The cyclic ground state structure of the HF trimer revealed by far infrared jet-cooled Fourier transform spectroscopy.

International audienceThe rovibrationally resolved Fourier transform (FT) far infrared (FIR) spectra of two intermolecular librations of (HF)3, namely the in-plane ν6 and out-of-plane ν4 bending fundamentals centered, respectively, at about 494 cm(-1) and 602 cm(-1), have been recorded for the first time under jet-cooled conditions using the supersonic jet of the Jet-AILES apparatus. The simultaneous rotational analysis of 245 infrared transitions belonging to both bands enabled us to determine the ground state (GS), ν6 and ν4 rotational and centrifugal distortion constants. These results provided definite experimental answers to the structure of such a weakly bound trimer: firstly the vibrationally averaged planarity of cyclic (HF)3, also supported by the very small value of the inertia defect obtained in the GS, secondly the slight weakening of the hydrogen bond in the intermolecular excited states evidenced from the center of mass separations of the HF constituents determined in the ground, ν6 = 1 and ν4 = 1 states of (HF)3 as well as the decrease of the fitted rotational constants upon excitation. Finally, lower bounds of about 2 ns on ν6 and ν4 state lifetimes could be derived from the deconvolution of experimental linewidths. Such long lifetimes highlight the interest in probing low frequency intermolecular motions of molecular complexes to get rid of constraints related to the vibrational dynamics of coupled anharmonic vibrations at higher energy, resulting in loss of rotational information

JET-COOLED SPECTROSCOPY ON THE AILES INFRARED BEAMLINE OF THE SYNCHROTRON RADIATION FACILITY SOLEIL

The Advanced Infrared Line Exploited for Spectroscopy (AILES) extracts the bright far infrared (FIR) synchrotron continuum of the third generation radiation facility SOLEIL. This beamline is equipped with a high resolution (10$^{-3}$ cm$^{-1}$) Bruker IFS125 Fourier transform spectrometer which can be operated in the FIR but also in the mid and near infrared by using its internal conventional sources. The jet-AILES consortium (IPR, PhLAM, MONARIS, SOLEIL) has implemented a supersonic-jet apparatus on the beamline to record absorption spectra at very low temperature (5-50 K) and in highly supersaturated gaseous conditions. Heatable slit-nozzles of various lengths and widths are used to set properly the stagnation conditions. A mechanical pumping (roots pumps) was preferred for its ability to evacuate important mass flow rates and therefore to boost the experimental sensitivity of the set-up, the counterpart being a non-negligible consumption of both carrier (argon, helium or nitrogen) and spectroscopic gases. Various molecular systems were investigated up to now using the Jet-AILES apparatus. The very low temperature achieved in the gas expansion was either used to simplify the rotation-vibration structure of monomers, such as SF_{6}footnote{High-resolution spectroscopy and analysis of the nub{2} + nub{3} combination band of SF_{6} in a supersonic jet expansion. V. Boudon, P. Asselin, P. Soulard, M. Goubet, T. R. Huet, R. Georges, O. Pirali, P. Roy, Mol. Phys. 111, 2154�2162 (2013)}, CF$_{4}$ or naphthalene footnote{The far infrared spectrum of naphthalene characterized by high resolution synchrotron FTIR spectroscopy and anharmonic DFT calculations. O. Pirali, M. Goubet, T.R. Huet, R. Georges, P. Soulard, P. Asselin, J. Courbe, P. Roy and M. Vervloet, Phys. Chem. Chem. Phys. 15, 10141-10150 (2013)}, or to stabilize the formation of weakly bonded molecular complexes such as the trimer of HFfootnote{The cyclic ground state structure of the HF trimer revealed by far-infrared jet-cooled Fourier transform spectroscopy. P. Asselin, P. Soulard, B. Madeb�ne, M. Goubet, T. R. Huet, R. Georges, O. Pirali and P. Roy, Phys. Chem. Chem. Phys. 16(10), 4797-806 (2014)} or the dimer of acetic acidfootnote{Standard free energy of the equilibrium between the trans-monomer and the cyclic-dimer of acetic acid in the gas phase from infrared spectroscopy. M. Goubet, P. Soulard, O. Pirali, P. Asselin, F. R�al, S. Gruet, T. R. Huet, P. Roy and R. Georges, Phys. Chem. Chem. Phys. DOI: 10.1039/c4cp05684a}. The nucleation of water vapor and the nuclear spin conversion of water were also investigated under free-jet conditions in the mid infrared

