Fast kinetics of the reactions of hydroxyl radicals with nitrone spin traps

The technique of spin trapping with nitrone spin traps nas gained wide acceptance as a method for estimating·OH yields in ESR studies. In our study, fast optical kinetic techniques applied to a series of these traps (PBN, 2-PyBN, 3-PyBN, 4-PyBN, 3-PyOBN and 4-PyOBN) reveal relaxation spectra that indicate two absorption maxima with different time constants, with all except 4-PyOBN showing second order behavior. These two spectral regions show different kinetics. Thus, two reaction sites are indicated, only one of which is necessarily a measure of initial · OH when ESR methods are used. One other trap (DMPO) after · OH reaction decays in one mode suggesting that its final product might be useful as a measure of initial · OH. Also, our ESR evidence shows that OH detection can be improved significantly by spin trapping -hydroxyalkyl radicals formed by · OH attack on alcohols.     

The kinetics of enhanced spin capturing polymerization: Influence of the nitrone structure

Several nitrones and one nitroso compound have been evaluated for their ability to control the molecular weight of polystyrene via the recently introduced radical polymerization method of enhanced spin capturing polymerization (ESCP). In this technique, molecular weight control is achieved (at ambient or slightly elevated temperatures) via the reaction of a growing radical chain with a nitrone forming a macronitroxide. These nitroxides subsequently react rapidly and irreversibly with propagating macroradicals forming polymer of a certain chain length, which depends on the nitrone concentration in the system. Via evaluation of the resulting number‐average molecular weight, M_n, at low conversions, the addition rate coefficient of the growing radicals onto the different nitrones is determined and activation energies are obtained. For the nitrones N‐tert‐butyl‐α‐phenylnitrone (PBN), N‐methyl‐α‐phenylnitrone (PMN), and N‐methyl‐α‐(4‐bromo‐phenyl) nitrone (pB‐PMN), addition rate coefficients, k_ad,macro, in a similar magnitude to the styrene propagation rate coefficient, k_p, are found with spin capturing constants C_SC (with C_SC = k_ad,macro/k_p) ranging from 1 to 13 depending on the nitrone and on temperature. Activation energies between 23.6 and 27.7 kJ mol⁻¹ were deduced for k_ad,macro, congruent with a decreasing C_SC with increasing temperature. Almost constant M_n over up to high monomer to polymer conversions is found when C_SC is close to unity, while increasing molecular weights can be observed when the C_SC is large. From temperatures of 100 °C onward, reversible cleavage of the alkoxyamine group can occur, superimposing a reversible activation/deactivation mechanism onto the ESCP system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1098–1107, 2009     

Inclusion complexes of PBN-type nitrone spin traps and their superoxide spin adducts with cyclodextrin derivatives: parallel determination of the association constants by NMR titrations and 2D-EPR simulations

(1)H NMR and electron paramagnetic resonance (EPR) titrations were used to determine the association constants of the complexes of alpha-phenyl-N-tert-butylnitrone (PBN) analogues and their superoxide spin adducts, respectively, with methylated beta-cyclodextrins. A 1:1 stoichiometry for the nitrones with randomly methylated beta-cyclodextrin and 2,6-di-O-methyl-beta-cyclodextrin and 1:1 and 1:2 stoichiometries for the corresponding cyclodextrin-nitroxide complexes were observed. After the superoxide radical spin trapping reaction, EPR titrations afforded the association constants of the corresponding cyclodextrin-nitroxide complexes. Two-dimensional EPR simulations indicated a bimodal inclusion of the nitroxide free radical spin adducts into the cyclodextrins. For all the nitrone-cyclodextrin and nitroxide-cyclodextrin complexes, the association constants were always higher for the nitroxide complexes than for the nitrone complexes. A cooperative system concerning the complexation of the nitroxide spin adduct with a cyclodextrin was evidenced by EPR titrations. The efficiency of the cyclodextrin inclusion technique to trap superoxide and to resist bioreduction by sodium l-ascorbate was also investigated.     

Effect of the phosphoryl substituent in the linear nitrone on the spin trapping of superoxide radical and the stability of the superoxide adduct: combined experimental and theoretical studies

A new phosphorylated linear nitrone N-(4-hydroxybenzyliene)-1-diethoxyphosphoryl-1-methylethylamine N-oxide (4-HOPPN) was synthesized, and its X-ray structure was determined. The spin trapping ability of various kinds of free radicals by 4-HOPPN was evaluated. Kinetic study of decay of the superoxide spin adduct (4-HOPPN-OOH) shows the half-life time of 8.8 min. On the basis of the X-ray structural coordinates, theoretical analyses using density functional theory (DFT) calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level were performed on spin-trapping reactions of superoxide radical with 4-HOPPN and PBN and three possible decay routes for their corresponding superoxide adducts. The comparative calculations on the spin-trapping reactions with superoxide radical predicted that both spin traps share an identical reaction type and have comparable potency when spin trapping superoxide radical. Analysis of the optimized geometries of 4-HOPPN-OOH and PBN-OOH reveals that an introduction of the phosphoryl group can efficiently stabilize the spin adduct through the intramolecular H-bonds, the intramolecular nonbonding attractive interactions, as well as the bulky steric protection. Examination of the decomposition thermodynamics of 4-HOPPN-OOH and PBN-OOH further supports the stabilizing role of the phosphoryl group to a linear phosphorylated spin adduct.     

