Ján Žabka

The oxidation of natural flavonoid quercetin

This study explains the controversies in the literature concerning the number of electrons involved in the oxidation of quercetin. This stems from inappropriate handling samples, which require strict anaerobic conditions. The redox potential of quercetin strongly depends on the pH and on the presence of dissociation forms in solution.

On the stability of the bioactive flavonoids quercetin and luteolin under oxygen-free conditions

AbstractThe natural flavonoid compounds quercetin (3,3′,4′,5,7-pentahydroxyflavone) and luteolin (3′,4′,5,7-tetrahydroxyflavone) are important bioactive compounds with antioxidative, anti-allergic, and anti-inflammatory properties. However, both are unstable when exposed to atmospheric oxygen, which causes degradation and complicates their analytical determinations. The oxidative change of these flavonoids was observed and followed by UV–visible spectrophotometry, both in aqueous and ethanolic solutions. The distribution of the degradation products in aqueous media was monitored by LC–MS and LC–DAD analysis. The amounts of oxidative reaction products increase with the exposure time. The oxidative degradation reduces the pharmacological efficiency of these antioxidants and renders analytical determination inaccurate. The oxidative changes in flavonoid test solutions can explain the inconsistent dissociation constants reported in the literature. Dissociation constants of quercetin and luteolin were determined both by alkalimetric titration and by UV–visible spectrophotometry under deaerated conditions. The values pK1 = 5.87 ± 0.14 and pK2 = 8.48 ± 0.09 for quercetin, and pK1 = 5.99 ± 0.32 and pK2 = 8.40 ± 0.42 for luteolin were found. FigureThe change of absorption spectra of quercetin during the exposure to the air oxygen

Reduction from copper(II) to copper(I) upon collisional activation of (pyridine)2CuCl

Electrospray ionization of dilute aqueous solutions of copper(II) chloride-containing traces of pyridine (py) as well as ammonia permits the generation of the gaseous ions (py)(2) Cu(+) and (py)(2) CuCl(+) , of which the latter is a formal copper(II) compound, whereas the former contains copper(I). Collision-induced dissociation of the mass-selected ions in an ion-trap mass spectrometer (IT-MS) leads to a loss of pyridine from (py)(2) Cu(+) , whereas an expulsion of atomic chlorine largely prevails for (py)(2) CuCl(+) . Theoretical studies using density functional theory predict a bond dissociation energy (BDE) of BDE[(py)(2) Cu(+) -Cl] = 125 kJ mol(-1) , whereas the pyridine ligand is bound significantly stronger, i.e. BDE[(py)CuCl(+) -py] = 194 kJ mol(-1) and BDE[(py)Cu(+) -py] = 242 kJ mol(-1) . The results are discussed with regard to the influence of the solvation on the stability of the Cu(I) /Cu(II) redox couple.

Surface-induced reactions and decomposition of the benzene molecular ion C6H6+: Product ion intensities, angular and translational energy distributions

Abstract Surface-induced decomposition of benzene molecular ions (impact energy range 19–30 eV) and hydrogen atom abstraction from surface adsorbates leading to the production of a protonated benzene ion, were investigated using two different types of scattering apparatus. In addition to determining relative abundances of the product ions (mass spectra), we have measured the angular and translational energy distributions of the major product ions. All product ions show a strongly inelastic scattering behavior, with a maximum in translational energy distribution at 2–6 eV and a maximum in angular distribution at angles (70–75°) lower than the specular angle (90°). For the projectile ion, besides inelastic scattering reactions, we also observed an elastic scattering channel of substantial abundance exhibiting its maximum at scattering angles (55–60°) lower than that for the other scattered ions. Differences in the translational energy spectra for C 6 H 6 + and C 6 H 7 + were used to deduce the reaction sequences leading to the various fragment ions observed in the product ion mass spectra.

Dynamics of chemical and charge transfer reactions of molecular dications: beam scattering and total cross section data on CF2D+ (CF2H+), CF2+, and CF+ formations in CF22+ + D2(H2) collisions

Abstract Dynamics of formation of the chemical rearrangement product CF 2 D + , the charge transfer product CF 2 + , and the dissociative products CF + and CFD + in collisions of the molecular dication CF 2 ++ with D 2 was investigated in crossed beam scattering experiments over the collision energy range 0.3–1.0 eV (center of mass). The scattering data show that coulomb repulsion between two singly charged products, CF 2 + + D + and CF 2 + + D 2 + , plays a dominant role in the nondissociative processes. A large fraction of the energy available (about 6 eV in the chemical reaction, about 4 eV in the charge transfer) goes into relative translational energy of the products. Relative total cross sections for formation of the nondissociative and dissociative products in collision of CF 2 ++ with D 2 and H 2 were determined over the collision energy range of 0.2–3.6 eV. The shape of the relative velocity dependence of the cross section for CF 2 + formation can be described by a simple model based on the Landau-Zener formalism. The data suggest that the dissociative product CF + is formed prevailingly in a subsequent dissociation of the charge transfer product CF 2 + . A potential surface model is described which accounts for competition of various processes in dication–neutral collisions.

