Hydrogen atom abstraction selectivity in the reactions of alkylamines with the benzyloxyl and cumyloxyl radicals. The importance of structure and of substrate radical hydrogen bonding

A time-resolved kinetic study on the hydrogen abstraction reactions from a series of primary and secondary amines by the cumyloxyl (CumO(?)) and benzyloxyl (BnO(?)) radicals was carried out. The results were compared with those obtained previously for the corresponding reactions with tertiary amines. Very different hydrogen abstraction rate constants (k(H)) and intermolecular selectivities were observed for the reactions of the two radicals. With CumO(?), k(H) was observed to decrease on going from the tertiary to the secondary and primary amines. The lowest k(H) values were measured for the reactions with 2,2,6,6-tetramethylpiperidine (TMP) and tert-octylamine (TOA), substrates that can only undergo N-H abstraction. The opposite behavior was observed for the reactions of BnO(?), where the k(H) values increased in the order tertiary < secondary < primary. The k(H) values for the reactions of BnO(?) were in all cases significantly higher than those measured for the corresponding reactions of CumO(?), and no significant difference in reactivity was observed between structurally related substrates that could undergo exclusive α-C-H and N-H abstraction. This different behavior is evidenced by the k(H)(BnO(?))/k(H)(CumO(?)) ratios that range from 55-85 and 267-673 for secondary and primary alkylamines up to 1182 and 3388 for TMP and TOA. The reactions of CumO(?) were described in all cases as direct hydrogen atom abstractions. With BnO(?) the results were interpreted in terms of the rate-determining formation of a hydrogen-bonded prereaction complex between the radical α-C-H and the amine lone pair wherein hydrogen abstraction occurs. Steric effects and amine HBA ability play a major role, whereas the strength of the substrate α-C-H and N-H bonds involved appears to be relatively unimportant. The implications of these different mechanistic pictures are discussed.     

Hydrogen atom abstraction reactions from tertiary amines by benzyloxyl and cumyloxyl radicals: influence of structure on the rate-determining formation of a hydrogen-bonded prereaction complex

A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO(?)) and benzyloxyl (BnO(?)) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (k(H)) were measured for the two radicals (k(H)(BnO(?))/k(H)(CumO(?)) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in k(H)(BnO(?))/k(H)(CumO(?)) ratios of 13 to 2027 times were observed. k(H) approaches the diffusion limit in the reactions between BnO(?) and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD(2)O(?), BnO(?)-d(2)) and the 3,5-di-tert-butylbenzyloxyl radical.     

Kinetic solvent effects on hydrogen abstraction reactions from carbon by the cumyloxyl radical. The importance of solvent hydrogen-bond interactions with the substrate and the abstracting radical

A kinetic study of the hydrogen atom abstraction reactions from propanal (PA) and 2,2-dimethylpropanal (DMPA) by the cumyloxyl radical (CumO?) has been carried out in different solvents (benzene, PhCl, MeCN, t-BuOH, MeOH, and TFE). The corresponding reactions of the benzyloxyl radical (BnO?) have been studied in MeCN. The reaction of CumO? with 1,4-cyclohexadiene (CHD) also has been investigated in TFE solution. With CHD a 3-fold increase in rate constant (k(H)) has been observed on going from benzene, PhCl, and MeCN to TFE. This represents the first observation of a sizable kinetic solvent effect for hydrogen atom abstraction reactions from hydrocarbons by alkoxyl radicals and indicates that strong HBD solvents influence the hydrogen abstraction reactivity of CumO?. With PA and DMPA a significant decrease in k(H) has been observed on going from benzene and PhCl to MeOH and TFE, indicative of hydrogen-bond interactions between the carbonyl lone pair and the solvent in the transition state. The similar k(H) values observed for the reactions of the aldehydes in MeOH and TFE point toward differential hydrogen bond interactions of the latter solvent with the substrate and the radical in the transition state. The small reactivity ratios observed for the reactions of CumO? and BnO? with PA and DMPA (k(H)(BnO?)/k(H)(CumO?) = 1.2 and 1.6, respectively) indicate that with these substrates alkoxyl radical sterics play a minor role.     