ESR-spin trapping in the presence of cyclodextrins. Detection of PBN-superoxide spin adduct

In phosphate buffer solution the half-life of the PBN-superoxide spin adduct is very short. However, as a result of the formation of inclusion complexes, its ESR signal was easily detected for up to 25 min when the trapping of superoxide anion with PBN was carried out in the presence of different cyclodextrins. Furthermore, the formation of these inclusion complexes results in a significant protection of the PBN-superoxide adduct against l-ascorbate monoanion reduction.     

Hydrostatic pressure effects on polymer cholesteric liquid-crystalline structure. I. Equilibrium structure of hydroxypropyl cellulose aqueous solution

The effect of pressure on the polymer cholesteric liquid-crystalline structure of hydroxypropyl cellulose aqueous solutions was studied using reflection spectra measurements. Pressures applied to the polymer liquid crystals ranged from 1 to 2000 bar. The equilibrium reflection spectrum of the cholesteric structure shifted to longer wavelengths, showing that the cholesteric pitch of the liquid-crystalline structure increases as the applied pressure increases. At pressures higher than 200 bar the maximum wavelength of reflection shifted linearly with the increase in applied pressure. At lower pressures, the cholesteric structure was influenced by the surface plane of the quartz window.     

Hydrostatic pressure effects on polymer cholesteric liquid-crystalline structure. I. Equilibrium structure of hydroxypropyl cellulose aqueous solution

Abstract

The effect of pressure on the polymer cholesteric liquid-crystalline structure of hydroxypropyl cellulose aqueous solutions was studied using reflection spectra measurements. Pressures applied to the polymer liquid crystals ranged from 1 to 2000 bar. The equilibrium reflection spectrum of the cholesteric structure shifted to longer wavelengths, showing that the cholesteric pitch of the liquid-crystalline structure increases as the applied pressure increases. At pressures higher than 200 bar the maximum wavelength of reflection shifted linearly with the increase in applied pressure. At lower pressures, the cholesteric structure was influenced by the surface plane of the quartz window.     

Dependence of the mobility of molecular probes in cellulose acetate solutions on complexation and the solution structure

The spin echo technique with a magnetic field pulse gradient was used to measure the translational diffusion coefficients D of molecular probes in solutions of cellulose acetates in dimethyl sulfoxide. A linear dependence of coefficient D on the extent of substitution of the cellulose acetate was established and the sensitivity of D to the supermolecular structure of the polymer solution was demonstrated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 470–472, February, 1990.     

Dependence of cyano bonded phase hydrolytic stability on ligand structure and solution pH

As part of our program to develop more stable cyano (CN) high-performance liquid chromatography (HPLC) column packings, we have evaluated hydrolytic stability as a function of ligand connectivity, chain length, and side group steric protection and the pH of the mobile phase. Three accelerated tests were used to evaluate stability: (1) A non-HPLC screening test measuring carbon loss in refluxing MeOH-100 mM KH2PO4 pH 4.5 (1:1, v/v) solution; (2) a continuous flow HPLC test measuring capacity factor maintenance in 1% trifluoroacetic acid in water (pH 1.02) at 80 degrees C; and (3) a continuous flow HPLC test measuring column efficiency maintenance in 50 mM triethylamine in water (pH 10.00) at 50 degrees C. The stability of the CN phases was found to be dependent on both ligand chemical structure and the pH of the test conditions. The starting screen test of intermediate pH was least able to differentiate the CN phases based on structure, because two different degradation mechanisms appear to offset each other (acid induced siloxane bond cleavage vs. base induced silica dissolution). A trifunctional and a sterically protected CN phase were notably stable under the acidic test conditions, but had poor stability under basic conditions. Conversely, chain extension afforded poor stability under acidic conditions, but did afford improved stability at higher pH. In total, the data indicate that good CN column stability can be achieved by using a trifunctional or a sterically protected phase in acidic mobile phases. However, as mobile phases of intermediate or higher pH are employed, shorter column lifetimes can be expected due to an accelerated dissolution of the underlying silica substrate. Materials were also compared chromatographically using a mixture of non-polar, polar, and basic analytes under reversed-phase conditions.     

NO spin trapping and EPR studies on the photochemistry of aliphatic aldehydes

Radicals, such as acyl, hydrated acyl, alkyl and ketyl radicals, from aliphatic aldehyde photochemistry were detected by NO spin trapping and EPR techniques. Deuterium effects on EPR spectra and the generation of radicals by 2-amido-2-propyl radical attack on substrate molecules in aqueous solution via hydrogen-atom abstraction were applied to identify radicals produced photochemically from aldehydes. Aliphatic aldehydes used in the present investigation were formaldehyde, acetaldehyde, acetaldehyde-d4, propionaldehyde, isobutyraldehyde, isopentanal and tert-pentanal. Possible reaction mechanisms are suggested.     

