complete basis set superposition error correction and of coupling between intra- and intermolecular motions.Les interactions intermoléculaires jouent un rôle important dans de nombreux processus tant dans le domaine de la physique que de la chimie. L'étude des systèmes de petite taille liés par ces interactions (complexes moléculaires) permet d'avoir des informations très précises à partir desquelles il est possible d'aller plus avant dans la compréhension des processus qui résultent des ces interactions (prédissociation, changement de structure des entités du complexe). Pour étudier expérimentalement les complexes moléculaires de petite taille, nous avons élaboré un dispositif combinant un jet moléculaire pulsé à fente et une diode laser accordable dans l'infrarouge. Ce dispositif nous a permis d'observer les spectres d'absorption infrarouges de complexes à basse température (10 K) et à haute résolution (5.10-3 cm-1) avec une grande sensibilité grâce au système de détection synchrone hétérodyne que nous utilisons. Nous avons observé les spectres du mode d'élongation H-Cl des complexes C2H2-HCl et C2H4-HCl. Les résultats expérimentaux que nous ont fournis l'analyse spectroscopique rovibrationnelle des ces vibrations intramoléculaires ont permis de remonter à des informations sur les interactions mises en jeu dans ces complexes (influence de l'excitation vibrationnelle sur l'interaction, estimation de l'énergie de liaison à partir d'un modèle pseudo-diatomique). Afin d'avoir une vision plus globale des systèmes étudiés, nous avons complétés notre étude expérimentale par des études de chimie quantique. Nous avons notament dépassé les approximations usuelles des calculs ab initio par un calcul du potentiel intermoléculaire sur une grille à deux dimensions. Nous avons ainsi pu obtenir des informations sur les effets d'anharmonicité, de la correction de l'erruer de superposition de base et de couplages entre les mouvements intramoléculaires et intermoléculaires

ETUDE PAR SPECTROSCOPIE INFRAROUGE DES COMPLEXES MOLECULAIRES C 2H 2-HC1 ET C 2H 4-HC1

ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Gas phase dynamics, conformational transitions and spectroscopy of charged saccharides: the oxocarbenium ion, protonated anhydrogalactose and protonated methyl galactopyranoside

International audienceProtonated intermediates are postulated to be involved in the rate determining step of many sugar reactions. This paper presents a study of protonated sugar species, isolated in the gas phase, using a combination of infrared multiple photon dissociation (IRMPD) spectroscopy, classical ab initio molecular dynamics (AIMD) and quantum mechanical vibrational self-consistent field (VSCF) calculations. It provides a likely identification of the reactive intermediate oxocarbenium ion structure in a D-galactosyl system as well as the saccharide pyrolysis product anhydrogalactose (that suggests oxocarbenium ion stabilization), along with the spectrum of the protonated parent species: methyl D-galactopyranoside-H +. Its vibrational fingerprint indicates intramolecular proton sharing. Classical AIMD simulations for galactosyl oxocarbenium ions, conducted in the temperature range ~300-350 K (using B3LYP potentials on-the-fly) reveal efficient transitions on the picosecond timescale. Multiple conformers are likely to exist under the experimental conditions and along with static VSCF calculations, they have facilitated the identification of the individual structural motifs of the galactosyl oxocarbenium ion and protonated anhydrogalactose ion conformers that contribute to the observed experimental spectra. These results demonstrate the power of experimental IRMPD spectroscopy combined with dynamics simulations and with computational spectroscopy at the anharmonic level to unravel conformer structures of protonated saccharides, and to provide information on their lifetimes

LASER DESORPTION SUPERSONIC JET SPECTROSCOPY OF HYDRATED TYROSINE

Author Institution: Chemical Resources Lab. Tokyo Tech. 4259 Nagatsuta-cho, Midoriku, Yokohama, 226-8503, Japan; ISMO. Universite Paris Sud XI Batiment 210, 91405, Orsay, Paris, FranceThe structure of tyrosine (tyr) consists of amino-acid chain and phenol, and it has roughly two possible binding sites for water, amino-acid site and phenolic OH site. Investigating how water molecule binds to tyr will give fundamental information for hydrations of peptide and protein. Resonance enhanced multi photon ionization (REMPI) spectrum of tyr-water 1:1 cluster has already been reported by de Vries and co-workers,, \textbf{115}, 6077 (2011).} however, no analysis on the hydrated structures has been reported. In the REMPI spectrum, two clusters of bands are observed; one appears at $\sim$$35600$ cm$^{-1}$ energy region which is the almost same with 0-0 transitions of tyr monomer,, (2013) DOI: 10.1039/c3cp43573c.} and another is observed at $\sim$$300$ cm$^{-1}$ lower than the former. Based on the electronic transition energy of phenylalanine and the hydrated clusters,, \textbf{8}, 4783 (2006).} the former is expected to be derived from a structure that water binds to amino acid site. On the other hand, it is plausibly predicted that the latter originates from a structure that water binds to phenolic OH group, because the electronic transition of mono hydrated phenol is $\sim$$300$ cm$^{-1}$ red-shifted from the monomer., \textbf{105}, 408 (1996).} We applied IR dip spectroscopy which can measure conformer selective IR spectra to the tyr-(H$_{2}$O)$_{1}$ clusters by using laser desorption supersonic jet technique to confirm the assignments. Especially in the phenolic OH bound isomer, it was found that the intra molecular hydrogen bond within amino-acid chain, which is far from the water molecule and cannot interact directly with each other, is strengthened by the hydration

Statistical versus non-statistical photo-fragmentation of protonated GWG tri-peptide induced by UV excitation

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HIGH-RESOLUTION UV SPECTRA OF BENZENE ISOTOPOMERS AND DIMERS IN HELIUM NANODROPLETS

Author Institution: Department of Chemistry, Princeton UniversityWe present high-resolution ultraviolet spectra of various benzene isotopomers and their dimers in helium nanodroplets in the region of the first Herzberg-Teller allowed vibronic transition $^{1}B_{2u} \leftarrow {^{1}}A_{1g} 6^{1}_{0}$ (a.k.a. the $A^{0}_{0}$ transition) at $\sim 260$ nm. Spectra were recorded in beam depletion and laser-induced fluorescence excitation. Unlike for many larger aromatic molecules, the monomer spectra consist of a single ``zero phonon'' line, blueshifted by about $30 cm ^{-1}$ from the gas phase value. The rotational moments of inertia of $C_{6}H_{6}$ are found to be at least 6 or 7 times larger than in the gas phase. The dimers present the same vibronic fine structure (though modestly compressed) as previously observed in the gas phase. The fluorescence lifetime and quantum yield of $(C_{6}H_{6})$ are found to be equal to those of $C_{6}H_{6}$, implying substantial inhibition of excimer formation in the dimer in helium nanodroplets

Comprehensive characterization of the photodissociation pathways of protonated tryptophan

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