Resonant 2-photon ionization study of the conformation and the binding of water molecules to 2-phenylethanethiol (PhCH2CH2SH).

The structures of 2-phenylethanethiol (PET, PhCH(2)CH(2)SH) and its 1:1 water clusters have been studied using resonant two-photon ionization spectroscopy including band contour analysis and UV-UV holeburning, combined with extensive ab initio calculations on ground and excited states. The most populated conformer, labeled Ggpi, has a gauche arrangement about the SCCC and HSCC bonds that permits a stabilizing SH...dpi type of hydrogen bond. The other observed conformer, Ag, is anti with respect to the SCCC bond. In the dominant 1:1 water cluster, a water molecule binds to the Ggpi conformer via an OH...S hydrogen bond and two significant CH...O interactions. There is also evidence for water binding to conformer Ag with a similar arrangement, and for a second Ggpi cluster where water inserts between the SH and the aromatic ring. The additional interactions to the water molecules result in net D(e) binding energies approximately double those resulting from a single thiol-water hydrogen bond. The (1)(pi,pi(*)) excited state lifetimes in the bare molecules are very short because of internal conversion to a dissociative (1)(n,pi(*)) state related to the thiol. In the dominant Gw(1) cluster, the lifetime is significantly increased from <1 to approximately 4 ns. Hydrogen bonding to the thiol, which raises the energy of the dissociative (1)(n,pi(*)) state, accounts for this behavior.

Photocatalytic ozonation of phenolic wastewaters: Syringic acid, tyrosol and gallic acid.

The photocatalytic ozonation of a mixture of 3 phenols (gallic acid, tyrosol and syringic acid) has been conducted under different operating conditions. The individual adsorption of the phenol type compounds onto titanium dioxide (photocatalyst) has been first evaluated. Equilibrium conditions are attained in less than an hour while the isotherm curves reveal that adsorption intensity increases in order: syringic acid < tyrosol < gallic acid. When the photocatalytic ozonation is applied, an optimum in titanium dioxide concentration is experienced (1.5 g L(-1)). Direct comparison of the photocatalytic ozonation to other less sophisticated oxidation systems (i.e., single ozonation, catalytic ozonation, photo-ozonation, etc.) indicates a higher efficiency of the former in terms of ozone uptake.

Mild photochemical synthesis of the antioxidant hydroxytyrosol via conversion of tyrosol.

Hydroxytyrosol, a naturally occurred orthodiphenolic antioxidant molecule found in olive oil and olive mill wastewaters, was obtained from the wet hydrogen peroxide photocatalytic oxidation of its monophenolic precursor tyrosol. The liquid-phase oxidation of tyrosol to hydroxytyrosol was performed by use of an iron-containing heterogeneous catalyst (Al-Fe)PILC with the assistance of UV irradiation at 254 nm and at room temperature. The spectroscopic and HPLC data of the synthesized compound proved to coincide fully with those of a pure sample obtained by continuous countercurrent extraction. This reaction was found to be light-induced. The hydroxytyrosol synthesis reaction reached its maximum yield of 64.36% under the optimized operating conditions of 3.6 mM tyrosol, 0.5 g L(-1) catalyst, and 10(-2) M H2O2 with the assistance of UV light. Increasing the initial hydrogen peroxide concentration more than 10(-2) M has a diminishing return on the reaction efficiency. Catalyst can be recuperated by means of filtration and then reused in a next run after regeneration since its activity did not significantly decrease (<10%). The reaction synthesis is operationally simple and could find application for industrial purposes.

The stereoselectivity of the Paternò-Büchi reaction between tertiary 2-furylmethanol derivatives and aromatic carbonyl compounds: on the nature of the hydroxy directing effect.

The photochemical reaction of 1-(2-furyl)-1-phenylethanol with benzaldehyde gave a mixture of regioisomeric products. The adduct obtained on the more hindered side of the molecule was obtained with complete diastereoselectivity. The same substrate with benzophenone gave only one product with a diastereoisomeric excess of 48%. The reaction of 2-(2-furyl)-3,3-dimethylbutan-2-ol with benzaldehyde and benzophenone gave the corresponding adducts on the more hindered side of the molecule with diastereoisomeric excesses of 42 and 71%, respectively. These results, and also those obtained using 2-furylphenylmethanol with benzophenone and acetone (complete diastereoselectivity and absence of diastereoselectivity, respectively), were explained assuming the attack of the excited carbonyl compound on the same side as the hydroxy group, through the formation of a hydrogen bond or of a complex. This type of attack gave the biradical intermediate in preferential conformations. The relative energies of these conformers account for the observed diastereoselectivity.