Energy partitioning in collisions of slow polyatomic ions with carbon surfaces

Abstract Energy transfer in collisions of slow polyatomic ions with carbon surfaces (Tore Supra carbon tile and highly oriented pyrolytic graphite) was investigated over the incident energy range 10–23 eV. Mass spectra and translational energy and angular distributions of product ions were used to determine the partitioning of the incident energy of the projectile ion into internal excitation of the projectile, product translational energy, and fraction absorbed by the surface. The ethanol molecular ion was used as a model polyatomic ion. For the incident angle of 60° (with respect to the surface normal) the peak values of the respective energy fractions were 6% for excitation of the projectile, 24%–28% for product ion translational energy, and 70%–66% absorbed by the surface. Similar values for energy transfer were found earlier for energy transfer at stainless steel surfaces covered by hydrocarbons and surfaces covered by a self-assembled monolayer (SAM) of C 12 alkane chains. The occurrence of chemical reaction products (protonated ethanol and its fragment ions) formed by H-atom transfer from the surface material for all the above mentioned surfaces (the carbon surfaces, stainless steel with a hydrocarbon layer, alkane SAM) indicated that the carbon surfaces were covered with a layer of hydrocarbons, too, and thus the present results provide information of interest for energy transfer on carbon tiles covered with hydrocarbon films used in fusion research.

Comparative study of mono- and dinuclear complexes of late 3d-metal chlorides with N,N-dimethylformamide in the gas phase.

Mono- and binuclear complexes of N,N-dimethylformamide (DMF) with chlorides of the divalent, late 3d metals M = Co, Ni, Cu, and Zn are investigated by means of electrospray ionization (ESI). Specifically, ESI leads to monocations of the type [(DMF)(n)MCl](+) and [(DMF)(n)M(2)Cl(3)](+), of which the species with n = 2 and 3 were selected for in-depth studies. The latter include collision-induced dissociation experiments, gas-phase infrared spectroscopy, and calculations using density functional theory. The mononuclear complexes [(DMF)(n)MCl](+) almost exclusively lose neutral DMF upon collisional activation with the notable exception of the copper complex, for which also a reduction from Cu(II) to Cu(I) concomitant with the release of atomic chlorine is observed. For the dinuclear clusters, there exists a competition between loss of a DMF ligand and cluster degradation via loss of neutral MCl(2) with decreasing cluster stability from cobalt to zinc. For the specific case of [(DMF)(n)ZnCl](+) and [(DMF)(n)Zn(2)Cl(3)](+), ion-mobility mass spectrometry indicates the existence of two isomeric cluster ions in the case of [(DMF)(2)Zn(2)Cl(3)](+) which corroborates parallel theoretical predictions.

Formation of Organoxenon Dications in the Reactions of Xenon with Dications Derived from Toluene

The bimolecular reactivity of xenon with C(7)H(n)(2+) dications (n=6-8), generated by double ionization of toluene using both electrons and synchrotron radiation, is studied by means of a triple-quadrupole mass spectrometer. Under these experimental conditions, the formation of the organoxenon dications C(7)H(6)Xe(2+) and C(7)H(7)Xe(2+) is observed to occur by termolecular collisional stabilization. Detailed experimental and theoretical studies show that the formation of C(7)H(6)Xe(2+)+H(2) from doubly ionized toluene (C(7)H(8)(2+)) and xenon occurs as a slightly endothermic, direct substitution of dihydrogen by the rare gas with an expansion to a seven-membered ring structure as the crucial step. For the most stable isomer of C(7)H(6)Xe(2+), an adduct between the cycloheptatrienyldiene dication and xenon, the computed binding energy of 1.36 eV reaches the strength of (weak) covalent bonds. Accordingly, electrophiles derived from carbenes might be particularly promising candidates in the search for new rare-gas compounds.

Reactivity of C2H5+ with benzene: formation of ethylbenzenium ions and implications for Titan's ionospheric chemistry.

The reaction of ethyl cation with benzene has been investigated in a combined experimental and theoretical approach. Under single collision conditions, proton transfer affording protonated benzene concomitant with neutral ethene represents the major reaction channel. From pressure-dependent measurements, an absolute cross section of 7 +/- 2 A(2) at hyperthermal energies (about 1.0 eV in the center of mass frame) is derived for this channel, from which a phenomenological rate constant of about 2.9 x 10(-10) cm(3) s(-1) is estimated at low energies. The energy behavior of the cross section as well as several side reactions leading to C-C coupling imply that the reaction of C(2)H(5)(+) with C(6)H(6) proceeds via a long-lived association product, presumably the covalently bound protonated ethylbenzene (ethylbenzenium ion). With regard to chemical processes in the atmosphere of Titan, present results imply that termolecular association of C(2)H(5)(+) with benzene to produce protonated ethylbenzene is very likely to occur. The condensation of alkyl cations with arenes thus provides an alternative route for the growth of larger hydrocarbon molecules.

Dissociative double photoionization of N2 using synchrotron radiation: Appearance energy of the N2+ dication

Photoionization cross sections for the production of the doubly charged ion N2+ from N2 have been measured by means of synchrotron radiation in the photon energy range from 50 to 110 eV. The appearance energy for N2+ has been determined as 55.2+/-0.2 eV, i.e., about 1.3 eV higher than the spectroscopic dissociation limit leading to the charge asymmetric dissociation channel N2+(2P)+N(4S) at 53.9 eV. The onset of a second threshold at 59.9+/-0.2 eV is detected and the energy dependence of photoion intensities near the threshold regions is interpreted in terms of the Wannier theory. The production of the N2+ dication is discussed in terms of direct and indirect mechanisms for dissociative charge asymmetric photoionization and by comparison with the potential energy curves of the intermediate N(2)2+ dication. Experimental evidences for the opening of the Coulomb explosion channel N2++N+ at high photon energies are provided by measuring the kinetic energy release spectra of N2+ fragments at selected photon energies.