Wolfgang Solar

Resolved multisite OH-attack on aqueous aniline studied by pulse radiolysis

Abstract The individual formation and decay kinetics as well as the absorption characteristics of the simultaneously formed primary species by OH attack on aniline in aqueous (pH 8–9.6), saturated with N2O, have been determined by pulse radiolysis combined with a computer optimization procedure. It was established that 36% OH radicals react directly with the -NH2 moiety (k = (5.0 ± 0.6) × 109dm3mol-1s-1 resulting in anilino radical, C6H5NH (ϵ300 = 2500 ± 100, ϵ355 = 950 ± 50, ϵ420 = 420 ± 40 dm3 mol-1cm-1)), which decays by a second order reaction with 2k = (1.8 × 0.2) × 109dm3mol-1s-1. The main primary process (54%) is the formation of the ortho-directed OH-adduct (k = (7.5 ± 0.8) × 109dm3mol-1s-1, ϵ300 = 1800±100, ϵ355 =3300±170dm3mol-1cm-1), which subsequently decays by water splitting resulting again in C6H5NH. The remaining 10% OH attack most probably the para position of the aniline ring (k = (1.5±0.2) × 109dm3mol-1s-1) forming the corresponding OH-adduct (ϵ300 = 1700±100, ϵ355 = 3600±200 dm3mol-1cm-1), which disappears with 2k = (1.1±0.3) × 109dm3mol-1cm-1; reactions with meta- and ipso-positions, however, cannot be excluded. H-atoms react with the neutral form of aniline with k =(1.9 ± 0.1) × 109dm3mol-1cm-1, forming H-adducts. Further the rate constant of e-aq with aniline was determined to (3.0 ± 0.1) × 107dm3mol-1cm-1. Qualitative analysis of final products were also performed.

Pulse radiolysis of methyl viologen in aqueous solutions

Pulse radiolysis of air-free aqueous methyl viologen (MV2+) solutions was carried out at various pH. The attack of e–aq on MV2+, with k(e–aq+ MV2+)= 7.5 × 1010 dm3 mol–1 s–1, leads to the formation of the long-lived radical cation (MV˙+), which possesses two absorption maxima at 392.5 nm (Iµ392.5= 4200 m2 mol–1) and 600 nm (Iµ600= 1450 m2 mol–1). The H-atoms react with MV2+ at pH 1 forming two species, which have superimposed absorption bands. By means of a computer simulation they are resolved in the absorptions belonging to: (1) a protonated form of the radical cation (MV˙+H+), which is produced with k(H + MV2+)=(3.5 ± 0.2)× 108 dm3 mol–1 s–1, has 2 absorption maxima at 390 nm (Iµ390= 1700 m2 mol–1) and 595 nm (Iµ595= 760 m2 mol–1) and decays by second-order kinetics with k= 3.5 × 109 dm3 mol–1 s–1; (2) an H-adduct (MV˙2+H) on the ring carbon, which is formed with k(H + MV2+)= 2.5 × 108 dm3 mol–1 s–1, absorbs at 310 nm (Iµ310= 900 m2 mol–1) and 470 nm (Iµ470= 630 m2 mol–1) and decays by conversion into MV˙+H+ in a first-order process with k= 6 × 103 s–1. For the equilibrium MV˙+H+⇌ MV˙++ H+ pK= 2.9 ± 0.1 was determined. The presented data explain, at least partly, the instability of MV2+ when used as an electron acceptor in various devices for the utilization of solar energy.

