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Short time scale dynamics in prototypes for hydrogen-bonded systems: highly resolved spectroscopy on (HF)2 and isotopomers

We use hydrogen fluoride dimer (HF)2 as prototype system to better understand and describe dynamics in hydrogen bonded systems. The particular simplicity of this dimer allows for high-resolution rotational-vibrational spectroscopy and also for the construction of an accurate fully 6-dimensional electronic potential energy hypersurface (PES).

The predissociation lifetime (tPD) of the dimer is highly vibrational-mode specific. In the fundamental stretching mode region of (HF)2 lifetimes have been measured from sub-Doppler resolution molecular beam spectroscopy: vibrational excitation of the hydrogen bonded HF stretching (nb) fundamental gives rise to a predissociation lifetime more than 20 times shorter than that of the “free” non hydrogen-bonded HF stretching (nf) fundamental. This mode selectivity in the vibrational predissociation is preserved in the N=2 polyad which corresponds to excitation of two quanta of HF stretching between 7500 and 8000 cm-1, i.e. more than seven times the dissociation threshold for breaking the hydrogen bond. The three different excitations lead to different dissociation dynamics: experimental measurements in our laboratory using FTIR spectroscopy have shown a non-RRKM behavior of the dissociation. More recently, predissociation lifetimes have also been measured using a high resolved cavity ring down experiment combined with a supersonic expansion: the excitation of the hydrogen bonded stretching mode gives rise to a predissociation lifetime by far shorter than the excitation of the free stretching mode, while the excitation of each of the two exhibits an intermediate behavior.

We have overhauled the complete experimental setup based on a supersonic expansion coupled with cw-Cavity Ring-Down spectroscopy (jet-CRDS setup). The optimization allows us to get a molecular beam colder than the previous measurements in the group. We also measured predissociation lifetimes of the isotopomers HF·DF and DF·HF.

Besides the experimental measurements, we have also performed classical molecular dynamics calculations of (HF)2 dimer on the 6-dimensional SO-3 and SQSBDE potential energy hypersurfaces (PES) to estimate predissociation lifetime tPD for various initial excitations. This is the first time that tPD is predicted for excitations involving two or three quanta of HF stretching vibrations. Our calculations can reproduce the mode selectivity in tPD observed experimentally. Excitations involving the hydrogen bonded stretching mode give rise to shorter tPD than those involving the free HF stretchnig mode. We also investigate the influence of the intermolecular stretching and bending modes on tPD. Moreover, our computed power spectra compare well with experimental infrared absorption spectra, both in band position and intensity ratio. Besides the results of the HF dimer, this study provides a benchmark system for classical calculations.

 

Investigation of the spectrum of methane in the overtone region

We have performed a temperature-dependent investigation of the rovibrational spectrum of the two main isotopomers of methane (12CH4 and 13CH4) within the region of the icosad. Our jet - CRDS setup allows us to cool supersonic jet expansions down to rotational temperature of 4 K and to perform semi-quantitative measurements.

The comparison of spectra at different rotational temperatures between 4 K and 300 K makes it possible to propose an unambiguous assignment for the Q and the R branches of the n2 + 2n3 combination band. We resolve previously observed discrepancies of assignments for 12CH4 and are able to present a definitive assignment for lines involving angular momentum quantum numbers up to J = 4.

 

Nuclear spin symmetry conservation or relaxation in small molecules

Molecules with several identical nuclei exhibiting a nonzero nuclear spin exist in several forms called nuclear spin isomers; for instance methane (CH4), ammonia (NH3) or water (H2O) are possible candidates to explore nuclear spin symmetry conservation and conversion. If the nuclear spin symmetry is conserved during a cooling process of the sample, the nuclear spin isomers keep there populations set before the cooling process and a superposition of several distribution is observed. On the other hand, in the case of nuclear spin symmetry relaxation, the nuclear spin states are allowed to change and a global thermal equilibrium among all the state is observed. We first used methane to investigate nuclear spin symmetry conservation in supersonic jet expansions with our jet - CRDS setup. The analysis of relative intensities in spectra taken at rotational and effective translational temperatures between 50 K and less than 10K indicate conservation of nuclear spin symmetry upon supersonic jet expansion, in agreement with previous results using other techniques and with theory of supersonic jet expansions.

Water is another interesting candidate: its nuclear spin symmetry relaxation would be interesting for interstellar measurements and has been observed in rare gas matrices. We have used the same approach as for methane to investigate the nuclear spin symmetry of water seeded in supersonic expansions of Ar or O2 in the 2n3 region at T<30 K. For the lowest concentrations of water that we have used, we have observed nuclear spin symmetry conservation, in agreement with our previous results with methane. The unexpected nuclear spin symmetry relaxation observed at “high” concentrations is explained in terms of water cluster formation. Similar results are observed when water is seeded in molecular oxygen instead of argon, revealing no significant effect of the magnetic dipole moment of the collision partner.

 

 

Intramolecular vibrational energy redistribution

In our group, we use two experimental approaches to investigate Intramolecular Vibrational energy Redistribution (IVR): the time resolved fs pump-probe approach and the high resolution spectroscopic approach without time resolution.