Strong influences of cocatalyst on ethylene/propylene copolymerization with a MgCl2/SiO2/TiCl4/diester type Ziegler-Natta catalyst

Using triethylaluminum (TEA), triisobutylaluminum (TIBA) or TEA/TIBA mixtures of molar ratio 75/25, 50/50 and 25/75 as the cocatalyst, five different ethylene-propylene copolymer samples were synthesized by a MgCl₂/SiO₂/TiCl₄/diester type Ziegler-Natta catalyst in a slurry polymerization process. The synthesized copolymers are strongly heterogeneous in chain structure and were fractionated into part of nearly random copolymer and part of segmented copolymer. Both polymerization activity and copolymer structure were found to be markedly changed when the cocatalyst was changed from TEA to TEA/TIBA mixtures or pure TIBA. As the content of TEA in cocatalyst increases, yield of the random part of product increases and the yield of the crystalline segmented copolymer part decreases. There is also a decrease in ethylene content of the whole product with increasing TEA amount. Copolymerization behaviors of the TEA/TIBA mixture activated catalysis systems are not simple superposition of those activated by pure TEA and TIBA. When a 50/50 TEA/TIBA mixture was used as cocatalyst, the copolymerization activity became the highest, and yields of both the random copolymer part and the segmented copolymer part are close to the highest level. On the other hand, both parts of the copolymer produced with a 50/50 TEA/TIBA mixture are relatively more blocky than the products of TEA or TIBA systems, and difference in ethylene content between the random part and the segmented part was the smallest. The segmented copolymer part of three typical samples was further fractionated by temperature-gradient extraction fractionation into fractions of different ethylene content and sequence distribution. Changing TEA content in the cocatalyst exerted strong influences also on the fraction distribution of the segmented part of copolymer.     

Novel Aluminoxanes Prepared from AlEt_3/Al(i-Bu)_3 Mixture as Cocatalyst for Metallocene Catalyzed Ethylene Polymerization

Methylalundnoxane(MAO)isthemostimportantcocatalystformetallocenecatalyzedolefinpolymerization.Intheearlyyearsofthefieldithadbeentheonlycocatalystwhichcanactivatemetallocenecatalyststoveryhighactivity.MAOispreparedbypartialhydrolysisofAl(CHs)s(TMA)undercontrolledconditions1.TMAisanextremelyreactiveanddangerousreagent,andthismakesMAOapreciouschendcalreagent.MAOaccountsforalargepercentageinthecostofthemetallocenecatalystsystem,becauseahighMAO/metalloceneratioisneededforpoIymerization'.I…     

Branching ratios in the hydroxyl reaction with propene

The branching ratios for the reactions of attachment of hydroxyl radical to propene and hydrogen-atom abstraction were measured at 298 K over the buffer gas pressure range 60-400 Torr (N(2)) using a subatmospheric pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass spectrometer. Isotopically enriched water H(2)(18)O was used to produce (18)O-labeled hydroxyl radicals in reaction with fluorine atoms. The β-hydroxypropyl radicals formed in the attachment reactions 1a and 1b , OH + C(3)H(6) → CH(2)(OH)C(?)HCH(3) (eq 1a ) and OH + C(3)H(6) → C(?)H(2)CH(OH)CH(3) (eq 1b ), were converted to formaldehyde and acetaldehyde in a sequence of secondary reactions in O(2)- and NO-containing environment. The (18)O-labeling propagates to the final products, allowing determination of the branching ratio for the attachment channels of reaction 1. The measured branching ratio for attachment is β(1b) = k(1b)/(k(1a) + k(1b)) = 0.51 ± 0.03, independent of pressure over the 60-400 Torr pressure range. An upper limit on the hydrogen-abstraction channel, OH + C(3)H(6) → H(2)O + C(3)H(5) (eq 1c ), was determined by measuring the water yield in reactions of OH and OD radicals (produced via H(D) + NO(2) → OH(OD) + NO reactions) with C(3)H(6) as k(1c)/(k(1a) + k(1b) + k(1c)) < 0.05 (at 298 K, 200 Torr N(2)).     