Pulse radiolysis of thionine and methylene blue in acid aqueous solutions

Abstract The attack of H-atoms on thionine and methylene blue in air-free aqueous acidic solutions (pH 0.8–4.2) was extensively investigated by an improved pulse-radiolysis technique combined with an extended simulation computation procedure. Thionine: The main product is semiquinone (SQ=89.5%; k(H+TH+)=(8.5±0.1)×109dm3mol−1s−1 for two protonated forms (ṪH32+, ṪH2+), which dismutates with 2k=(2.4±0.1)×109dm3mol−1s−1 at pH 1 and 2k=(2.8±0.1)×109dm3mol−1s−1 at pH 4. Three absorption maxima of thionine SQ were determined. At pH 1, λ=790nm (ϵ790=775m2 mol−1), λ = 385 nm (ϵ385 = 690 m2 mol−1) and λ = 260 nm (ϵ260 = 2600 m2 mol−1); at pH 4, λ = 770 nm (ϵ770 = 1870 m2 mol−1). λ = 400 nm (ϵ400 = 750 m2mol−1) and λ = 260 nm (ϵ260 = 2650 m2 mol−1). A pK-value of 1.8±0.1 was determined for these two protonated forms. In addition, the formation of an H-adduct on the aromatic ring (H-adduct = 10.5%; k (H + TH+) = (1±0.2) × 109 dm3 mol−1 s−1) was observed (λmax = 310 nm; ϵ310 = 560 m2mol−1), which disappears according to a pseudo-first order reaction with k = (1±0.2) × 103 s−1. The rate constant for the reaction of thionine with eaq− at pH 7.6 is k(TH+ + eaq−) = (1.1±0.1) × 1010 dm3 mol−1 s−1 and at pH 12.6 k(T + eaq−) = (1.4±0.1) × 1010 dm3 mol−1 s−1. Methylene Blue: Reactions with H-atoms in the pH range 1–4 leads to the formation of semiquinone (SQ = 86%; k(H + MB+) = 9.5±0.1) × 109 dm3 mol−1 s−2), H-adducts on the aromatic ring carbon ( H D H + = 5.4%; k = (0.6±0.02) × 10 9 dm 3 mol −1 s −1 ) and radical species resulting from attack on the -S-position (R = 8.6%: k = (0.95±0.02) × 109 dm3 mol−1 s−1). A pK2 of 1.9±0.1 for two forms of the protonated SQ was determined, each protonated form having three absorption maxima. At pH 1, MḂH22+ exhibits maxima at λ = 880 nm (ϵ880 = 1190 m2mol−1), λ = 380 nm (ϵ380 = 900 m2 mol−1) and λ = 290 nm (ϵ290 = 3750 m2 mol−1) dismutating with 2k = (1.6±0.2) × 109 dm3 mol−1 s−1. The corresponding data for MḂH+ (pH 4) are: λ = 880 nm (ϵ880 = 2350 m2mol−1), λ = 395 nm (ϵ395 = 910 m2 mol−1), λ = 290 nm (ϵ290 = 3800 m2 mol−1) and 2k = (1.8±0.2) × 109 dm3 mol−1 s−1. The HḊH+ species have a λmax = 315 nm (ϵ315 = 580 m2 mol−1) and disappear according to a pseudo-first order reaction, k = (2±0.2) × 103 s−1. The R. transients have absorption maxima at λ = 285 nm (ϵ285 = 4850 m3 mol−1), λ = 350 nm (ϵ350 = 6500 m2 mol−1) and λ = 400 nm (ϵ400 = 5400 m2 mol−1) decaying according to a pseudo-first order reaction (k = (1±0.2) × 105 s−1). The spectroscopic data of HḊH+ and R. do not alter in the pH range investigated.

