Ultraviolet photochemistry of trichlorovinylsilane and allyltrichlorosilane: vinyl radical (HCCH2) and allyl radical (H2CCHCH2) production in 193 nm photolysis

The absolute gas phase ultraviolet absorption spectra of trichlorovinylsilane and allyltrichlorosilane have been measured from 191 to 220 nm. Over this region the absorption spectra of both species are broad and relatively featureless, and their cross sections increase with decreasing wavelength. The electronic transitions of trichlorovinylsilane were calculated by ab initio quantum chemical methods and the observed absorption bands assigned to the A(1)A'<-- X[combining tilde](1)A' transition. The maximum absorption cross section in the region, at 191 nm, is sigma = (8.50 +/- 0.06) x 10(-18) cm(2) for trichlorovinylsilane and sigma = (2.10 +/- 0.02) x 10(-17) cm(2) for allyltrichlorosilane. The vinyl radical and the allyl radical are formed promptly from the 193 nm photolysis of their respective trichlorosilane precursors. By comparison of the transient visible absorption and the 1315 nm I atom absorption from 266 nm photolysis of vinyl iodide and allyl iodide, the absorption cross sections at 404 nm of vinyl radical ((2.9 +/- 0.4) x 10(-19) cm(2)) and allyl radical ((3.6 +/- 0.8) x 10(-19) cm(2)) were derived. These cross sections are in significant disagreement with literature values derived from kinetic modeling of allyl or vinyl radical self-reactions. Using these cross sections, the vinyl radical yield from trichlorovinylsilane was determined to be phi = (0.9 +/- 0.2) per 193 nm photon absorbed, and the allyl radical yield from allyltrichlorosilane phi = (0.7 +/- 0.2) per 193 nm photon absorbed.     

Photohydroxylation of 1,4-benzoquinone in aqueous solution revisited

In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH(2) which are formed with a quantum yield of Phi=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (lambda(max)=282 and approximately 410 nm) which is the precursor of these products increases nonlinearly (k=(2-->3.8) x 10(6) s(-1)) with increasing Q concentration ((0.2-->10) mM). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q(2)* (K=5500+/-1000 M(-1)). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)(3), Q(2)* decays by electron transfer and water addition yielding benzosemiquinone (.)QH and (.)OH adduct radicals (.)QOH. The latter enolizes to the 2-hydroxy-1,4-semiquinone radical (.)Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of (.)QH. On the post-millisecond time scale, that is, when (.)QH has decayed, Ph(OH)(3) is oxidized by Q yielding HOQ and QH(2) as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. This shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a non-radical one at low Q concentration. In agreement with this, the yield of Ph(OH)(3) is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H(2)PO(4) (-) giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that (.)OH is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the (.)OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q(2)* oxidizes DMPO (k approximately 1 x 10(8) M(-1) s(-1)) to its radical cation which subsequently reacts with water. Q*/Q(2)* react with alcohols by H abstraction (rates in units of M(-1) s(-1)): methanol (4.2 x 10(7)), ethanol (6.7 x 10(7)), 2-propanol (13 x 10(7)) and tertiary butyl alcohol ( approximately 0.2 x 10(7)). DMSO (2.7 x 10(9)) and O(2) ( approximately 2 x 10(9)) act as physical quenchers.     

Photoionization versus photoheterolysis of all-trans-retinol. The effects of solvent and laser radiation intensity

