title: Influence of cage confinement on the photochemistry of matrix-isolated E-β-ionone: FT-IR and DFT study.
abstract: β-ionone, a model compound of carotenoids ring structure, was investigated by FT-IR spectroscopy in a low-temperature argon matrix as well as using B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) quantum-chemical calculations. The spectrum of matrix-isolated E-β-ionone was analyzed and attributed to six conformers of the compound. Then, matrix-isolated E-β-ionone was submitted to UV irradiation using either a broadband source (with different cutoff filters) or a narrowband laser/MOPO system (at various wavelengths). Upon 240 nm narrowband irradiation, the formation of both Z-retro-γ-ionone and Z-β-ionone was observed, the reactant and the photoproducts being in a photostationary equilibrium. Under these conditions, the matrix environment was found to hamper subsequent reactions of Z-retro-γ-ionone and Z-β-ionone, so that this last species could be observed directly for the first time. Furthermore, the formation of Z-retro-γ-ionone was shown to occur directly via an intramolecular [1,5] H-atom shift and thereby, under the constraints imposed by the matrix confinement, the conformations assumed by this photoproduct were found to be strictly determined by those initially assumed by the reactant molecules. Broadband irradiation resulted in the completion of the reaction (disappearance of the reactant) and the sole observation of Z-retro-γ-ionone. These results imply that under these conditions the Z-β-ionone is unstable, very likely decaying to additional conformers of Z-retro-γ-ionone, as reflected in the broader bands due to this photoproduct observed in the infrared spectra of the broadband irradiated matrix.
authors.affiliation: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland. kaczor@chemia.uj.edu.pl
authors.forename: A
authors.initials: A
authors.lastname: Kaczor
authors.forename: I
authors.initials: I
authors.lastname: Reva
authors.forename: R
authors.initials: R
authors.lastname: Fausto
mesh_terms: D015394:Molecular Structure; D045792:Norisoprenoids / Q000737:chemistry*; D055668:Photochemical Processes; D011789:Quantum Theory; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D045792:Norisoprenoids