Resolved multisite OH-attack on aqueous tryptophan studied by pulse radiolysis

By means of a combined pulse radiolysis-computer optimization procedure it was possible to resolve the individual formation and decay kinetics as well as the absorption spectra of different transients produced by OH-attack on tryptophan in aqueous solutions at pH = 6.5–8.5. The main part of the Oh-species (<60%) attack the pyrrole ring of the molecule, preferentially at C2-position, with k = (7.5±0.5) × 109dm3mol-1s-1, resulting in transients with λmax at 325, 340 and 520 nm (ϵ325 = 375±30 m2mol-1, ϵ340 = 295±25 m2 mol-1 and ϵ520 = 75±5 m2 mol-1). They disappear following second order kinetics with k = (2.3±0.2) × 108dm3mol-1s-1. About 40% of the OH-radicals are forming OH-adducts on the benzene ring of the trytophan molecule with k = (5.0±0.3) × 109 dm3 mol-1 s-1, having absorption maxima at 310 nm and 410 nm (ϵ310 = 400±30 m2mol-1 and ϵ410 = 220±20 m2mol-1) and decay by second order reaction with k = (2.0±0.3) × 109dm3mol-1s-1, The data are of interest for radiation biology and in respect of sterilization and conservation of foodstuffs by irradiation.

A kinetic study of multiple H-attack on methylene blue in aqueous solution☆

Abstract The attack of H-atoms on several positions of the methylene blue molecule (5.10-6-2.10-5 mol dm-3 MB+) in acid aqueous solution (pH1) was investigated by pulse radiolysis. A least-squares fitting procedure was used for the determination of specific rate constants of these reactions as well as of ϵ-values of the resulting species. It was found that 88% of the H-atoms attacking MB+, with k = (1.2±0.1) × 1010 dm3 mol-1s-1, lead to the formation of semiquinone (MBH22+; SQ), which shows two absorption bands at 400 nm (ϵ400 = 670 m2 mol-1) and 880 nm (ϵ880 = 1770 m2 mol-1) and which decays by dismutation to MB+ and leuco dye with 2k = (2.0±0.3) × 109 dm3 mol-1 s-1. 8% of the H-atoms attack the chromophore group with k = (1.1±0.2) × 109 dm3 mol-1 s-1, leading to a species (R·) with ϵ400 = 5000 m2 mol-1. This unspecified transient disappears probably by a pseudo-first order reaction, with k = (1.4±0.3) × 105 s-1. The rest of the H-atoms (4%) form H-adducts on the aromatic rings with k(0.6±0.2) × 109dm3mol-1s-1.

A pulse radiolysis-computer simulation method for resolving of complex kinetics and spectra

Abstract The presented combined pulse radiolysis-computer simulation method permits the individual resolving of superimposed spectra and kinetic data of mixed transients, produced by multisite radical attack on a substrate in solution. The method also renders a critical examination of the optimized parameters (k-and e-values) of an assumed reaction model by means of a sensitivity matrix. The last one helps to establish whether the available experimental data principally allow a determination of the unknown parameters. If this is not possible, further experiments are recommended, however, with somewhat changed experimental conditions (dose rate, solute concentration, wavelength). The performance of the combined method is illustrated by a practical example. It is also applicable for resolving of mixed kinetic and spectroscopic data obtained by laser and flash photolysis.

Reactivity of OH and O– with aqueous methyl viologen studied by pulse radiolysis

The behaviour of aqueous MV2+ towards oxidizing radicals (OH and O–) has been investigated in the pH range from 6 to 14 by means of pulse radiolysis. A semi-linear optimization method was applied for resolving the complex reaction mechanism. In the pH range from 6 to 8 the rate constant for attack by OH is k=(2.5±0.2)× 108 dm3 mol–1 s–1. The resulting transient absorbs at λmax= 470 nm (Iµ470= 1600±70 m2 mol–1) and decays with 2k=(1.3±0.2)× 108 dm3 mol–1 s–1.In strongly alkaline solutions (pH 13.8) the O– radical anion reacts preferentially by hydrogen abstraction from the methyl group, k=(1.4±0.2)× 109 dm3 mol–1 s–1, forming a radical which then decays by reaction with OH–(k= 2.8 × 106 dm3 mol–1 s–1) to produce a modified radical cation; this has absorption maxima at 392 and 605 nm (Iµ392= 4300 m2 mol–1, Iµ605= 1500 m2 mol–1) and is relatively long lived.The remaining part (< 10%) of O– attacks MV2+ at the ring carbon atom, k=(1.0±0.4)× 108 dm3 mol–1 s–1, resulting in an O– adduct, which has λmax= 470 nm (Iµ470= 2200±100 m2 mol–1).

