Type I and type II photosensitization by the antibacterial drug nalidixic acid. A laser flash photolysis study.

The 355 nm laser flash photolysis of nalidixic acid at pH 9.2 leads to the formation of the nalidixate anion triplet state (absorption lambda max = 620 nm; 5700 less than or equal to epsilon T less than or equal to 9000 M-1cm-1; 0.6 less than or equal to phi T less than or equal to 1). The first order triplet state decay (kT = 7.7 x 10(3) s-1) is accompanied by a diffusion controlled triplet-triplet annihilation. Oxygen efficiently quenches the triplet state (k = 3.2 x 10(9) M-1s-1). The nalidixate radical dianion (absorption lambda max = 650 nm; epsilon = 3000 M-1cm-1) is produced by the diffusion controlled reductive quenching of the triplet state by tryptophan and tyrosine. The superoxide anion (O2-.) is produced by diffusion controlled reaction of the radical dianion with oxygen. The O2-. is characterized by its reactions with ferricytochrome c and superoxide dismutase. The physiological form of nalidixic acid is thus a good Type I and Type II photosensitizer.     

Simultaneous determination of ethanol and glycerol in wines by a flow injection-pervaporation approach with in parallel photometric and fluorimetric detection

A simultaneous method for the determination of ethanol and glycerol in wines based on the coupling of pervaporation-chemical derivatisation-photometric detection for ethanol and biochemical derivatisation-fluorimetric detection for glycerol is proposed. After separation by pervaporation the ethanol is collected in a K(2)Cr(2)O(7) acceptor stream and the Cr(3+) formed is driven to the spectrophotometer and monitored at 600 nm. The determination of glycerol is based on its oxidation by oxidised beta-nicotinamide adenine dinucleotide (beta-NAD(+)) catalysed by glycerol dehydrogenase immobilised on controlled-pore glass, the reduced form of the coenzyme (NADH) being spectrofluorimetrically monitored (lambda(ex)=340 nm, lambda(em)=460 nm). The linear determination range is between 1-20% for ethanol and 2-8 g l(-1) for glycerol, with RSDs 3 and 2%, respectively. The method applied to red and white Spanish wines, compares well with the official methods for these analytes.     

Effect of structural modification on photodynamic activity of hypocrellins

Aluminum ion complexed 5,8-di-Br-hypocrellin B is a new water-soluble perylenequinonoid derivative with enhanced absorption over hypocrellin B (HB) in the phototherapeutic window (600-900 nm). Electron paramagnetic resonance and 9,10-diphenyl-anthracene bleaching methods were used to investigate the photosensitizing activity of [AL2(5,8-di-Br-HB)Cl4]n in the presence of oxygen. Singlet oxygen, superoxide anion radical and hydroxyl radical can be generated by [AL2(5,8-di-Br-HB)CL4]n photosensitization. Singlet oxygen (1O2) is formed via energy transfer from triplet-state [AL2(5,8-di-Br-HB)CL4]n to ground-state molecular oxygen. 1O2 participates in the generation of a portion of superoxide anion radical (O2.-). Besides superoxide anion radical (O2.-) may originate from the electron transfer between the triplet-state [AL2(5,8-di-Br-HB)CL4]n and the ground-state molecular oxygen. OH is formed through the Fenton-Haber-Weiss reaction and the decomposition of DMPO-1O2 adduct. Compared with HB [AL2(5,8-di-Br-HB)CL4]n primarily remains and enhances the generation efficiency of superoxide anion radical and hydroxyl radical but that of singlet oxygen decreases.     

Vitamin B6 (pyridoxine) and its derivatives are efficient singlet oxygen quenchers and potential fungal antioxidants