The time-resolved formation of the retinyl carbocation from all-trans-retinol and all-trans-retinol acetate was studied by use of picosecond flash photolysis. From both precursors, the retinyl cation is produced by heterolytic C-O bond cleavage in solvents of medium polarity (acetonitrile, tetrahydrofuran, propanol with Reichardt polarity parameter ET(N) approximately 0.5) and high polarity (EtOH, MeOH, TFE, HFIP, ET(N) > 0.6) during the laser pulse (< or =5 ps) where its lifetime is >10 ns. The absorption maximum of the cation at early times (t < 100 ps) is at lambda = 590-600 nm; it shifts to shorter wavelengths (Deltalambda = 5-10 nm) within 1-10 ns. This spectral shift is suggested to be due to contact ion pair --> solvent-separated ion pair --> free-ion transformation. The quantum yield of cation formation phi(cat) is independent of excitation wavelength (213, 266 or 355 nm). Photoheterolysis proceeds via a one-quantum process. In chlorinated solvents, i.e. n-BuCl, 1,2-dichloroethane, chloroform or CCl(4), formation of the retinol radical cation (which is characterized by a peak at 610 nm and further absorption maxima at approximately 840 and approximately 940 nm) by intermolecular electron transfer to the solvent molecules was detected. The radical cation lifetime in all these solvents is 1.5-2 ns, except for CCl(4) where it is 0.25 ns. The formation of the radical cation or cation was not detected in the low polarity solvents: cyclohexane, hexane, dioxane and p-xylene. However, in solvents of medium and high polarity, at high radiation intensities the radical cation may form in addition to the cation (as a result of two-quantum ionization). DFT calculations confirm our experimental results. The rate of retinol S(1) depopulation (k = 0.3-1 x 10(9) s(-1)) is almost independent of the solvent polarity in the range from cyclohexane to methanol. In highly polar solvents (ET(N) > 0.9) the rate increases to (0.5-5) x 10(10) s(-1).     

Coumarin 314 free radical cation: formation, properties, and reactivity toward phenolic antioxidants

We have explored the photogeneration of the coumarin 314 radical cation by using nanosecond laser excitation at wavelengths longer than 400 nm in benzene, acetonitrile, dichloromethane, and aqueous media. In addition, time-resolved absorption spectroscopy measurements allowed detection of the triplet excited state of coumarin 314 (C(314)) with a maximum absorption at 550 nm in benzene. The triplet excited state has a lifetime of 90 μs in benzene. It is readily quenched by oxygen (k(q) = 5.0 × 10(9) M(-1) s(-1)). From triplet-triplet energy transfer quenching experiments, it is shown that the energy of this triplet excited state is higher than 35 kcal/mol, in accord with the relatively large singlet oxygen quantum yield (Φ(Δ) = 0.25). However, in aqueous media, the coumarin triplet was no longer observed, and instead of that, a long-lived (160 μs in air-equilibrated solutions) free radical cation with a maximum absorbance at 370 nm was detected. The free radical cation generation, which has a quantum yield of 0.2, occurs by electron photoejection. Moreover, density functional theory (DFT) calculations indicate that at least 40% of the electronic density is placed on the nitrogen atom in aqueous media, which explains its lack of reactivity toward oxygen. On the other hand, rate constant values close to the diffusion rate limit in water (>10(9) M(-1) s(-1)) were found for the quenching of the C(314) free radical cation by phenolic antioxidants. The results have been interpreted by an electron-transfer reaction between the phenolic antioxidant and the radical cation where ion pair formation could be involved.     

Scavenging of acetylperoxyl radicals and quenching of triplet diacetyl by beta-carotene: mechanisms and kinetics

Beta-carotene scavenges triplet diacetyl generated by laser flash photolysis with a second-order rate constant of 9.1+/-0.9 x 10(9) M(-1) s(-1) in deaerated benzene at 20 degrees C. In the presence of oxygen diacetyl dissociates to generate acetylperoxyl radicals. It is demonstrated that diacetyl does not dissociate to any appreciable extent in the absence of oxygen. The acetylperoxyl radical is scavenged by beta-carotene with second-order rate constant 9.2+/-0.6 x 10(8) M(-1) s(-1) in aerated benzene at 20 degrees C to give an adduct between the acetylperoxyl radical and beta-carotene, whereas no evidence of oxidation of beta-carotene by the strongly oxidizing acetylperoxyl radical to give the beta-carotene radical cation is found. This adduct decays with first-order rate constant 1.35+/-0.16 x 10(3) s(-1) to give (presumably) a beta-carotene epoxide and the acetyloxyl radical.     

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.     

