Peer-reviewed Publications |
Alata, I., Broquier, M., Dedonder, C., Jouvet, C., & Marceca, E. (2012). Electronic excited states of protonated aromatic molecules: Protonated Fluorene. CHEMICAL PHYSICS, 393(1).
Résumé: The photo-fragmentation spectrum of protonated fluorene has been recorded in the visible spectral region, evidencing an absorption that appears largely red shifted in comparison to that of the neutral molecule fluorene. The spectrum shows two different vibrational progressions, separated by 0.19 eV. As in the case of protonated linear polycyclic aromatic hydrocarbons (PAHs), comparison of the measured spectra with ab initio calculations allows to associate the observed absorption shift with the charge transfer character of the excited state. The spectra can be properly simulated by geometry optimization of the ground and excited states, followed by Franck Condon analysis. The two vibrational bands progressions observed are assigned, with relatively good confidence, to the existence of two different conformers. (C) 2011 Elsevier B. V. All rights reserved.
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Alata, I., Omidyan, R., Broquier, M., Dedonder, C., & Jouvet, C. (2012). Protonated salicylaldehyde: Electronic properties. CHEMICAL PHYSICS, 399, 224–231.
Résumé: The excitation spectrum of protonated salicylaldehyde has been recorded in the 20,800-22,400 cm(-1) region (480-450 nm). The first excited state of protonated salicylaldehyde is a pi pi* state, largely red shifted as compared to the pi pi* transition of its neutral analogue. Like protonated benzaldehyde and in contrast to some other protonated aromatic molecules such as benzene or tryptophan in which the excited state dynamics is so fast that no vibrational structure can be observed, the vibrational bands are well resolved and assigned. This molecule has many low energy isomers and the simulations of the electronic spectrum via ab initio excited state optimizations and Franck-Condon calculations are precise enough to assign the observed electronic spectrum to one of the isomers. (C) 2011 Elsevier B.V. All rights reserved.
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Arabei, S. M., Galaup, J. - P., McCaffrey, J. G., Shafizadeh, N., & Crepin, C. (2012). Electronic spectroscopy, stimulated emission, and persistent spectral hole burning of cryogenic nitrogen matrices doped with tetrabenzoporphin. LOW TEMPERATURE PHYSICS, 38(8), 727–731.
Résumé: This paper deals with our on-going work on the electronic spectroscopy of tetrapyrrole molecules embedded in cryogenic hosts. Under nanosecond laser excitation of free-base tetrabenzoporphin molecules in a nitrogen matrix at 8 K, increasing laser pulse energy results in essential enhancement of the intensity of one or several vibronic emission lines in the fine-structure fluorescence spectrum. Some characteristics of stimulated radiation are realized in the observed intense monochromatic: radiation as a result of a transition from the purely electronic S-1 level to a vibrational sublevel of the S-0 state, corresponding to the excitation of C-C stretching vibrations of tetrapyrrole methane bridges. We also report on persistent spectral hole-burning effects. Hole and antihole formation analysis gives some insight on the photochemical properties of tetrabenzoporphin in a nitrogen matrix. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4746794]
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Henchy, C., Murray, C., Crepin, C., & McCaffrey, J. G. (2012). A DFT study of reversed isotope shifts in H/D substitution of free-base porphyrin and related free-base tetrapyrroles. Can. J. Chem.-Rev. Can. Chim., 90(12), 1078–1091.
Résumé: DFT/B3LYP calculations are used to analyse the occurrence of reverse isotope shift ratios (ISR) in H/D substitution of the free-base tetrapyrroles, in situations where the frequency ratio v(H)/v(D) is less than 1. The reverse ISR effect is found to be most evident in the out-of-plane bending modes (b(2g) and b(3u), symmetry) involving some N-H motion for the four molecules studied, viz., porphine (H2P), tetraaza-porphine (H(2)TAP), tetrabenzo-porphine (H2TBP), and phthalocyanine (H2Pc). It was analysed by following the evolution of the normal mode frequencies with incremental variation of the H atom masses from 1 to 2 amu. This method allows direct, unambiguous mode correlations to be established between the light and the heavy isotopologues. When the NH(D) motion is predominant, the H to D frequency evolution decreases in a continuous manner for a particular normal mode. In the case of two modes of the same symmetry and whose frequencies are similar, their frequency evolutions could cross, depending on the extent of NH(D) motion involved in them. The evolution diagrams may show avoided crossings of various extents, which thereby reflects the degree of the NH(D) motion in the modes. The reverse ISR effect is directly correlated to these avoided crossings. Because the isotope shifts are quite small (<10 cm(-1)) and occur in the congested 1500-500 cm(-1) spectral region, high-resolution methods yielding narrow line transitions are required for experimental analysis. The matrix isolation technique is particularly well suited for this work and is proposed for use in a search for this effect.