One-electron oxidation of acetohydroxamic acid: the intermediacy of nitroxyl and peroxynitrite

The pharmacological effects of hydroxamate derivatives have been attributed not only to metal chelation or enzyme inhibition but also to their ability to serve as nitroxyl (HNO/NO(-)) and nitric oxide (NO) donors. However, the mechanism underlying the formation of these reactive nitrogen species is not clear and requires further elucidation. In the present study, one-electron oxidation of acetohydroxamic acid (aceto-HX) by (?)OH, (?)N(3), (?)NO(2), CO(3)(?-), and O(2)(?-) radicals was investigated using pulse radiolysis. It is demonstrated that only (?)OH, (?)N(3), and CO(3)(?-) radicals attack effectively and selectively the deprotonated form of the hydroxamate moiety, yielding the respective transient nitroxide radical. This nitroxide radical is a weak acid (CH(3)C(O)NHO(?), pK(a) = 9.1), which decays via a pH-dependent second-order reaction, 2k(2CH(3)C(O)NO(?-)) = (5.6 ± 0.4) × 10(7) M(-1) s(-1) (I = 0.002 M), 2k(CH(3)C(O)NO(?-) + CH(3)C(O)NHO(?)) = (8.3 ± 0.5) × 10(8) M(-1) s(-1)), and 2k(2CH(3)C(O)NHO(?)) = (8.7 ± 1.3) × 10(7) M(-1) s(-1). The second-order decomposition of the nitroxide yields transient species, one of which decomposes via a first-order reaction whose rate increases linearly upon increasing [CH(3)C(O)NHO(-)] or [OH(-)]. One-electron oxidation of aceto-HX under anoxia does not give rise to nitrite even after exposure to O(2), indicating that NO is not formed during the decomposition of the nitroxide radical. The presence of oxidants such as Tempol or O(2) during CH(3)C(O)NO(?-) decomposition had no effect on the reaction kinetics. Nevertheless, in the presence of Temopl, which does not react with NO but does with HNO, the formation of the hydroxylamine Tempol-H was observed. In the presence of O(2), about 60% of CH(3)C(O)NO(?-) yields ONOO(-), indicating that 30% NO(-) is formed in this system. It is concluded that under pulse radiolysis conditions, the transient nitroxide radicals derived from one-electron oxidation of aceto-HX decompose bimoleculary via a complex mechanism forming nitroxyl rather than NO.     

Improvement of Structure and Properties of Polypropylene/Poly(ethylene-co-propylene) In-reactor Alloy by Modifying the Cocatalyst

Summary : A series of polypropylene/poly(ethylene-co-propylene) in-reactor alloy were synthesized by a TiCl₄/MgCl₂/SiO₂/diester type Ziegler-Natta catalyst, using triethylaluminium (TEA), triisobutylaluminium (TIBA) or TEA/TIBA mixtures of different molar ratio as cocatalyst. Mechanical properties of the alloy are strongly influenced by the cocatalyst. Toughness-stiffness balance of the alloy synthesized using a 50/50 TEA/TIBA mixture as cocatalyst is much better than that of the alloy based on pure TEA cocatalyst. Changes in copolymer chain structure and composition distribution are thought to be the main reason for this improvement of properties.     