Electron transfer within 2,7-dinitronaphthalene radical anion

The optical spectrum of 2,7-dinitronaphthalene radical anion generated by Na(Hg) reduction in acetonitrile containing a large excess of cryptand[2.2.2] exhibits a Hush-type intervalence charge-transfer band at 1070 nm, estimated to correspond to an off-diagonal matrix coupling element of 310 cm(-)(1). The interpolated rate constant for intramolecular electron transfer at 293 K measured by ESR between 225 and 320 K for this solution is 3.1(+/-0.2) x 10(9) s(-)(1). Rate constants estimated in two ways from the optical parameters using the Marcus-Hush assumption that the diabatic surfaces should be parabolae are 1.0 and 0.11 x 10(9) s(-1), and those using diabatic surfaces that fit the observed charge-transfer band are 9.6 and 3.4 x 10(9) s(-)(1), when used with an electron-transfer distance on the adiabatic surfaces of 6.42 A. Similar measurements and comparisons were also carried out using dimethylformamide and butyronitrile as solvents. The success of simple, classical two-state Marcus-Hush theory precludes an electron-hopping mechanism. UHF calculations predict a planar unsymmetrical gas-phase structure for 1,3-dinitrobenzene radical anion but give serious spin contamination. Semiempirical AM1 calculations using singles excitation configuration interaction with an active space of 70 orbitals and the COSMO solvent model also give a planar unsymmetrical structure. These calculations make the internal vibrational component of the reorganization energy nearly constant, and much smaller than the solvent reorganizational component, and predict the transition energy to lie between that observed in acetonitrile (9360 cm(-1)) and those observed in dimethylformamide (8100 cm(-1)) and butyronitrile (8040 cm(-1)).     

Kinetics and mechanism of *NO2 reacting with various oxidation states of myoglobin

Nitrogen dioxide ((*)NO(2)) participates in a variety of biological reactions. Of great interest are the reactions of (*)NO(2) with oxymyoglobin and oxyhemoglobin, which are the predominant hemeproteins in biological systems. Although these reactions occur rapidly during the nitrite-catalyzed autoxidation of hemeproteins, their roles in systems producing (*)NO(2) in the presence of these hemeproteins have been greatly underestimated. In the present study, we employed pulse radiolysis to study directly the kinetics and mechanism of the reaction of oxymyoglobin (MbFe(II)O(2)) with (*)NO(2). The rate constant of this reaction was determined to be (4.5 +/- 0.3) x 10(7) M(-1)s(-1), and is among the highest rate constants measured for (*)NO(2) with any biomolecule at pH 7.4. The interconversion among the various oxidation states of myoglobin that is prompted by nitrogen oxide species is remarkable. The reaction of MbFe(II)O(2) with (*)NO(2) forms MbFe(III)OONO(2), which undergoes rapid heterolysis along the O-O bond to yield MbFe(V)=O and NO(3-). The perferryl-myoglobin (MbFe(V)=O) transforms rapidly into the ferryl species that has a radical site on the globin ((*)MbFe(IV)=O). The latter oxidizes another oxymyoglobin (10(4) M(-1)s(-1) < k(17) < 10(7) M(-1)s(-1)) and generates equal amounts of ferrylmyoglobin and metmyoglobin. At much longer times, the ferrylmyoglobin disappears through a relatively slow comproportionation with oxymyoglobin (k(18) = 21.3 +/- 5.3 M(-1)s(-1)). Eventually, each (*)NO(2) radical converts three oxymyoglobin molecules into metmyoglobin. The same intermediate, namely MbFe(III)OONO(2), is also formed via the reaction peroxynitrate (O(2)NOO(-)/O(2)NOOH) with metmyoglobin (k(19) = (4.6 +/- 0.3) x 10(4) M(-1)s(-1)). The reaction of (*)NO(2) with ferrylmyoglobin (k(20) = (1.2 +/- 0.2) x 10(7) M(-1)s(-1)) yields MbFe(III)ONO(2), which in turn dissociates (k(21) = 190 +/- 20 s(-1)) into metmyoglobin and NO(3-). This rate constant was found to be the same as that measured for the decay of the intermediate formed in the reaction of MbFe(II)O(2) with (*)NO, which suggests that MbFe(III)ONO(2) is the intermediate observed in both processes. This conclusion is supported by thermokinetic arguments. The present results suggest that hemeproteins may detoxify (*)NO(2) and thus preempt deleterious processes, such as nitration of proteins. Such a possibility is substantiated by the observation that the reactions of (*)NO(2) with the various oxidation states of myoglobin lead to the formation of metmyoglobin, which, though not functional in the gas transport, is nevertheless nontoxic at physiological pH.     

