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Molecules and complexes with hydrogen bond : solvation and photoreactivity in cryogenic matrices

Abstract : The hydrogen bond interaction is an important stabilizing interaction present in many kinds of molecular systems, from small water clusters to the big DNA molecule. The study of the specific case of the intramolecular hydrogen bond (IHB) is of special interest because of the important role of this kind of interaction in internal hydrogen transfer processes, photodynamic behavior and structural conformation. The β-dicarbonyl family of molecules is a unique model system with relatively small amount of degrees of freedom and where all the processes just mentioned are clearly present. The main aim of this work is to study the link between the isotopic and electronic structure of β-dicarbonyl molecules (model IHB system) with the strength of intramolecular hydrogen bond, selectivity on the photoisomerization process and coupling of hydrogen transfer with other large amplitude motions. Experiments are supported with quantum chemical calculations. Four molecules from the β-diketone family (double deuterated acetylacetone, 3-chloroacetylacetone, hexafluoroacetylacetone and trifluoroacetylacetone) and one from the β-dialdehyde (2-chloromalonaldehyde) are studied in low temperature inert environments by means of electronic and vibrational spectroscopy (FT-IR and Raman). Neon and para-hydrogen were mainly used as host matrices allowing clear spectroscopic analysis. The β-dicarbonyl molecules can be present in two tautomeric forms: keto and enol, but the latter largely predominates in the gas phase, and as a consequence, in the deposited isolated samples. Eight different enol conformers can exist, but the one with intramolecular hydrogen bond (chelated enol) is the most stable. The enol conformers can be divided in four pairs in which each couple shares the same skeletal structure and differs only in the hydroxyl conformation.In the deposited sample of all the molecules under study, the closed enolic conformer was found as the most stable species. The influence of the environment and the hydrogen bond strength on spectroscopic variables like bandwidth, intensity and frequency position are discussed. Experimental evidence of Nuclear Spin Conversion process in para-hydrogen matrix of the closed enol form of double deuterated acetylacetone was also found. Different open enol conformers were produced in each system after UV laser excitation. The open enol conformers show different energy ordering for each halogenated analog because of the existence of specific non covalent interactions, as revealed by theoretical calculations. Nevertheless, in all cases, the produced conformers are the most stable conformers of their enolic pairs. This is explained by a tunneling driven process in the hydroxyl hydrogen, as observed experimentally in deuterated isotopologues. From the experimental results we proposed a general mechanism to explain the photoisomerization in these systems. On the other hand, helium droplets technique was also used to allow recording valuable spectroscopic information (ro-vibrational spectrum) about strong and weak complexes in inert media. The role of water as donor or acceptor proton species in a complex can easily change by an imbalance of the interaction forces at play. Preliminary results about the intermolecular hydrogen bonded system propyne-water in helium droplets are presented.
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Alejandro Gutiérrez Quintanilla. Molecules and complexes with hydrogen bond : solvation and photoreactivity in cryogenic matrices. Chemical Physics [physics.chem-ph]. Université Paris Saclay (COmUE), 2016. English. ⟨NNT : 2016SACLS561⟩. ⟨tel-01686957⟩

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