High pressure studies of the static permittivity tensor components in the nematic phase of 6CB

The dielectric permittivity tensor components, ε_II and ε_⊥, in the nematic phase of 6CB (4‐n‐hexyl‐4′‐cyanobiphenyl) were measured in the pressure range 0.1–130 MPa and the temperature range 12–58°C. The dielectric anisotropy, Δε(p, V, T) = ε_II ‐ ε_⊥, was analysed in isothermal, isobaric and isochoric conditions taking into account the pVT data and the well known Maier and Meier equation. On that basis the nematic order parameter S(p, V, T) was determined. This was used to calculate the parameter Γ relating the interaction potential with the volume (density). Its value Γ = 4.1 agrees very well with other estimates.     

High pressure studies of the static permittivity tensor components in the nematic phase of 6CB

The dielectric permittivity tensor components, ε_II and ε_⊥, in the nematic phase of 6CB (4-n-hexyl-4'-cyanobiphenyl) were measured in the pressure range 0.1-130 MPa and the temperature range 12-58°C. The dielectric anisotropy, Δε(p, V, T) = ε_II - ε_⊥, was analysed in isothermal, isobaric and isochoric conditions taking into account the pVT data and the well known Maier and Meier equation. On that basis the nematic order parameter S(p, V, T) was determined. This was used to calculate the parameter Γ relating the interaction potential with the volume (density). Its value Γ = 4.1 agrees very well with other estimates.     

超氧负离基研究: 无水乙腈中超氧负离子基的稳定性及其捕获

考察了影响无水乙腈中超氧负离子基(O2^-)稳定性的因素,发现氧促使乙腈中O2^-浓度迅速减小,通过产物鉴定和UV动力学分析提出氧促O2^-与乙腈反应的可能机理。研究了O2^-与2,2,6,6-四甲基-4-羟基-N-溴哌啶(NB)的反应,表明NB可望成为非水溶剂中O2^-的专一捕获剂。     

EPR/spin trapping study of the spontaneous addition of dioxygen on a dienol

The spontaneous addition of triplet oxygen on dienol 1, yielding endoperoxide 2, was followed by EPR/spin trapping. The use of nitroso and nitrone spin traps allowed the detection of two radical centers, showing that this reaction could likely follow a radical pathway.     

Dependence of the solute retention on the column pressure in reversed-phase HPLC

In the course of reversed-phase HPLC analysis, increase in pressure at the column inlet can affect the retention of sorbates only when they penetrate into the grafted phase. Due to conformational changes in the alkyl groups of the loose grafted layer, the solvation of sorbates by these alkyl groups would increase, leading to a growth of the retention factors. In the case of the surface sorption of solutes, the effect is not observed.     

Pressure effects on the electron spin relaxation of several radicals in solution

The pressure effect on the decay rate of chemically induced dynamic electron spin polarization (CIDEP) was investigated on several free-radical intermediates in photolysis, and the spin-lattice relaxation times for these radicals were estimated from the decay rates of CIDEP signals at various pressures. The spin-lattice relaxation rates were retarded by increasing external pressure. From the pressure dependence of the spin-lattice relaxation rates the activation volume was estimated. The activation volumes of these radicals divide into two groups; ≈30 cm³ mol⁻¹ for negative ions and ≈10 cm³ mol⁻¹ for neutral radicals.     

ESR spectroscopic study on the addition of PhCONHCHCO2Me radicals to alkenes and spin traps

Using ESR spectroscopy, the rate constants for the addition of PhCONHCHCO₂Me radicals to alkenes CH₂=CXY (X = Me, Y = Ph; X = H, Y = Ph; X = Me, Y = CO₂Me; X = H, Y = CO₂Me; X = H, Y = CN) and nitrosodurene were determined at 22 °C. It is shown that a linear dependence exists between the donor-acceptor properties of the substituents at the vinyl group and the rate constants for the addition.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2124–2126, August, 1996.     

Effect of pressure on the rotational mobility of spin probes in polymers

The effects of pressure on the rotational mobility of three nitroxyl radicals (spin probes) in natural rubber, polyethylene, and butadiene-nitrile rubber SRN-26 have been studied. The activation volumes, activation energies, and pre-exponential factors of spin probe rotation at constant pressure and volume have been determined. The activation volumes of probe rotation (20–70 cm³/mol) increase with increasing size of radicals and differ insignificantly from the activation volumes of the β-relaxation process. In the polar polymer SRN-26, the activation volumes of rotation of radicals are appreciably more than in the nonpolar polymers, natural rubber and polyethylene. These features are apparently due to different volumes of the kinetic chain segment controlling probe rotation. The activation volumes of radical rotation around different molecular axes differ significantly. The activation energy of probe rotation at constant volume is appreciably less than at constant pressure. It has also been shown that the energy necessary for the formation of a fluctuation hole is the main factor that determines temperature dependence of the rotational mobility of low-molecular particles in the polymer.