Unusually fast 1,6-h shifts of enolic hydrogens in peroxy radicals: formation of the first-generation C2 and C3 carbonyls in the oxidation of isoprene

In a theoretical investigation using the CBS-QB3//UB3LYP/6-31+G** method supported by higher-level computations such as CBS-QB3//UQCISD/6-31+G**, the 1,6-H shifts of the enolic hydrogen in peroxy radicals of the type Z-HO-CH═CH-CH(2)-OO(?) were found to face exceptionally low energy barriers of only about 11 kcal mol(-1)--i.e., 6-9 kcal mol(-1) lower than the barriers for similar shifts of alkane hydrogens--such that they can proceed at unequaled rates of order 10(5) to 10(6) s(-1) at ambient temperatures. The unusually low barriers for enolic 1,6-H shifts in peroxy radicals, characterized here for the first time to our knowledge, are rationalized. As cases in point, the secondary peroxy radicals Z-HO-CH═C(CH(3))-CH(OO(?))-CH(2)OH (case A) and Z-HO-CH═CH-C(CH(3))(OO(?))-CH(2)OH (case B) derived from the primary Z-δ-hydroxy-peroxy radicals in the oxidation of isoprene, are predicted to undergo 1,6-H shifts of their enolic hydrogens at TST-calculated rates in the range 270-320 K of k(T)(A) = 5.4 × 10(-4) × T(5.04) × exp(-1990/T) s(-1) and k(T)(B) = 109 × T(3.13) × exp(-3420/T) s(-1), respectively, i.e., 2.0 × 10(6) and 6.2 × 10(4) s(-1), respectively, at 298 K, far outrunning in all relevant atmospheric and laboratory conditions their reactions with NO proposed earlier as their dominant pathways (Dibble J. Phys. Chem. A 2004, 108, 2199). These fast enolic-H shifts are shown to provide the explanation for the first-generation formation of methylglyoxal + glycolaldehyde, and glyoxal + hydroxyacetone in the oxidation of isoprene under high-NO conditions, recently determined by several groups. However, under moderate- and low-NO atmospheric conditions, the fast interconversion and equilibration of the various thermally labile, initial peroxy conformers/isomers from isoprene and the isomerization of the initial Z-δ-hydroxy-peroxy radicals, both recently proposed by us (Peeters et al. Phys. Chem. Chem. Phys. 2009, 11, 5935), are expected to substantially reduce the yields of the small carbonyls at issue.     

Isothermal evaporation of ethanol in a dynamic gas atmosphere

Optimization of evaporation and pyrolysis conditions for ethanol are important in carbon nanotube (CNT) synthesis. The activation enthalpy (ΔH(?)), the activation entropy (ΔS(?)), and the free energy barrier (ΔG(?)) to evaporation have been determined by measuring the molar coefficient of evaporation, k(evap), at nine different temperatures (30-70 °C) and four gas flow rates (25-200 mL/min) using nitrogen and argon as carrier gases. At 70 °C in argon, the effect of the gas flow rate on k(evap) and ΔG(?) is small. However, this is not true at temperatures as low as 30 °C, where the increase of the gas flow rate from 25 to 200 mL/min results in a nearly 6 times increase of k(evap) and decrease of ΔG(?) by ~5 kJ/mol. Therefore, at 30 °C, the effect of the gas flow rate on the ethanol evaporation rate is attributed to interactions of ethanol with argon molecules. This is supported by simultaneous infrared spectroscopic analysis of the evolved vapors, which demonstrates the presence of different amounts of linear and cyclic hydrogen bonded ethanol aggregates. While the amount of these aggregates at 30 °C depends upon the gas flow rate, no such dependence was observed during evaporation at 70 °C. When the evaporation was carried out in nitrogen, ΔG(?) was almost independent of the evaporation temperature (30-70 °C) and the gas flow rate (25-200 mL/min). Thus the evaporation of ethanol in a dynamic gas atmosphere at different temperatures may go via different mechanisms depending on the nature of the carrier gas.     