Electronic spectroscopy of transient species in plasma discharges

The work was focused on the spectroscopic study of carbon chain species as well as small boron clusters in the gas phase. The experimental apparatus was built before, however, in order to produce boron species. The source had to be modified as in this case a solid sample of precursor had to be used. Up to now the apparatus could be used to study the electronic transitions of variety of the species producing only by discharging a suitable gas precursor. The heating system built for this purpose worked flawlessly and the first gas-phase electronic spectrum of B3 could be obtained. This experiment showed that solid-state precursors can be also used for the production of the exotic species in the gas phase, which increases significantly the potential of the CRD experiment. The heated source can be promising for further studies of the bare carbon chains C6+, C7+ and C9+. Their spectra were already measured in the neon matrix and solid precursor (perchloronaphtalene) was used in this case. Since C10Cl8 worked very well in the matrix experiment, it seems reasonable to assume that in the CRD experiment this precursor could work as well. As mentioned before, cavity ringdown spectroscopy combined with a supersonic slit jet plasma have been applied to obtain gas-phase spectra of the variety of carbon chain species. They ranged from rather short ones like HC4H+, to relatively long such as HC10H+. Most of them are linear, however, spectra of the nonlinear carbon chains were also measured (C4H4+, C6H4+ and C8H4+). High sensitivity of the cavity ringdown method and low temperature attained in a jet, in many cases allowed to obtain and analyze rotationally resolved electronic spectra. One of the main results of the work was obtaining the electronic spectrum of the pentaacetylene cation. It turned out to be the largest polyacetylene cation measured in the gas phase so far. Astrophysical relevance of the HC10H+ was considered, however, the work showed that HC10H+ can not be a carrier of the DIBs. There was no match with the hitherto reported diffuse interstellar bands in the near infrared. Apart from carbon chain species, the spectrum of the cyclic B3 molecule was also measured. It was the first detection of this molecule in the gas phase. The analysis of the rotational structure of the spectrum allowed the geometry of the molecule to be inferred, which turned out to be the first structural information on cyclic boron trimer in the gas phase

Molecular aggregation of thiols and alcohols: study of non-covalent interactions by microwave spectroscopy