Studies of the Anodic Oxidation of 1,4-Diazabicyclo

The title compound (1) was studied at platinum and gold electrodes in acetonitrile. A reversible oxidation peak occurs at +0.30 V vs the standard potential for ferrocenium ion/ferrocene. This process is followed by a second irreversible anodic peak that is due to the oxidation of the initially formed radical cation to the dication. The principal ultimate product of the first oxidation, the conjugate acid of 1, is also oxidized over the range of potentials corresponding to the second anodic peak. The rate of disappearance of the radical cation of 1 has been determined by cyclic voltammetry. The results are best interpreted in terms of parallel pseudo-first-order decay (k(1) = 0.6 s(-)(1)) and second-order reactions. The first of these second-order reactions is either proton transfer from the radical cation to neutral 1 or hydrogen atom abstraction by the radical cation from neutral 1, reactions that give the same products (k(2) = 100 M(-)(1) s(-)(1)) and are kinetically indistinguishable. The other second-order reaction is the hydrogen-atom-transfer disproportionation of the radical cation giving the conjugate acid of 1 and the immonium ion (k(3) = 100 M(-)(1) s(-)(1)). Both second-order processes must be included to account for the results. The present results are thought to be the first experimental evidence for the occurrence of hydrogen-atom-transfer disproportionation of amine radical cations.     

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.     

Photooxidation of guanine by a ruthenium dipyridophenazine complex intercalated in a double-stranded polynucleotide monitored directly by picosecond visible and infrared transient absorption spectroscopy

Transient species formed by photoexcitation (400 nm) of [Ru(dppz)(tap)2]2+ (1) (dppz = dipyrido[3,2-a:2',3'-c]phenazine; tap=1,4,5,8-tetraazaphenanthrene) in aqueous solution and when intercalated into a double-stranded synthetic polynucleotide, [poly(dG-dC)]2, have been observed on a picosecond timescale by both visible transient absorption (allowing monitoring of the metal complex intermediates) and transient infrared (IR) absorption spectroscopy (allowing direct study of the DNA nucleobases). By contrast with its behavior when free in aqueous solution, excitation of 1 when bound to [poly(dG-dC)]2 causes a strong increase in absorbance at 515 nm due to formation of the reduced complex [Ru(dppz)(tap)2]+ (rate constant=(2.0+/-0.2) x 10(9) s(-1)). The subsequent reformation of 1 proceeds with a rate constant of (1.1+/-0.2) x 10(8) s(-1). When the process is carried out in D2O, the rates of formation and removal of [Ru(dppz)(tap)2]+ are reduced (rate constants (1.5+/-0.3) x 10(9) and (0.7+/-0.2) x 10(8) s(-1) respectively) consistent with proton-coupled electron transfer processes. Picosecond transient IR measurements in the 1540-1720 cm(-1) region in D2O solution confirm that the reduction of 1 intercalated into [poly(dG-dC)]2 is accompanied by bleaching of IR ground-state bands of guanine (1690 cm(-1)) and cytosine (1656 cm(-1)), each with similar rate constants.     

Bimolecular hole transfer from the trimethoxybenzene radical cation in the excited state