Deuterated methyliodide CHD2I is an appropriate candidate for the investigation of IVR after excitation in the region of the CH-stretching overtone to the low frequency CI modes. fs-pump-probe experiments in our group have shown that CHD2I has different IVR times from fs to ps, revealing different intramolecular coupling mechanisms. On the other hand, our spectroscopic work at modest resolution has already highlighted the strong Fermi-resonance coupling between the CH-stretching and bending modes in CHD2I, demonstrating very fast redistribution times on the order of 100 fs. The present work will focus on the rovibrational analysis of the very high resolution spectra of fundamentals recorded with our Fourier transform infrared spectrometer: a systematic investigation should make it possible to establish weaker couplings between the modes and to understand the slower redistribution times in the fs-pump-probe experiment. Our investigation of the region of the n1 CH-stretching mode as well as the region around 1000 cm-1 has revealed weaker couplings which may be related to the slow redistribution processes observed in the fs-pump-probe experiments.

 

Hydrogen/proton transfer along a hydrogen-bonded wire

Proton and hydrogen transfer are among the fundamental and best studied reactions in biology, especially proton transport through membrane ion channels (“proton wires”). 7-hydroxyquinoline is an aromatic molecule which combines an O-H group as H donor with a quinolinic N acceptor site and is employed as a framework for a hydrogen-bonded wire, the two hydrogen bonding sites being spaced far enough apart to stretch the wire.

The 7-hydroxyquinoline(NH3)3 cluster has been used as a model to study excited state H-atom transfer (ESHAT) along a hydrogen bonded NH3 • • • NH3 • • • NH3 wire. Excitation of the supersonically cooled cluster to its vibrationless S1 state produces no reaction, whereas excitation of ammonia-wire vibrations induces the yellow fluorescence of the excited 7-ketoquinoline(NH3)3 cluster, revealing the excited state enol-keto tautomerization. With the use of ab initio calculations, this reaction is assumed to occur via successive H transfers along the ammonia wire in a Grotthus like process, the proton and electron movement along the wire being closely coupled. The rate-controlling S1 state barriers arise from crossing of a pp* and a Rydberg-type ps* electronic excited state and mode selectivity has been observed in the excited state: the intermolecular modes accelerate the ESHAT, whereas the intramolecular modes are less efficient. Current studies focus on the effect of the solvent and of the solvation.

 

Physisorption on amorphous ice surfaces

N2, CO, Ar, Kr, CH4 et CF4 physisorption on amorphous ice surfaces in the range 40-100 K is studied by coupling adsorption isotherm manometry and FT-IR spectroscopy. Plotting "infrared isotherms" has evidenced the modifications in three infrared signals related to the three kinds of surface molecules.

Ice samples obtained present large specific surface areas (larger than 100 m2g-1), but has been shown to be non-microporous.

Comparative studies of adsorption have indicated that interactions between gas and ice are weak and a hydrogen bond has been evidenced for N2 and CO. The adsorbed amount at monolayer completion is correlated to the quadrupole moment of the adsorbate. Periodic calculations based on Hartree-Fock method have put in evidence a vibrational Stark effect which explains the shift of the free OH stretching band.

In the case of CO, extensive experimental study and calculations based on density functional theory has permitted us to complete the assignment of the infrared signals of CO adsorbed on amorphous ice and to model three adsorption sites ; it has also evidenced the existence of stabilizing L-type interactions in the adsorbed layer.

We have also probed the N2 and CF4 adsorption on crystalline ice using X ray absorption spectroscopy. The preliminary results are complementary with our FT-IR measurements.

 

Reaction/isomerization induced by selective UV and IR irradiations

Acetylacetone and malonaldehyde in rare gas matrices

We have used malonaldehyde and acetylacetone as prototypes for systems with intramolecular hydrogen bond. The chelated form of the acetylacetone is stable in a cryogenic matrix. UV
irradiations in the cryogenic matrices
made it possible to measure the UV absorption spectra of its high-energy non-chelated isomers. These have then been identified by coupling selective
UV- and IR-induced isomerizations
, infrared spectroscopy, and harmonic vibrational frequency calculations as well as calculated UV transition energies and dipole oscillator strengths.

We also compared the structure and reactivity of the eight enolic isomers of acetylacetone and malonaldehyde using both theoretical and experimental data. The hydrogen bond strength of the chelated forms is estimated by the energy difference between chelated and non-chelated forms, and its enhancement due to methyl-induced electron release is computed to 1.7 kcal mol-1. IR irradiations interconvert the non-chelated isomers for both molecules; the eciency and rate of the interconversion depend on the gas of the cryogenic matrix, as well as the presence of oxygen in the matrix, revealing a different reactivity than in the gas phase.

Nitromethane in rare gas matrices and in condensed phase

We have studied the UV photoinduced reactivity of nitromethane in order to characterize by FTIR spectroscopy the aci and aci ion forms. The dissociation strongly depends on its environment: For the molecule in a densed phase (pure liquid or solid), we observed the formation of these two forms. Conversely, when the nitromethane is isolated in cryogenic matrix, its photolysis leads to the formation of methylnitrite.