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Lozada-Garcia, R. R., Ceponkus, J., Chevalier, M., Chin, W., Mestdagh, J. - M., & Crepin, C. (2012). Nuclear Spin Conversion to Probe the Methyl Rotation Effect on Hydrogen-Bond and Vibrational Dynamics. Angewandte Chemie-International Edition, 51(28), 6947–6950.
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Lozada-Garcia, R. R., Ceponkus, J., Chevalier, M., Chin, W., Mestdagh, J. - M., & Crepin, C. (2012). Photochemistry of acetylacetone isolated in parahydrogen matrices upon 266 nm irradiation. Physical Chemistry Chemical Physics, 14(10), 3450–3459.
Résumé: The photochemistry of the chelated enol form of acetylacetone (AcAc) was investigated by UV excitation of the S2 state at 266 nm in parahydrogen matrices, complemented by experiments in neon and normal hydrogen matrices. Infrared (IR) spectroscopy, combined with theoretical calculations, was used to identify the photoproducts. Isomerization towards various non-chelated forms (no intramolecular H-bond) of AcAc is the dominant channel whereas fragmentation is very minor. The isomerization kinetics is monitored by IR spectroscopy. Among the seven non-chelated conformers of AcAc, only three are formed in parahydrogen matrices, whereas four are observed in normal hydrogen matrices. This difference suggests that an active tunnelling process between conformers occurs in parahydrogen but is quenched in normal hydrogen where guest-host interactions are stronger. Fragmentation and isomerization of excited AcAc are discussed in the light of these new data. The role of the intermediate triplet state in the S-2 -> S-0 relaxation is confirmed, as the importance of phonons in the condensed phase.
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Malis, M., Loquais, Y., Gloaguen, E., Biswal, H. S., Piuzzi, F., Tardivel, B., Brenner, V., Broquier, M., Jouvet, C., Mons, M., Doslic, N., & Ljubic, I. (2012). Unraveling the mechanisms of nonradiative deactivation in model peptides following photoexcitation of a phenylalanine residue. J Am Chem Soc, 134(50), 20340–20351.
Résumé: The mechanisms of nonradiative deactivation of a phenylalanine residue after near-UV photoexcitation have been investigated in an isolated peptide chain model (N-acetylphenylalaninylamide, NAPA) both experimentally and theoretically. Lifetime measurements at the origin of the first pipi* state of jet-cooled NAPA molecules have shown that (i) among the three most stable conformers of the molecule, the folded conformer NAPA B is approximately 50-times shorter lived than the extended major conformer NAPA A and (ii) this lifetime is virtually insensitive to deuteration at the NH(2) and NH sites. Concurrent time-dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations in the full dimensionality, carried out for the NAPA B conformer, provided direct insights on novel classes of ultrafast deactivation mechanisms, proceeding through several conical intersections and leading in fine to the ground state. These mechanisms are found to be triggered either (i) by a stretch of the N(Phe)H bond, which leads to an H-transfer to the ring, or (ii) by specific backbone amide distortions. The potential energy surfaces of the NAPA conformers along these critical pathways have been characterized more accurately using the coupled cluster doubles (CC2) method and shown to exhibit barriers that can be overcome with moderate excess energies. These results analyzed in the light of the experimental findings enabled us to assign the short lifetime of NAPA B conformer to a number of easily accessible exit channels from the initial pipi* surface, most importantly the one involving a transfer of electronic excitation to an npi* surface, induced by distortions of the backbone peptide bond.
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Savee, J. D., Soorkia, S., Welz, O., Selby, T. M., Taatjes, C. A., & Osborn, D. L. (2012). Absolute photoionization cross-section of the propargyl radical. J. Chem. Phys., 136(13), 134307.
Résumé: Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C(3)H(3)) radical, sigma(propargyl) (ion)(E), relative to the known absolute cross-section of the methyl (CH(3)) radical. We generated a stoichiometric 1:1 ratio of C(3)H(3):CH(3) from 193 nm photolysis of two different C(4)H(6) isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of sigma(propargyl)(ion)(10.213 eV)=(26.1+/-4.2) Mb and sigma(propargyl)(ion)(10.413 eV)=(23.4+/-3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of sigma(propargyl)(ion)(10.213 eV)=(23.6+/-3.6) Mb and sigma(propargyl)(ion)(10.413 eV)=(25.1+/-3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.