Ultrafast intersystem crossing and spin dynamics of zinc meso-tetraphenylporphyrin covalently bound to stable radicals

tert-Butylphenylnitroxide (BPNO(?)) and α,γ-bisdiphenylene-β-phenylallyl (BDPA(?)) stable radicals are each attached to zinc meso-tetraphenylporphyrin (ZnTPP) at a fixed distance using one of the ZnTPP phenyl groups. BPNO(?) and BDPA(?) are oriented para (1 and 3, respectively) or meta (2 and 4, respectively) relative to the porphyrin macrocycle. Following photoexcitation of 1-4, transient optical absorption spectroscopy is used to observe excited state quenching of (1)*ZnTPP by the radicals and time-resolved electron paramagnetic resonance (TREPR) spectroscopy is used to monitor the spin dynamics of the paramagnetic product states. The presence of BPNO(?) or BDPA(?) accelerates the intersystem crossing rate of (1)*ZnTPP about 10- to 500-fold in 1-4 depending on the structure compared to that of (1)*ZnTPP itself. In addition, the lifetime of (3)*ZnTPP in 1 is shorter than that of (3)*ZnTPP itself as a result of enhanced intersystem crossing (EISC) from (3)*ZnTPP to the ground state. The TREPR spectra of the three unpaired spins produced within 1 and 2 show spin-polarized excited doublet (D(1)) and quartet (Q) states and subsequent formation of a spin-polarized ground state radical (D(0)). All three signals are absorptive for 1 and emissive for 2. Polarization inversion of the Q state is observed on a tens of nanoseconds time scale in 2, while no polarization inversion is observed for 1. The lack of polarization inversion in 1 is attributed to the short lifetime of the doublet-quartet manifold as a result of the very large exchange interaction. The TREPR spectra of 3 and 4 show ground state radical polarization at X-band (9.5 GHz) at room temperature, but not at 85 K, and similarly no polarization is observed at W-band (94 GHz). No evidence of excited doublet or quartet states is observed, indicating that the exchange interaction is both weak and temperature dependent. These results show that although ultrafast EISC produces (3)*ZnTPP within 1-4, the magnitude of the exchange interactions between the three relevant spins in the resulting (3)*ZnTPP-BPNO(?) and (3)*ZnTPP-BDPA(?) systems dramatically alters their spin dynamics.     

Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides: absolute rate constants derived from pulse radiolysis and laser flash photolysis

The intramolecular reaction of cysteine thiyl radicals with peptide and protein alphaC-H bonds represents a potential mechanism for irreversible protein oxidation. Here, we have measured absolute rate constants for these reversible hydrogen transfer reactions by means of pulse radiolysis and laser flash photolysis of model peptides. For N-Ac-CysGly6 and N-Ac-CysGly2AspGly3, Cys thiyl radicals abstract hydrogen atoms from Gly with k(f) = (1.0-1.1 x 10(5) s(-1), generating carbon-centered radicals, while the reverse reaction proceeds with k(r) = (8.0-8.9) x 10(5) s(-1). The forward reaction shows a normal kinetic isotope effect of k(H)/k(D) = 6.9, while the reverse reaction shows a significantly higher normal kinetic isotope effect of 17.6, suggesting a contribution of tunneling. For N-Ac-CysAla2AspAla3, cysteine thiyl radicals abstract hydrogen atoms from Ala with k(f) = (0.9-1.0) x 10(4) s(-1), while the reverse reaction proceeds with k(r) = 1.0 x 10(5) s(-1). The order of reactivity, Gly > Ala, is in accord with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that k(f) < k(r) suggests some secondary structure of the model peptides, which prevents the adoption of extended conformations, for which calculations of homolytic bond dissociation energies would have predicted k(f) > k(r). Despite k(f) < k(r), model calculations show that intramolecular hydrogen abstraction by Cys thiyl radicals can lead to significant oxidation of other amino acids in the presence of physiologic oxygen concentrations.     