Vitamin B6 (pyridoxine, 1) and its derivatives: pyridoxal (2), pyridoxal 5-phosphate (3) and pyridoxamine (4) are important natural compounds involved in numerous biological functions. Pyridoxine appears to play a role in the resistance of the filamentous fungus Cercospora nicotianae to its own abundantly produced strong photosensitizer of singlet molecular oxygen (1O2), cercosporin. We measured the rate constants (kq) for the quenching of 1O2 phosphorescence by 1-4 in D2O. The respective total (physical and chemical quenching) kq values are: 5.5 x 10(7) M-1 s-1 for 1; 7.5 x 10(7) M-1 s-1 for 2, 6.2 x 10(7) M-1 s-1 for 3 and 7.5 x 10(7) M-1 s-1 for 4, all measured at pD 6.2. The quenching efficacy increased up to five times in alkaline solutions and decreased approximately 10 times in ethanol. Significant contribution to total quenching by chemical reaction(s) is suggested by the degradation of all the vitamin derivatives by 1O2, which was observed as declining absorption of the pyridoxine moiety upon aerobic irradiation of RB used to photosensitize 1O2. This photodegradation was completely stopped by azide, a known physical quencher of 1O2. The pyridoxine moiety can also function as a redox quencher for excited cercosporin by forming the cercosporin radical anion, as observed by electron paramagnetic resonance. All B6 vitamers fluoresce upon UV excitation. Compounds 1 and 4 emit fluorescence at 400 nm, compound 2 at 450 nm and compound 3 at 550 nm. The fluorescence intensity of 3 increased approximately 10 times in organic solvents such as ethanol and 1,2-propanediol compared to aqueous solutions, suggesting that fluorescence may be used to image the distribution of 1-4 in Cercospora to understand better the interactions of pyridoxine and 1O2 in the living fungus.     

THE PEROXIDASE/OXIDASE ACTIVITY OF SOYBEAN LIPOXYGENASE – II. TRIPLET CARBONYLS AND RED PHOTOEMISSION DURING POLYUNSATURATED FATTY ACID AND GLUTATHIONE OXIDATION

During the aerobic reaction of soybean lipoxygenase with polyunsaturated fatty acids (linoleic, linolenic, and arachidonic acid) oxygen uptake is followed by excited carbonyl photoemission. The chemiluminescence yield of phi cl = 10(-10) photons/O2 molecule consumed is enhanced 2-3 orders of magnitude by the carbonyl sensitizers 9,10-dibromo-anthracene-2-sulfonate (kET tau 0 = 10(4) M-1; phi cl = 10(-8) photons/O2) and chlorophyll-a (kET tau 0 = 10(6) M-1; phi cl = 10(-7) photons/O2), respectively. alpha,beta-Saturated triplet excited carbonyls as from 1,2-dioxetane cleavage are discussed to arise from a secondary peroxidase/oxidase reaction with aldehydes formed in the course of enzymic lipid peroxidation. When 1 mM glutathione is added to the aerobic lipoxygenase/arachidonate reaction, carbonyl emission (375-455 nm) is replaced by intense red bands (630-645 nm and 695-715 nm) resembling the characteristic spectrum of (1 delta g)O2-singlet oxygen dimol-emission. The quantum yield (phi cl = 10(-8) photons/O2) remains unaffected by chlorophyll indicating that the red emission is independent of excited carbonyls. The effect of GSH is attributed to dioxetane interception and subsequent glutathione peroxidation generating 1O2 by electron transfer from the superoxide anion radical to a peroxysulfenyl radical.     

Kinetic study of the electron-transfer oxidation of the phenolate anion of a vitamin E model by molecular oxygen generating superoxide anion in an aprotic medium

Electron-transfer reduction of molecular oxygen (O2) by the phenolate anion (1-) of a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), occurred to produce superoxide anion, which could be directly detected by a low-temperature EPR measurement. The rate of electron transfer from 1- to O2 was relatively slow, since this process is energetically unfavourable. The one-electron oxidation potential of 1- determined by cyclic voltammetric measurements is sufficiently negative to reduce 2,2-bis(4-tert-octylphenyl)-1-picrylhydrazyl radical (DOPPH*) to the corresponding one-electron reduced anion, DOPPH-, suggesting that 1- can also act as an efficient radical scavenger.     

