Peer-reviewed Publications |
Gutiérrez-Quintanilla, A., Chevalier, M., Platakyte, R., Ceponkus, J., & Crépin, C. (2023). Revisiting Photoisomerization in Fluorinated Analogues of Acetylacetone Trapped in Cryogenic Matrices. EPJD, 77, 158.
Résumé: UV-induced processes are commonly studied in acetylacetone analogues. In this contribution,
we revisit the existing work on the photoisomerization process in some of the fluorinated analogues of
acetylacetone, i.e., trifluoroacetylacetone (F3-acac) and hexafluoroacetylacetone (F6-acac). We performed
selective UV laser excitation of these molecules trapped in soft cryogenic matrices, namely neon and parahydrogen,
and probed by vibrational spectroscopy. Clear spectroscopy of 3 isomers of F6-acac and 6 isomers
of F3-acac is obtained, including the first characterization of a second open enol isomer of hexafluoroacetylacetone.
In addition, we present the electronic absorption spectra of both molecules in cryogenic matrices
before and after specific UV irradiations, giving new data on the electronic transitions of photoproducts.
Vibrational and electronic experimental results are analyzed and discussed within comparisons with DFT
and TD-DFT calculations. Our findings contribute to a deeper understanding of the photoisomerization
process in these molecules after electronic excitation in gas and condensed phase.
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Imani, Z., Mundlapati, V. R., Brenner, V., Gloaguen, E., Le Barbu-Debus, K., Zehnacker, A., Robin, S., Aitken, D. J., & Mons, M. (2023). Non-covalent interactions reveal the protein chain δ conformation in a flexible single-residue model. ChemComm, 59, 1161–1164.
Résumé: The δ conformation is a local secondary structural feature in proteins that implicates a πamide N-H···N interaction between a backbone N atom and the NH of the following residue. Small molecule probes of this conformation have been limited so far to rigid proline-type models that may over-emphasize the significance of the interaction. We show here that, in thiacyclic amino acid derivatives with a sulphur atom in the γ-position, specific side-chain/backbone N-H···S interactions stabilize the δ conformation sufficiently to allow it to compete with classical C5 and C7 H-bonding conformers. With support from quantum chemistry, the δ-folded conformers have been characterized by IR spectroscopy in the gas phase. In solution, the IR absorption of the πamide N-H appears at 3450 cm-1, notably less red-shifted than in proline-type models, in a frequency range often considered as implicating a free NH motif and suggestive of very weak hydrogen bonding at best.
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Nejad, A., Mellor, A. P. F., Lange, M., Alata, I., Zehnacker, A., & Suhm, M. A. (2023). Subtle hydrogen bond preference and dual Franck-Condon activity – the interesting pairing of 2-naphthol with anisole. PCCP, 25(15), 10427–10439.
Résumé: The hydrogen-bonded complexes between 2-naphthol (or beta-naphthol) and anisole are explored by detecting their IR absorption in the OH stretching range as well as their UV absorption by means of laser-induced fluorescence and resonance-enhanced two-photon UV ionisation. For the more stable cis and the metastable trans conformations of the OH group in 2-naphthol, hydrogen bonding to the oxygen atom of anisole is consistently detected in different supersonic jet expansions. Alternative hydrogen bonding to the aromatic ring of anisole remains elusive, although the majority of state-of-the-art hybrid DFT functionals with London dispersion correction and – less surprisingly – MP2 wavefunction theory predict it to be slightly more stable at zero-point level, unless three-body dispersion correction is added to the B3LYP-D3(BJ) approach. This changes at the CCSD(T) level, which forecasts an energy advantage of 1-3 kJ mol(-1) for the classical hydrogen bond arrangement even after including (DFT) zero-point energy contributions. The UV and IR spectra of the cis complex exhibit clear evidence for intensity redistribution of the primary OH stretch oscillator to combination states with the same low-frequency intermolecular bending mode by Franck-Condon-type vertical excitation mechanisms. This rare case of dual (vibronic and vibrational) Franck-Condon activity of a low-frequency mode invites future studies of homologues where aromatic ring docking of the OH group may be further stabilised, e.g. through anisole ring methylation.
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Pollet, R., & Chin, W. (2023). In silico Investigation of the Thermochemistry and Photoactivity of Pyruvic Acid in an Aqueous Solution of NaCl. Chem. Eur. J., 2922(5555).
Résumé: Abstract The photochemistry of oxocarboxylic acids contributes significantly to the complex chemistry occurring in the atmosphere. In this regard, pyruvic acid undergoes photoreactions that lead to many diverse products. The presence of sodium cation near pyruvic acid in an aqueous solution, or its conjugate base in non-acidic conditions, influences the hydration equilibrium and the photosensitivity to UV-visible light of the oxocarboxylic acid. We performed an ab?initio metadynamics simulation which serves two purposes: first, it unveils the mechanisms of the reversible hydration reaction between the keto and the diol forms, with a free-energy difference of only 2?kJ/mol at 300?K, which shows the influence of sodium on the keto/diol ratio; second, it provides solvent-shared ion pairing (SSIP) and contact ion pairing (CIP) structures, including Na+ coordinated to carbonyl, for the calculations of the electronic transition energies to an antibonding π* orbital, which sheds light on the photoactivity of these two forms in the actinic region.
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