Kinetic study of the thermal inactivation of glucose oxidase in the presence of denaturant and stabilizer by means of bioelectrocatalysis method

The thermal inactivation of glucose oxidase (GOD) in aqueous solution has been studied by the electrochemical method to follow the bioelectrocatalytic current due to the oxidation of glucose by GOD. Exponential time-dependent decrease in bioelectrocatalytic current, that is, the decrease in the enzymatic activity of GOD, was observed at given temperatures to determine the rate constant (k) of a simple inactivation process: GOD (active) → GOD (inactive). The ln[k] vs. T(-1) plots gave straight lines with all solution conditions tested, so that the resulting Arrhenius activation parameters including ΔH(?) and ΔS(?) can be compared with each other. In the 50 mmol/L phosphate buffer at 70°C, k was determined to be (6.6 ± 1.6)× 10(-4) s(-1), and ΔH(?) and ΔS(?) were calculated to be 202 ± 13 kJ mol(-1) and 282 ± 39 J K(-1) mol(-1), respectively. By addition of 3 mol/L guanidine hydrochloride, the k was increased to (4.7 ± 0.6)× 10(-3) s(-1), indicating that the denaturant accelerates the thermal inactivation. In this case, ΔH(?) was significantly reduced. By addition of 1 g/L ε-poly-L-lysine, which may adsorb onto the GOD surface to reduce the local disorder, k was decreased to (1.8 ± 0.6)× 10(-4) s(-1). In this case, ΔS(?) was reduced but ΔH(?) was not decreased much. This can be used as an important indication for selection of the enzyme stabilizer in solution.     

Ab Initio Kinetics for Thermal Decomposition of CH(3)N(•)NH(2), cis-CH(3)NHN(•)H, trans-CH(3)NHN(•)H, and C(•)H(2)NNH(2) Radicals

The thermal decomposition of the CH(3)N(?)NH(2), cis-CH(3)NHN(?)H, trans-CH(3)NHN(?)H, and C(?)H(2)NNH(2) radicals, which are the four radical products from the H-abstraction reactions of monomethylhydrazine, were theoretically studied by using ab initio Rice-Ramsperger-Kassel-Marcus (RRKM) transition-state theory and master equation analysis. Various decomposition pathways were identified by using either the QCISD(T)/cc-pV∞Z//CASPT2/aug-cc-pVTZ or the QCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) quantum chemistry methods. The results reveal that the β-scission of NH(2) to form methyleneimine is the predominant channel for the decomposition of the C(?)H(2)NNH(2) radical due to its small energy barrier of 13.8 kcal mol(-1). The high pressure limit rate coefficient for the reaction is fitted by 3.88 × 10(19)T(-1.672) exp(-9665.13/T) s(-1). In addition, the pressure dependent rate coefficients exhibit slight temperature dependence at temperatures of 1000-2500 K. The cis-CH(3)NHN(?)H and trans-CH(3)NHN(?)H radicals are the two distinct spatial isomers with an energy barrier of 26 kcal mol(-1) for their isomerization. The β-scission of CH(3) from the cis-CH(3)NHN(?)H radical to form trans-diazene has an energy barrier of 35.2 kcal mol(-1), and the β-scission of CH(3) from the trans-CH(3)NHN(?)H radical to form cis-diazene has an energy barrier of 39.8 kcal mol(-1). The CH(3)N(?)NH(2) radical undergoes the β-scission of methyl hydrogen and amine hydrogen to form CH(2)═NNH(2), trans-CH(3)N═NH, and cis-CH(3)N═NH products, with the energy barriers of 42.8, 46.0, and 50.2 kcal mol(-1), respectively. The dissociation and isomerization rate coefficients for the reactions were calculated via the E/J resolved RRKM theory and multiple-well master equation analysis at temperatures of 300-2500 K and pressures of 0.01-100 atm. The calculated rate coefficients associated with updated thermochemical property data are essential components in the development of kinetic mechanisms for the pyrolysis and oxidation of MMH and its derivatives.     