Transients in the oxidative and H-atom-induced degradation of 1,3,5-trithiane. Time-resolved studies in aqueous solution

The (*)OH-induced oxidation of 1,3,5-trithiacyclohexane (1) in aqueous solution was studied by means of pulse radiolysis with optical and conductivity detection. This oxidation leads, via a short-lived (*)OH radical adduct (<1 micros), to the radical cation 1(*+) showing a broad absorption with lambda(max) equal to 610 nm. A defined pathway of the decay of 1(*+) is proton elimination. It occurs with k = (2.2 +/- 0.2) x 10(4) s(-1) and yields the cyclic C-centered radical 1(-H)(*). The latter radical decays via ring opening (beta-scission) with an estimated rate constant of about 10(5) s(-1). A distinct, immediate product (formed with the same rate constant) is characterized by a narrow absorption band with lambda(max) = 310 nm and is attributed to the presence of a dithioester function. The formation of the 310 nm absorption can be suppressed in the presence of oxygen, the rationale for this being a reaction of the C-centered cyclic radical 1(-H)(*) with O(2). The disappearance of the 310 nm band (with a rate constant of 900 s(-1)) is associated with the hydrolysis of the dithioester functionality. A further aspect of this study deals with the reaction of H(*) atoms with 1 which yields a strongly absorbing, three-electron-bonded 2sigma/1sigma* radical cation [1(S therefore S)-H](+) (lambda(max) = 400 nm). Its formation is based on an addition of H(*) to one of the sulfur atoms, followed by beta-scission, intramolecular sulfur-sulfur coupling (constituting a ring contraction), and further stabilization of the S therefore S bond thus formed by protonation. [1(S therefore S)-H](+) decays with a first-order rate constant of about 10(4) s(-1). Its formation can be suppressed by the addition of oxygen which scavenges the H(*) atoms prior to their reaction with 1. Complementary time-resolved conductivity experiments have provided information on the quantification of the 1(*+) radical cation yield, the cationic longer-lived follow-up species, extinction coefficients, and kinetics concerning deprotonation processes as well as further reaction steps after hydrolysis of the transient dithioesters. The results are also discussed in the light of previous photochemical studies.     

Photoreactivity of biologically active compounds. XVI. Formation and reactivity of free radicals in mefloquine

The formation and reactivity of the triplet state and free radicals of mefloquine hydrochloride (MQ) have been investigated by pulse radiolysis and flash photolysis. The excited triplet, cation radical and anion radical have been produced and their absorption characteristics determined. The triplet-triplet absorption spectrum of MQ showed a maximum at 430 nm, with a molar absorption coefficient of 3600 M(-1) cm(-1) and the quantum yield for intersystem crossing was determined to be close to unity. Deactivation of the triplet, in the absence of oxygen, led to the formation of MQ cation and/or anion radicals. The molar absorption coefficient of the cation radical at 330 nm was determined to be 2300 M(-1) cm(-1), whilst that for the anion radical was 2400 M(-1) cm(-1) at 620 nm and 3600 M(-1) cm(-1) at 350 nm. The molar absorption coefficients of the proposed neutral radical at 320 nm and 520 nm were 4000 M(-1) cm(-1) and 1300 M(-1) cm(-1) respectively. The quantum yield for the formation of singlet oxygen, sensitized by MQ triplet, was determined to be close to unity. Aqueous solutions of MQ were found to photoionize to yield hydrated electron and cation radical of MQ in a biphotonic process. The influences of pH, buffer concentration, oxygen concentration and addition of sodium azide on the formation and reactivity of the transients were evaluated. The reactions between MQ and solvated electrons and superoxide anion were also studied.     

Photodynamic action of amino substituted hypocrellins: EPR studies on the photogenerations of active oxygen and free radical species

A novel method has been employed to prepare 2-butylamino-2-demethoxy hypocrellin A (BADMHA) and 2-butylamino-2-demethoxy hypocrellin B (BADMHB). Both compounds exhibit stronger absorption at the phototherapeutic window (600-900 nm). The spin trapping and spin counteraction studies have shown that they are both efficient generators of the active oxygen (1O2, O2*-) in the aerobic condition. Under the anaerobic condition they generate non-oxygen free radical (semiquinone radical anion), and the active oxygen mechanism of photosensitization can be converted into non-oxygen free radical mechanism with the depletion of oxygen. The quantum yields of 1O2 generation of BADMHA and BADMHB are 0.46 and 0.44, respectively. Both are lower than those of their parent compounds HA and HB. But the productions of superoxide anion are enhanced significantly compared with HA and HB, indicating they are both favorable Type I phototherapeutic agents.     