Reactivity of H, OH and e-aq with nicotinic acid: A pulse radiolysis study

Abstract The reactivity of aqueous nicotinic acid (NA) towards OH, e-aq and H-atoms has been investigated in the pH-range 0.3–13.8. The OH attack on NA [k = (2.5 ± 0.2) × 109 M-1 s-1] and its N-protonated forms [k = (2.2 ± 0.2) × 107 M-1 s-1] gives OH-adducts with pH-dependent optical spectra [NĊ5H4(OH)COO-:λmax = 310 nm, ϵ310 = 1800 ± 200 M-1 cm-1; +HNĊ5H4(OH)COO-:λmax = 325 nm, ϵ325 = 2000 ± 200 M-1 cm-1]. pKa = 4.5 ± 0.2 was determined for these two transients. The reaction of H-atoms with NA in the pH-range 6–12 [k = (6.0 ± 0.5) × 108 M-1 s-1] results in the formation of one type of transient, the H-adducts on ring carbons (λmax = 315 nm, ϵ315 = 4500 ± 200 M-1 s-1; pK = 6.7 ± 0.2). With the N-protonated forms of NA, however, two kinds of radicals are produced: pyridinyl (λmax = 285 nm, ϵ285 = 7800 ± 200 M-1 cm-1; λmax = 410 nm, ϵ410 = 3100 ± 100 M-1 cm-1) and H-adduct on ring carbons (λmax = 340 nm, ϵ340 = 4600 ± 200 M-1 cm-1). The reaction of e-aq with NA was reinvestigated. The kinetic and spectroscopic data are in good agreement with those previously reported.

A semi-linear optimization model for resolving fast processes

A new algorithmic procedure for resolving superimposed spectra and kinetic data of transients produced in fast complex chemical processes is presented. The optimization model allows partitioning of the unknown model parameters into two groups: linear spectroscopic parameters and non-linear kinetic parameters. When solving the inverse problem, the field of iterations is confined to the kinetic parameters and so no initial information regarding spectroscopic behaviour of the transients is required and the computing time is drastically reduced, as compared with a previous method. We illustrate the optimization model for the reaction of multisite hydrogen attack on three dyes (acridine, proflavin and acridine orange) studied by pulse radiolysis. The reliability of the optimized parameters (k, Iµ) is critically examined by means of a sensitivity matrix. The optimization procedure, combined with pulse radiolysis, laser or flash photolysis, will be helpful in providing an insight into the reaction mechanisms of fast complex chemical processes.

Hydrogen-atom attack on methyl viologen in aqueous solution studied by pulse radiolysis

Using hydrogen at high pressures of up to 150 bar (0.12 mol dm–3 H2) as an OH scavenger in aqueous MV2+ solutions (pH 1) it is possible to differentiate between two kinds of transient formed simultaneously by H-atom attack on methyl viologen. One of them is assigned to an H adduct on the N atom, MV˙+H+(k= 3.1 × 108 dm3 mol–1 s–1), with absorption bands identical to those of the radical cation, MV˙+, but with Iµ392.5= 3200 m2 mol–1 and Iµ600= 1100 m2 mol–1. The MV˙+H+ species deprotonates with k= 2 × 104 s–1, forming the long-lived radical cation, MV˙+. The second type of transient produced, with k= 2.9 × 108 dm3 mol–1 s–1, is attributed to an H-adduct on the ring carbon, MV˙2+H, with Iµ310= 570 m2 mol–1 and Iµ470= 920 m2 mol–1, decaying by second-order kinetics with 2k=(6.0±1)× 108 dm3 mol–1 s–1. The formation of MV˙+ by electron transfer from the propan-2-ol radical has been reinvestigated (pH 0–7); its absorption spectrum does not change in this pH range.