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Sen, A., Bouchet, A., Lepere, V., Le Barbu-Debus, K., Scuderi, D., Piuzzi, F., & Zehnacker-Rentien, A. (2012). Conformational Analysis of Quinine and Its Pseudo Enantiomer Quinidine: A Combined Jet-Cooled Spectroscopy and Vibrational Circular Dichroism Study. JOURNAL OF PHYSICAL CHEMISTRY A, 116(32), 8334–8344.
Résumé: Laser-desorbed quinine and quinidine have been studied in the gas phase by combining supersonic expansion with laser spectroscopy, namely, laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and IR-UV double resonance experiments. Density functional theory (DFT) calculations have been done in conjunction with the experimental work. The first electronic transition of quinine and quinidine is of pi-pi* nature, and the studied molecules weakly fluoresce in the gas phase, in contrast to what was observed in solution (Qin, W. W.; et al. J. Phys. Chem. C 2009, 113, 11790). The two pseudo enantiomers quinine and quinidine show limited differences in the gas phase; their main conformation is of open type as it is in solution. However, vibrational circular dichroism (VCD) experiments in solution show that additional conformers exist in condensed phase for quinidine, which are not observed for quinine. This difference in behavior between the two pseudo enantiomers is discussed.
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Soorkia, S., Leone, S. R., & Wilson, K. R. (2012). Radical-neutral chemical reactions studied at low temperature with VUV synchrotron photoionization mass spectrometry. AIP Conference Proceedings, 1501, 1365–1372.
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Sunil Kumar, S., Lucas, B., Soorkia, S., Barat, M., & Fayeton, J. A. (2012). Calpha-Cbeta chromophore bond dissociation in protonated tyrosine-methionine, methionine-tyrosine, tryptophan-methionine, methionine-tryptophan and their sulfoxide analogs. Phys. Chem. Chem. Phys., 14(29), 10225–32.
Résumé: C(alpha)-C(beta) chromophore bond dissociation in some selected methionine-containing dipeptides induced by UV photons is investigated. In methionine containing dipeptides with tryptophan as the UV chromophore, the tryptophan side chain is ejected either as an ion or as a neutral fragment while in dipeptides with tyrosine, the tyrosine side chain is lost only as a neutral fragment. Mechanisms responsible for these fragmentations are proposed based on measured branching ratios and fragmentation times, and on the results of DFT/B3-LYP calculations. It appears that the C(alpha)-C(beta) bond cleavage is a non-statistical dissociation for the peptides containing tyrosine, and occurs after internal conversion for those with tryptophan. The proposed mechanisms are governed by the ionization potential of the aromatic side chain compared to that of the rest of the molecule, and by the proton affinity of the aromatic side chain compared to that of the methionine side chain. In tyrosine-containing peptides, the presence of oxygen on sulfur of methionine presumably reduces the ionization potential of the peptide backbone, facilitating the loss of the side chain as a neutral fragment. In tryptophan-containing peptides, the presence of oxygen on methionyl-sulfur expedites the transfer of the proton from the side chain to the sulfoxide, which facilitates the loss of the neutral side chain.
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Turowski, M., Crepin, C., Couturier-Tamburelli, I., Pietri, N., & Kolos, R. (2012). Low-temperature phosphorescence of dicyanoacetylene in rare gas solids. LOW TEMPERATURE PHYSICS, 38(8), 723–726.
Résumé: A strong visible a(similar to 3)Sigma(+)(u) – X-similar to 1 Sigma(+)(g) luminescence was observed upon UV excitation of cryogenic rare gas (argon, krypton, and xenon) matrices doped with dicyanoacetylene (NC4N). Spectra and life-times of this phosphorescence have been measured. A detailed analysis of resolved vibronic bands is presented. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745884]
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Turowski, M., Crepin, C., Douin, S., Gronowski, M., Couturier-Tamburelli, I., Pietri, N., Wasiak, A., & Kolos, R. (2012). Low temperature Raman spectra of cyanobutadiyne (HC5N). VIBRATIONAL SPECTROSCOPY, 62, 268–272.
Résumé: Low temperature Raman scattering spectra of cyanobutadiyne, either condensed from the vapour as a solid film, or isolated in a cryogenic argon matrix, have been measured – leading to the identification of all vibrational fundamentals, together with several overtones and combination modes. The analysis is based on previous experimental data, including the vibrationally resolved phosphorescence spectra, and on the comparison to available theoretical predictions. (C) 2012 Elsevier B.V. All rights reserved.
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