Identification and reactivity of the triplet excited state of 5-hydroxytryptophan

Both the neurotransmitter serotonin and the unnatural amino acid 5-hydroxytryptophan (5HT), contain the 5-hydroxyindole chromophore. The photochemistry of 5HT is being investigated in relation to the multiphoton excitation of this chromophore to produce a characteristic photoproduct with green fluorescence ('hyperluminescence'). Laser flash photolysis (308 nm) of 5HT in aqueous solution at neutral pH produces both the neutral 5-indoloxyl radical (lambda(max) 400-420 nm) and another transient absorption with lambda(max) 480 nm and lifetime of 2 micros in deaerated solutions. Based on quenching by oxygen and beta-carotene, the species at 480 nm is identified as the triplet excited state of 5HT. In acidic solution a new oxygen-insensitive intermediate with lambda(max) 460 is assigned to the radical cation of 5HT. Time-resolved measurements of luminescence at 1270 nm have shown that the triplet state of 5HT is able to react with oxygen to form singlet excited oxygen (1O2*) with a quantum yield of approximately 0.1. However, 5HT has also been found to be an effective quencher of singlet oxygen with a second order rate constant of 1.3 x 10(8) dm3 mol(-1) s(-1). The results are discussed in the light of recent observations on the multiphoton-excited photochemistry of serotonin.     

Thermochromism of metal ion complexes of semiquinone radical anions. Control of equilibria between diamagnetic and paramagnetic species by Lewis acids

Metal ion complexes of semiquinone radical anions exhibit different types of thermochromism depending on metal ions and quinones. Metal ion complexes of 1,10-phenanthroline-5,6-dione radical anion (PTQ(.-)) produced by the electron-transfer reduction of PTQ by 1,1'-dimethylferrocene (Me(2)Fc) in the presence of metal ions (Mg(2+) and Sc(3+)) exhibit the color change depending on temperature, accompanied by the concomitant change in the ESR signal intensity. In the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is in equilibrium, when the concentration of the PTQ(.-)-Mg(2+) complex (lambda(max) = 486 nm) increases with increasing temperature because of the positive enthalpy for the electron-transfer equilibrium. In contrast to the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is complete in the presence of Sc(3+), which is a much stronger Lewis acid than Mg(2+), to produce the PTQ(.-)-Sc(3+) complex (lambda(max) = 631 nm). This complex is in disproportionation equilibrium and the concentration of the PTQ(.-)-Sc(3+) complex increases with decreasing temperature because of the negative enthalpy for the proportionation direction, resulting in the remarkable color change in the visible region. On the other hand, the p-benzosemiquinone radical anion (Q(.-)) forms a 2:2 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(2)-Q] with Q and Sc(3+) ions at 298 K (yellow color), which is converted to a 2:3 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(3)-Q] with a strong absorption band at lambda(max) = 604 nm (blue color) when the temperature is lowered to 203 K. The change in the number of binding Sc(3+) ions depending on temperature also results in the remarkable color change, associated with the change in the ESR spectra.     

The reactions of cytidine and 2'-deoxycytidine with SO4.- revisited. Pulse radiolysis and product studies