Role of water and carbonates in photocatalytic transformation of CO2 to CH4 on titania

Using the electron paramagnetic resonance technique, we have elucidated the multiple roles of water and carbonates in the overall photocatalytic reduction of carbon dioxide to methane over titania nanoparticles. The formation of H atoms (reduction product) and (?)OH radicals (oxidation product) from water, and CO(3)(-) radical anions (oxidation product) from carbonates, was detected in CO(2)-saturated titania aqueous dispersion under UV illumination. Additionally, methoxyl, (?)OCH(3), and methyl, (?)CH(3), radicals were identified as reaction intermediates. The two-electron, one-proton reaction proposed as an initial step in the reduction of CO(2) on the surface of TiO(2) is supported by the results of first-principles calculations.     

CRYSTAL STRUCTURE OF OXAZOLIDINE-2-THIONE

<正> Crystals of. oxazolidine-2-thione, G3H5SNO, are triclinic,P1, a = 5.738(3), b = 5.928(4), c = 7.763(5) A, (?) = 83.05(6)°, α= 67.68(5)°, r = 77.85(5)°, Z = 2, V = 235.8A3, Dx = 1.45 g.cm-3, P(000) = 108, Mr = 103.14, m.p. = 96.2°-97.2°C, (MoK(?)) = 0.71069A, T = 293K, μ(MoK(?)) =0.051 mm-1. The structure was solved by direct methods and. difference Fourier synthesis. The final R was 0.033 for 655 unique observed reflections. It is a new hydrolysis and rearranging product of 2-hydryoxyl ethyl glucosinolate from the seeds of Copparis masaikai Levl.. The study of its physiological activity is in progress.     

13C, 18O, and D fractionation effects in the reactions of CH3OH isotopologues with Cl and OH radicals

A relative rate experiment is carried out for six isotopologues of methanol and their reactions with OH and Cl radicals. The reaction rates of CH2DOH, CHD2OH, CD3OH, (13)CH3OH, and CH3(18)OH with Cl and OH radicals are measured by long-path FTIR spectroscopy relative to CH3OH at 298 +/- 2 K and 1013 +/- 10 mbar. The OH source in the reaction chamber is photolysis of ozone to produce O((1)D) in the presence of a large excess of molecular hydrogen: O((1)D) + H2 --> OH + H. Cl is produced by the photolysis of Cl2. The FTIR spectra are fitted using a nonlinear least-squares spectral fitting method with measured high-resolution infrared spectra as references. The relative reaction rates defined as alpha = k(light)/k(heavy) are determined to be: k(OH + CH3OH)/k(OH + (13)CH3OH) = 1.031 +/- 0.020, k(OH + CH3OH)/k(OH + CH3(18)OH) = 1.017 +/- 0.012, k(OH + CH3OH)/k(OH + CH2DOH) = 1.119 +/- 0.045, k(OH + CH3OH)/k(OH + CHD2OH) = 1.326 +/- 0.021 and k(OH + CH3OH)/k(OH + CD3OH) = 2.566 +/- 0.042, k(Cl + CH3OH)/k(Cl + (13)CH3OH) = 1.055 +/- 0.016, k(Cl + CH3OH)/k(Cl + CH3(18)OH) = 1.025 +/- 0.022, k(Cl + CH3OH)/k(Cl + CH2DOH) = 1.162 +/- 0.022 and k(Cl + CH3OH)/k(Cl + CHD2OH) = 1.536 +/- 0.060, and k(Cl + CH3OH)/k(Cl + CD3OH) = 3.011 +/- 0.059. The errors represent 2sigma from the statistical analyses and do not include possible systematic errors. Ground-state potential energy hypersurfaces of the reactions were investigated in quantum chemistry calculations at the CCSD(T) level of theory with an extrapolated basis set. The (2)H, (13)C, and (18)O kinetic isotope effects of the OH and Cl reactions with CH3OH were further investigated using canonical variational transition state theory with small curvature tunneling and compared to experimental measurements as well as to those observed in CH4 and several other substituted methane species.     