El estudio y comprensión de las interacciones no covalentes a nivel molecular es un campo que está en continuo desarrollo y cobra vital importancia para determinar el comportamiento estructural de muchas moléculas de interés químico, tecnológico o biológico. En esta tésis doctoral se han analizado las interacciones intermoleculares implicadas en la formación de agregados moleculares neutros, tanto dímeros como productos de microsolvatación, en fase gas. Los complejos intermoleculares se han generado mediante expansiones supersónicas pulsadas, caracterizándose posteriormente mediante espectroscopía de rotación. Este trabajo ha utilizado dos técnicas espectroscópicas, incluyendo un espectrómetro de microondas con transformada de Fourier (FTMW) de tipo Balle-Flygare en el rango de frecuencias 8-20 GHz, y un espectrómetro de transformada de Fourier de banda ancha con excitación multifrecuencia (CP-FTMW) cubriendo el rango espectral de 2-8 GHz. Los complejos intermoleculares estudiados han incluido moléculas con grupos alcohol y/o tiol, con objeto de analizar las diferencias entre las interacciones intermoleculares que implican átomos de oxígeno o azufre, en especial el enlace de hidrógeno. Se han estudiado moléculas incluyendo tanto sistemas cíclicos alifáticos (ciclohexanol, ciclohexanotiol) como aromáticos (furfuril alcohol, furfuril mercaptano, tienil alcohol, tienil mercaptano). Los enlaces de hidrógeno analizados han comprendido especialmente interacciones de tipo O-H···O, O-H···S y S-H···S. La formación de los complejos intermoleculares ha revelado en algunos de ellos una gran variedad conformacional, como la observación de seis isómeros del dímero de ciclohexanol. En el caso de los monohidratos se han observado en algunos casos desdoblamientos asociados a movimientos internos de gran amplitud, como la rotación de la molécula de agua en los monohidratos de ciclohexanol y tienil mercaptano. En los casos de moléculas quirales la dimerización ha permitido observar la estabilidad relativa de los diastereoisómeros homo o heteroquirales. El estudio experimental se ha completado con diferentes cálculos teóricos de orbitales moleculares, en especial teoría del funcional de la densidad, a fin de caracterizar las interacciones estructuralmente, energéticamente y mediante análisis topológico de la densidad electrónica. El conjunto de datos experimentales y teóricos permite aumentar la información existente sobre enlaces de hidrógeno con átomos de azufre, generalmente poco estudiados, y su comparación con los análogos oxigenados.The study and understanding of non-covalent interactions at molecular level is a field in continuous development and essential to determine the structural behavior of many molecules of chemical, technological or biological interest. In this PhD thesis, the intermolecular interactions involved in the formation of neutral molecular aggregates, both dimers and microsolvation products, have been analyzed in the gas phase. The intermolecular complexes were generated by pulsed supersonic expansions, and later characterized by rotational spectroscopy. This work has used two spectroscopic techniques, including a Balle-Flygare Fourier-Transform Microwave (FTMW) spectrometer in the 8-20 GHz frequency range, and a broadband Chirped-Pulse Fourier Transform Microwave (CP-FTMW) spectrometer covering the 2-8GHz spectral range. The intermolecular complexes studied have included molecules with alcohol and / or thiol groups, in order to analyze the differences between the intermolecular interactions involving oxygen or sulfur atoms, especially hydrogen bonds. Molecules that comprise both aliphatic (cyclohexanol) and aromatic (furfuryl alcohol, furfuryl mercaptan, thenyl alcohol, thenyl mercaptan) ring systems have been studied. The analyzed hydrogen bonds included especially O-H···O, O-H···S and S-H···S interactions. The formation of intermolecular complexes has revealed a great conformational diversity in some of them, such as the observation of six isomers of the cyclohexanol dimer. With regard to the monohydrates, tunnelling splittings associated with internal large amplitude motions have been observed in some cases, such as the rotation of the water molecule in the monohydrates of cyclohexanol, thenyl alcohol and thenyl mercaptan. In the case of chiral molecules, dimerization has made it possible to observe the relative stability of homo- or heterochiral diastereoisomers. The experimental study has been supported by different theoretical molecular orbital calculations, in particular Density Functional Theory (DFT) calculations, in order to characterize the interactions structurally, energetically and by a topological analysis of electron density. The set of experimental and theoretical data will advance the existing information on hydrogen bonds involving sulfur atoms, generally scarcely studied, and their comparison with the oxygenated analogues.Departamento de Química Física y Química InorgánicaDoctorado en Físic

Molecules in Superfluid Helium Nanodroplets

This open access book covers recent advances in experiments using the ultra-cold, very weakly perturbing superfluid environment provided by helium nanodroplets for high resolution spectroscopic, structural and dynamic studies of molecules and synthetic clusters. The recent infra-red, UV-Vis studies of radicals, molecules, clusters, ions and biomolecules, as well as laser dynamical and laser orientational studies, are reviewed. The Coulomb explosion studies of the uniquely quantum structures of small helium clusters, X-ray imaging of large droplets and electron diffraction of embedded molecules are also described. Particular emphasis is given to the synthesis and detection of new species by mass spectrometry and deposition electron microscopy

Vibrational dynamics in O–H···O connected aggregates: FTIR spectroscopy from the near to the far infrared

This work focuses on the vibrational dynamics of molecular clusters featuring intermolecular O–H···O hydrogen bonds. For this purpose the clusters are generated in supersonic jet expansions and probed via FTIR spectroscopy in different spectral regions. The dimers of simple diols are characterized in the O–H stretching region. The O–H stretching overtones of the dimers of methanol, ethanol, tert-butyl alcohol and hexafluoroisopropanol are observed in the near infrared and anharmonicity constants are deduced. The librational vibration of the O–H group can be found in the far infrared and is observed for clusters of methanol, tert-butyl alcohol, trifluoroethanol and methyl lactate and partly for their deuterated isotopologues. The cold absorption spectrum of formic acid dimer and its deuterated isotopologues is characterized in the region from 200 to 1900 cm-1. Fundamental transitions and some combination tones are assigned for the species