The reactions of SO4.- with 2'-deoxycytidine 1a and cytidine 1b lead to very different intermediates (base radicals with 1a, sugar radicals with 1b). The present study provides spectral and kinetic data for the various intermediates by pulse radiolysis as well as information on final product yields (free cytosine). Taking these and literature data into account allows us to substantiate but also modify in essential aspects the current mechanistic concept (H. Catterall, M. J. Davies and B. C. Gilbert, J. Chem. Soc., Perkin Trans. 2, 1992, 1379). SO4.- radicals have been generated radiolytically in the reaction of peroxodisulfate with the hydrated electron (and the H. atom). In the reaction of SO4.- with 1a (k = 1.6 x 10(9) dm3 mol-1 s-1), a transient (lambda max = 400 nm, shifted to 450 nm at pH 3) is observed. This absorption is due to two intermediates. The major component (lambda max approximately 385 nm) does not react with O2 and has been attributed to an N-centered radical 4a formed upon sulfate release and deprotonation at nitrogen. The minor component, rapidly wiped out by O2, must be due to C-centered OH-adduct radical(s) 6a and/or 7a suggested to be formed by a water-induced nucleophilic replacement. These radicals decay by second-order kinetics. Free cytosine is only formed in low yields (G = 0.14 x 10(-7) mol J-1 upon electron-beam irradiation). In contrast, 1b gives rise to an intermediate absorbing at lambda max = 530 nm (shifted to 600 nm in acid solution) which rapidly decays (k = 6 x 10(4) s-1). In the presence of O2, the decay is much faster (k approximately 1.3 x 10(9) dm3 mol-1 s-1) indicating that this species must be a C-centered radical. This has been attributed to the C(5)-yl radical 8 formed upon the reaction of the C(2')-OH group with the cytidine SO4(.-)-adduct radical 2b. This reaction competes very effectively with the corresponding reaction of water and the release of sulfate and a proton generating the N-centered radical. Upon the decay of 8, sugar radical 11 is formed with the release of cytosine. The latter is formed with a G value of 2.8 x 10(-7) mol J-1 (85% of primary SO4.-) at high dose rates (electron beam irradiation). At low dose rates (gamma-radiolysis) its yield is increased to 7 x 10(-7) mol J-1 due to a chain reaction involving peroxodisulfate and reducing free radicals. Phosphate buffer prevents the formation of the sugar radical at the SO4(.-)-adduct stage by enhancing the rate of sulfate release by deprotonation of 2b and also by speeding up the decay of the C(5)-yl radical into another (base) radical. Cytosine release in cytidine is mechanistically related to strand breakage in poly(C). Literature data on the effect of dioxygen on strand breakage yields in poly(C) induced by SO4.- (suppressed) and upon photoionisation (unaltered) lead us to conclude that in poly(C) and also in the present system free radical cations are not involved to a major extent. This conclusion modifies an essential aspect of the current mechanistic concept.     

Fluorescence quenching method for the determination of sodium carboxymethyl cellulose with acridine yellow or acridine orange

In near neutral to weak basic media, sodium carboxymethyl cellulose (NaCMC) will dissociate to become a macro polymeric anion, which can react with acridine yellow (AY) or acridine orange (AO) to form an ion-association complex resulting in fluorescence quenching of the acridine dyes. The maximum fluorescence quenching wavelength is 505 nm (lambda(ex)=440 nm) for AY system and 530 nm (lambda(ex)=493 nm) for AO system, respectively. The fluorescence quenching values (DeltaF) are directly proportional to the concentrations of NaCMC and the linear ranges are 20.0-4000 microg/L for AY system and 20.0-7000 microg/L for AO system, separately. This method has high sensitivity and the detection limits for NaCMC are 58.0 microg/L (AY system) and 157.2 microg/L (AO system). The effects of coexistent substance have been investigated, and the results show that this method has a relatively good selectivity. A fluorescence quenching method for the determination of NaCMC based on the ion-association reactions of CMC polymeric anion with a basic acridine dye was developed. The method is sensitive, simple and fast.     

Photoinduced adsorption of hydrogen and methane on gamma-alumina. the photoinduced chesorluminescence (PhICL) effect

Adsorption of hydrogen and methane on a preirradiated surface of gamma-Al2O3 produces an afterglow, which has been described as a photoinduced chesorluminescence (PhICL), whose spectral features identify with the intrinsic photoluminescence of alumina. The emission spectrum consists of at least four overlapping single emission bands. For methane adsorption, the PhICL phenomenon is seen only if the solid is preirradiated in the presence of oxygen. Emission decay kinetics of the PhICL effect for gamma-Al2O3 reveal two wavelength regimes: a short wavelength regime at lambda = 300-370 nm (decay time tau = 1.1 +/- 0.2 s; signal width = 2.8 s), and a longer wavelength regime at lambda = 380-700 nm (decay time tau = 2.1 +/- 0.1 s; signal width = 4.3 s). A model is proposed in which there exist two different emission centers and, thus, two different pathways for emission decay. In the first, emission originates with electron trapping by such deep energy traps as anion vacancies {e- + Va --> F+ + hv1} to yield electron F-type color centers, whereas in the second, emission originates from electron/trapped hole recombination {e- + Os*- --> Os2- + hv2}. The first common step of the pathways is homolytic dissociative chemisorption of hydrogen and methane upon interaction with surface-active hole centers Os*-, produced upon preirradiation of alumina, to give atomic hydrogen H* and methyl radicals CH3*. Thermoprogrammed desorption spectra of photoadsorbed or postsorbed oxygen show that adsorbed oxygen interacts with atomic hydrogen and methyl radicals. The products of thermodesorption were H2O for hydrogen and H2O, CO2, and CH3CH3 for methane. The Solonitsyn memory effect coefficient was also evaluated for oxygen photoadsorption.     