Probing lipid peroxidation by using linoleic acid and benzophenone

A thorough mechanistic study has been performed on the reaction between benzophenone (BZP) and a series of 1,4-dienes, including 1,4-cyclohexadiene (CHD), 1,4-dihydro-2-methylbenzoic acid (MBA), 1,4-dihydro-1,2-dimethylbenzoic acid (DMBA) and linoleic acid (LA). A combination of steady-state photolysis, laser flash photolysis (LFP), and photochemically induced dynamic nuclear polarization (photo-CIDNP) have been used. Irradiation of BZP and CHD led to a cross-coupled sensitizer-diene product, together with 6, 7, and 8. With MBA and DMBA as hydrogen donors, photoproducts arising from cross-coupling of sensitizer and diene radicals were found; compound 7 was also obtained, but 6 and o-toluic acid were only isolated in the irradiation of BZP with MBA. Triplet lifetimes were determined in the absence and in the presence of several diene concentrations. All three model compounds showed similar reactivity (k(q) ≈10(8) M(-1) s(-1)) towards triplet excited BZP. Partly reversible hydrogen abstraction of the allylic hydrogen atoms of CHD, MBA, and DMBA was also detected by photo-CIDNP on different timescales. Polarizations of the diamagnetic products were in full agreement with the results derived from LFP. Finally, LA also underwent partly reversible hydrogen abstraction during photoreaction with BZP. Subsequent hydrogen transfer between primary radicals led to conjugated derivatives of LA. The unpaired electron spin population in linoleyl radical (LA(.)) was predominantly found on H(1-5) protons. To date, LA-related radicals were only reported upon hydrogen transfer from highly substituted model compounds by steady-state EPR spectroscopy. Herein, we have experimentally established the formation of LA(.) and shown that it converts into two dominating conjugated isomers on the millisecond timescale. Such processes are at the basis of alterations of membrane structures caused by oxidative stress.     

Switch-over in photochemical reaction mechanism from hydrogen abstraction to exciplex-induced quenching: interaction of triplet-excited versus singlet-excited acetone versus cumyloxyl radicals with amines

The fluorescence and phosphorescence quenching of acetone by 13 aliphatic amines has been investigated. The bimolecular rate constants lie in the range of 10(8)-10(9) M(-1) s(-1) for singlet-excited acetone and 10(6)-10(8) M(-1) s(-1) for the triplet case. The rate data indicate that a direct hydrogen abstraction process dominates for triplet acetone, while a charge-transfer mechanism, namely, exciplex-induced quenching, becomes important for singlet-excited acetone. Pronounced stereoelectronic effects toward H abstraction, e.g., for 1,4-diazabicyclo[2.2.2]octane (DABCO), and significant steric hindrance effects, e.g., for N,N-diisopropyl-3-pentylamine, are observed. A negative activation energy (E(a) = -0.9 +/- 0.2 kcal mol(-1) for triethylamine and DABCO) and the absence of a significant solvent effect on the fluorescence quenching of acetone are indicative of the involvement of exciplexes. Full electron transfer can be ruled out on the basis of the low reduction potential of acetone, which was found to lie below -3.0 V versus SCE. The participation of H abstraction for triplet acetone is corroborated by the respective quenching rate constants, which resemble the reaction rate constants for cumyloxyl radicals. The latter were measured for all 13 amines and showed also a dependence on the electron donor properties of the amines. It is suggested that the H abstraction proceeds directly and not through an exciplex or ion pair. Further, abstraction from N-H bonds in addition to alpha C-H bonds has been corroborated as a significant pathway for excited acetone. Product studies and quantum yields for photoreduction of singlet- and triplet-excited acetone by triethylamine (8% for S(1) versus 24% for T(1)) are in line with the suggested mechanisms of quenching through an exciplex and photoreduction through direct H abstraction.