Bimolecular hole transfer quenching of the 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) by hole quenchers (Q) such as biphenyl (Bp), naphthalene (Np), anisole (An), and benzene (Bz) with higher oxidation potentials than that of TMB was directly observed during the two-color two-laser flash photolysis at room temperature. From the linear relationships between the inverse of the transient absorption changes of TMB*+ during the second 532-nm laser excitation versus the inverse of the concentration of Q, the rate constant of the hole transfer from TMB*+* to Q was estimated to be (8.5 +/- 0.4) x 10(10), (1.4 +/- 0.7) x 10(11), (1.3 +/- 0.6) x 10(11), and (6.4 +/- 0.3) x 10(10) M(-1)s(-1) for Bp, An, Np, and Bz, respectively, in acetonitrile based on the lifetime of TMB*+*. The estimated rate constants are larger than the diffusion-controlled rate constant in acetonitrile. Short lifetime, high energy, and high oxidation potential of TMB*+* cause the lifetime-dependent quenching process or static quenching process as the major process during the quenching of TMB*+* by Q as indicated by the Ware's theoretical model. The subsequent hole transfer from Q*+ to TMB, giving TMB*+, was found to occur at the diffusion-controlled rate for Bp and An as Q. For Q such as Np and Bz, the dimerization of Q*+ with Q to give dimer radical cation (Q2*+) occurred competitively with the hole transfer from Q*+ to TMB.     

Direct observation of a carbene-alcohol ylide

Carboethoxycarbene reacts with methanol-OD to form an ylide. The formation and decay of this ylide was monitored by ultrafast time-resolved IR spectroscopy. The formation and decay of the ylide is linearly dependent on the concentration of methanol-OD in acetonitrile with second-order rate constants of ylide formation (8.4 × 10(9) M(-1) s(-1)) and decay (1.4 × 10(9) M(-1) s(-1)). Similar results were obtained with 1-butanol.     

Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.     

Primary photophysical properties of moxifloxacin--a fluoroquinolone antibiotic

Abstract The photophysical properties of the fluoroquinolone antibiotic moxifloxacin (MOX) were investigated in aqueous media. MOX in water, at pH 7.4, shows two intense absorption bands at 287 and 338 nm (epsilon = 44 000 and 17 000 dm(3) mol(-1) cm(-1), respectively). The absorption and emission properties of MOX are pH-dependent, pK(a) values for the protonation equilibria of both the ground (6.1 and 9.6) and excited singlet states (6.8 and 9.1) of MOX were determined spectroscopically. MOX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Phosphorescence from the excited triplet state in frozen ethanol solution has a quantum yield of 0.046. Laser flash photolysis and pulse radiolysis studies have been carried out to characterize the transient species of MOX in aqueous solution. On laser excitation, MOX undergoes monophotonic photoionization with a quantum yield of 0.14. This leads to the formation of a long-lived cation radical whose absorption is maximum at 470 nm (epsilon(470) = 3400 dm(3) mol(-1) cm(-1)). The photoionization process releases hydrated electron which rapidly reacts (k = 2.8 x 10(10) dm(3) mol(-1) s(-1)) with ground state MOX, yielding a long-lived anion radical with maximum absorption at 390 nm (epsilon(390) = 2400 dm(3) mol(-1) cm(-1)). The cation radical of MOX is able to oxidize protein components tryptophan and tyrosine. The bimolecular rate constants for these reactions are 2.3 x 10(8) dm(3) mol(-1) s(-1) and 1.3 x 10(8) dm(3) mol(-1) s(-1), respectively. Singlet oxygen sensitized by the MOX triplet state was also detected only in oxygen-saturated D(2)O solutions, with a quantum yield of 0.075.     

Kinetics and mechanism of methane, methanol, and dimethyl ether C-H activation with electrophilic platinum complexes