Kinetic analysis of superoxide anion radical-scavenging and hydroxyl radical-scavenging activities of platinum nanoparticles

There are few reports on the physiological effects of metal nanoparticles (nps), especially with respect to their functions as scavengers for superoxide anion radical (O2(.-)) and hydroxyl radical (.OH). We tried to detect the scavenging activity of Pt nps using a hypoxanthine-xanthine oxidase system for O2(.-) and using a Fenton and a UV/H2O2 system for .OH. Electron spin resonance analysis revealed that 2 nm particle size Pt nps have the ability to scavenge O2(.-) and .OH. The calculated rate constant for the O2(.-)-scavenging reaction was 5.03 +/- 0.03 x 10(7) M (-1) s (-1). However, the analysis of the Fenton and UV/H 2O 2 system in the presence of Pt nps suggested that the .OH-scavenging reaction cannot be determined in both systems. Among particle sizes tested from 1 to 5 nm, 1 nm Pt nps showed the highest O2(.-)-scavenging ability. Almost no cytotoxicity was observed even after adherent cells (TIG-1, HeLa, HepG2, WI-38, and MRC-5) were exposed to Pt nps at concentrations as high as 50 mg/L. Pt nps scavenged intrinsically generated reactive oxygen species (ROS) in HeLa cells. Additionally, Pt nps significantly reduced the levels of intracellular O2(.-) generated by UVA irradiation and subsequently protected HeLa cells from ROS damage-induced cell death. These findings suggest that Pt nps may be a new type of antioxidant capable of circumventing the paradoxical effects of conventional antioxidants.     

Activation of electron-transfer reduction of oxygen by hydrogen bond formation of superoxide anion with ammonium ion

A hydrogen bond formed between the superoxide anion and the ammonium ion (NH4+) accelerates electron transfer from the C60 radical anion to oxygen significantly, whereas the tetra-n-butylammonium ion has no ability to form a hydrogen bond with the superoxidie anion, exhibiting no acceleration of the electron-transfer reduction of oxygen. The second-order rate constant of electron transfer from C60*- to O2 increases linearly with increasing concentration of NH4+. This indicates that O2*- produced in the electron transfer from C60 to O2 is stabilized by 1:1 complex formation between O2*- and NH4+. The 1:1 complex formed between O2*- and NH4+ was detected by ESR. The binding of O2*- with NH4+ results in a positive shift of the reduction potential of O2 with increasing concentration of NH4+, leading to the acceleration of electron transfer from C60*- to O2.     

Photophysical properties of 2,5-diphenyl-1,6,6a-trithiapentalene revealed by time-resolved spectroscopy

The fluorescence decay from S2(pi, pi*) state of 2,5-diphenyl-1,6,6a-trithiapentalene (DP-TTP) in cyclohexane, tetrahydrofuran and acetonitrile solutions of a quantum yield of approximately 0.02-0.04 were measured. The results indicate that, the dominant process of radiationless deactivation of the S2 state, is internal conversion to the S1 state. Upon laser pulse excitation (lambda(ex) = 532 nm) from the S1(pi, pi*) state, DP-TTP in deoxygenated benzonitrile, acetonitrile, ethanol and tetrahydrofuran solutions give rise to transient triplet triplet absorption (lambdaTmax = 700-720 nm). Kinetic data are presented for intrinsic triplet lifetimes, self-quenching and quenching by oxygen.     

含柠檬酸配体的钼硫簇合物的合成与晶体结构的测定

合成了含有Mo—S簇阴离子 [Mo2 O2 ( μ S) 2 (C6H5O7) (C6H4 O7) ]5-柠檬酸簇合物 ,并通过元素分析、红外光谱、紫外 可见光谱和X射线光电子能谱对配合物进行了表征 ,用X射线衍射法测定了该化合物的晶体结构 .结果表明 ,该簇合物属于单斜晶系 ,空间群P2 1/c,a =2 3 766( 5)nm ,b =1 3 2 74 ( 3 )nm ,c =2 2 4 71 ( 5)nm ,α=γ =90°,β=1 1 8 2 1°,V =6 2 4 7( 2 )nm3 ,Z =8,一致性因子R =0 0 83 1 ,R2 ,w=0 1 536.该簇合物阴离子中每一个Mo原子都采取扭曲的八面体结构 ,柠檬酸通过羟基、α 羧基、一个 β 羧基与Mo配位 .晶体结构表明在一个结晶学上不对称的结构单元内有两个结晶学上独立的分子 .     

One-electron reduction of aqueous nitric oxide: a mechanistic revision

The pulse radiolysis of aqueous NO has been reinvestigated, the variances with the prior studies are discussed, and a mechanistic revision is suggested. Both the hydrated electron and the hydrogen atom reduce NO to yield the ground-state triplet (3)NO(-) and singlet (1)HNO, respectively, which further react with NO to produce the N(2)O(2)(-) radical, albeit with the very different specific rates, k((3)NO(-) + NO) = (3.0 +/- 0.8) x 10(9) and k((1)HNO + NO) = (5.8 +/- 0.2) x 10(6) M(-)(1) s(-)(1). These reactions occur much more rapidly than the spin-forbidden acid-base equilibration of (3)NO(-) and (1)HNO under all experimentally accessible conditions. As a result, (3)NO(-) and (1)HNO give rise to two reaction pathways that are well separated in time but lead to the same intermediates and products. The N(2)O(2)(-) radical extremely rapidly acquires another NO, k(N(2)O(2)(-) + NO) = (5.4 +/- 1.4) x 10(9) M(-)(1) s(-)(1), producing the closed-shell N(3)O(3)(-) anion, which unimolecularly decays to the final N(2)O + NO(2)(-) products with a rate constant of approximately 300 s(-)(1). Contrary to the previous belief, N(2)O(2)(-) is stable with respect to NO elimination, and so is N(3)O(3)(-). The optical spectra of all intermediates have also been reevaluated. The only intermediate whose spectrum can be cleanly observed in the pulse radiolysis experiments is the N(3)O(3)(-) anion (lambda(max) = 380 nm, epsilon(max) = 3.76 x 10(3) M(-)(1) cm(-)(1)). The spectra previously assigned to the NO(-) anion and to the N(2)O(2)(-) radical are due, in fact, to a mixture of species (mainly N(2)O(2)(-) and N(3)O(3)(-)) and to the N(3)O(3)(-) anion, respectively. Spectral and kinetic evidence suggests that the same reactions occur when (3)NO(-) and (1)HNO are generated by photolysis of the monoprotonated anion of Angeli's salt, HN(2)O(3)(-), in NO-containing solutions.     

Study of photoinduced N-hydroxy-arylnitroxide radicals (ArNO*OH) by time-resolved EPR

The photoreduction of aromatic nitro compounds by alcohols is a well-known reaction; however, the first stages of its mechanism remain controversial. This study aims at characterizing the "primary" radicalar transients involved in this reaction by EPR spectroscopy. Laser flash photolysis (lambda = 266 nm) of nitrobenzene, 5-nitrouracil, p-nitroacetophenone, o-propylnitrobenzene, and 2-nitroresorcinol in ethylene glycol was followed by time-resolved EPR spectroscopy. In all reported TR-EPR spectra, except those obtained from the photolysis of 2-nitroresorcinol, the key intermediate N-hydroxy-arylnitroxide radicals (ArNO*OH, 1-4) could be identified unambiguously. In 2-nitroresorcinol, the radical anion (ArNO*O(-), 5) and a sigma iminoxy radical (6) were observed, and a third radical (7) remains unidentified. These observations indicate that two radicalar mechanisms (by H* transfer and by electron transfer) are competing in the photoreduction mechanism. The attribution of the EPR spectra was helped by DFT calculations of the hyperfine coupling constants (hcc's).