The relative rates of C-H activation of methane, methanol, and dimethyl ether by [(N-N)PtMe(TFE-d(3))](+) ((N-N) = ArN=C(Me)-C(Me)=NAr; Ar = 3,5-di-tert-butylphenyl, TFE-d(3) = CF(3)CD(2)OD) (2(TFE)) were determined. Methane activation kinetics were conducted by reacting 2(TFE)-(13)C with 300-1000 psi of methane in single-crystal sapphire NMR tubes; clean second-order behavior was obtained (k = 1.6 +/- 0.4 x 10(-3) M(-1) s(-1) at 330 K; k = 2.7 +/- 0.2 x 10(-4) M(-1) s(-1) at 313 K). Addition of methanol to solutions of 2(TFE) rapidly establishes equilibrium between methanol (2(MeOD)) and trifluoroethanol (2(TFE)) adducts, with methanol binding preferentially (K(eq) = 0.0042 +/- 0.0006). C-H activation gives [(N-N)Pt(CH(2)OD)(MeOD)](+) (4), which is unstable and reacts with [(RO)B(C(6)F(5))(3)](-) to generate a pentafluorophenyl platinum complex. Analysis of kinetics data for reaction of 2 with methanol yields k = 2.0 +/- 0.2 x 10(-3) M(-1) s(-1) at 330 K, with a small kinetic isotope effect (k(H)/k(D) = 1.4 +/- 0.1). Reaction of dimethyl ether with 2(TFE) proceeds similarly (K(eq) = 0.023 +/- 0.002, 313 K; k = 5.5 +/- 0.5 x 10(-4) M(-1) s(-1), k(H)/k(D) = 1.5 +/- 0.1); the product obtained is a novel bis(alkylidene)-bridged platinum dimer, [(diimine)Pt(mu-CH(2))(mu-(CH(OCH(3)))Pt(diimine)](2+) (5). Displacement of TFE by a C-H bond appears to be the rate-determining step for all three substrates; comparison of the second-order rate constants (k((methane))/k((methanol)) = 1/1.3, 330 K; k((methane))/k((dimethy)(l e)(ther)) = 1/2.0, 313 K) shows that this step is relatively unselective for the C-H bonds of methane, methanol, or dimethyl ether. This low selectivity agrees with previous estimates for oxidations with aqueous tetrachloroplatinate(II)/hexachloroplatinate(IV), suggesting a similar rate-determining step for those reactions.     

One-electron oxidation of alcohols by the 1,3,5-trimethoxybenzene radical cation in the excited state during two-color two-laser flash photolysis

One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*.     

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.     

Rates of electronic excitation hopping in anisotropic ionic crystals of [Ru(2,2'-bipyridine)3]X2 (X = ClO4-, PF6-, SbF6-); Monte Carlo simulation of single- and multi-exponential emission decays

Emission decays of triplet metal-to-ligand charge transfer states in anisotropic crystals of [Ru(1 - x)Os(x)(bpy)(3)]X(2) (bpy = 2,2'-bipyridine, X = PF(6)-, ClO(4)-, SbF(6)-, and 0.115 > x > 0.001) at approximately 300 K were measured by means of time-correlated single-photon counting. Rates of excitation hopping calculated on the basis of an interaction between transition dipoles of a donor cation and an acceptor cation are insufficient to simulate the single-exponential decays (x = 0.0099) and the multiexponential decays (x = 0.060 and 0.115) of the PF(6)- salt crystals. A limiting rate of excitation hopping to an imaginary cation at the van der Waals distance via a super-exchange interaction between d orbitals through the bpy ligands was determined to be 0.83 x 10(10) s(-1) on average by means of a step-by-step Monte Carlo simulation, assuming an distance-attenuation factor, beta, of the exchange interaction of 10 nm-1. The total rate of excitation hopping via both a dipole-dipole mechanism and a super-exchange mechanism to the neighboring sites of the cation was calculated to be 5.4 x 10(9) s(-1) for the PF(6)- crystal. Anisotropic diffusion constants estimated from the hopping rates and lengths in the PF(6)- crystal are 9.3 x 10(-6), 9.1 x 10(-6), and 1.4 x 10(-6) cm(2)s(-1) along the a axis, the b axis, and the c axis, respectively, which are compared with an isotropic diffusion constant, 1.3 x 10(-6) cm(2) s(-1), estimated from the pseudo-bimolecular rate constant of excitation transfer to [Os(bpy)(3)](2+), using an isotropic Smoluchowski equation. A multiexponential emission decay of [Ru(0.885)Os(0.115)(bpy)(3)](PF(6))(2) was also simulated to determined the limiting rate of excitation transfer to [Os(bpy)(3)](2+) at the van der Waals distance (2.6 x 10(11) s(-1)). The magnitude of beta determined is 6.5 and 11.5 nm(-1) for the ClO(4)- and the SbF(6)- salt crystals, respectively, on reference to that of beta (10 nm(-1)) for the PF(6)- salt crystal.