p-Nitrobenzenesulfenate esters as precursors for laser flash photolysis studies of alkyl radicals

A series of p-nitrobenzenesulfenate esters was used in laser flash photolysis (LFP) studies to generate alkoxyl radicals that fragmented to give the (2,2-diphenylcyclopropyl)methyl radical. Rate constants for the beta-scission reactions increased as a function of the carbonyl compound produced in the fragmentation reaction in the order CH2O < MeCHO < Me2CO < PhCHO < Ph2CO and increased with increasing solvent polarity. For alkoxyl radicals that fragment to produce benzaldehyde and benzophenone, the beta-scission reactions are faster than 1,5-hydrogen atom abstractions when the incipient carbon radical is as stable as a secondary alkyl radical, and this entry to carbon radicals can be used in LFP kinetic studies.     

Photochemistry of carboxylic acids containing the phenyl and thioether groups: Steady-state and laser flash photolysis studies

The mechanisms for the direct photolysis of phenylthioacetic acid (PTAA) and S-benzylthioglycolic acid (SBTGA) in acetonitrile were investigated using steady-state and laser flash photolysis. For both compounds, a variety of stable photoproducts were found under steady-state, 254 nm irradiation of acetonitrile solutions. The products from the direct photolysis of PTAA included carbon dioxide (photodecarboxylation), acetic acid, diphenyl disulfide, diphenyl sulfide, thiophenol, thioanisole, di(phenylthio) methane, and S-phenyl benzenethiosulfate. The products from the direct photolysis of SBTGA included carbon dioxide, toluene, dibenzyl, dibenzyl sulfide, dibenzyl disulfide, thioglycolic acid, benzyl mercaptan, benzyl alcohol, and benzaldehyde. These stable photoproducts were identified and characterized using HPLC, GC, GC–MS, and UV–vis methods. Quantum yields were determined for the formation of the various stable products following steady-state irradiations in the absence and in the presence of oxygen. In laser flash photolysis (266 nm Nd:YAG laser) experiments, a variety of transients (e.g., phenylthiyl radical, benzyl radical, etc.) was found. For both substrates (PTAA and SBTGA), photoinduced CS bond cleavage was the main primary process. It was also found that for both acids, photoinduced CC bond cleavage occurred, but as a minor process relative to CS bond cleavage. Detailed mechanisms of the primary and secondary processes are proposed and discussed. The validity of these proposed mechanisms was supported by an analysis of the quantum yields of stable products and their transient precursors. Supplementary observations on reactions between the radicals (e.g., C₆H₅–S, C₆H₅–CH₂) and oxygen are also consistent with the proposed mechanisms.     

Photodissociation of naphthalene dimer radical cation during the two-color two-laser flash photolysis and pulse radiolysis-laser flash photolysis

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.     

Solvent effects on the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. A laser flash photolysis study

[reaction: see text] A laser flash photolysis study has been carried out to assess solvent effects on the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. The rearrangement rate constant k decreases by increasing solvent polarity and an excellent correlation with negative slope is obtained between log k and the solvent polarity parameter E(T)N. These evidences are in full agreement with the previous indication that the extent of internal charge separation decreases on going from the starting 1,1-diarylalkoxyl radical to the transition state.     

Reaction of thianthrene cation radical with grignard reagents : Evidence for electron transfer and trapping of alkyl radicals by the thianthrene cation radical

Reactions are reported between RMgCl and thianthrene cation radical perchlorate (Th^.+ClO^-₄) suspended in ether and tetrahydrofuran (THF). In ether solution reactions R = Bu, s-Bu, t-Bu, 5-hexenyl, and cyclopentylmethyl. Major products were the alkane, the alkene R(-H) in some cases, and, in the cases of R = Bu, 5-hexenyl, and cyclopentylmethyl, the 5-alkylthianthrenium perchlorate (ThR⁺ClO^-₄). When 5-hexenylMgCl was used a mixture of 5-(5-hexenyl)- and 5-(cyclopentylmethyl)thianthrenium per-chlorates in the ratio of approximately 2 was obtained. Since the ratio of 5-hexenyl/cyclopentylmethyl in the Grignard reagent was 10.4, it is concluded that the C₆ sulfonium ions were formed by radical trapping by Th^.+ after single electron transfer from Grignard to cation radical had occurred, thus allowing for cyclization of 5-hexenyl radical. Formation of ThBu⁺ClO^-₄ is attributed to the trapping of butyl radical by Th^·+, while formation of RH and R(-H) is in all cases also attributed to alkyl radical reactions. Reactions in THF(R = Me, i-Pr, Bu, s-Bu, t-Bu, Ph) led almost exclusively to RH and Th. Polymerization of THF was also initiated and took place slowly giving rise to low molecular weight poly(THF). By using THF-d₈, as solvent for reaction between BuMgCl and Th^.+, it was possible to find Bu groups (¹H-NMR) in the poly(THF-d₈). Polymerization of THF is attributed, in some cases (R = Me, Bu), to alkyl-cation transfer from ThR⁺ to THF. In other cases initiation of polymerization by R⁺ and THF(-H)⁺ is considered.     

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.     

On the electrophilicity of hydroxyl radical: a laser flash photolysis and computational study

The rate coefficients for reactions of hydroxyl radical with aromatic hydrocarbons were measured in acetonitrile using a novel laser flash photolysis method. Comparison of kinetic data obtained in acetonitrile with those obtained in aqueous solution demonstrates an unexpected solvent effect on the reactivity of hydroxyl radical. In particular, reactions of hydroxyl radical with benzene were faster in water than in acetonitrile, and by a significant factor of 65. Computational studies, at the B3LYP and CBS-QB3 levels, have confirmed the rate enhancement of hydroxyl radical addition to benzene via calculation of the transition states in the presence of explicit solvent molecules as well as a continuum dielectric field. The origin of the rate enhancement lies entirely in the structures of the transition states and not in the pre-reactive complexes. The calculations reveal that the hydroxyl radical moiety becomes more anionic in the transition state and, therefore, looks more like hydroxide anion. In the transition states, solvation of the incipient hydroxide anion is more effective with water than with acetonitrile and provides the strong energetic advantage for a polar solvent capable of hydrogen bonding. At the same time, the aromatic unit looks more like the radical cation in the transition state. The commonly held view that hydroxyl radical is electrophilic in its reactions with DNA bases is, therefore, strongly dependent on the ability of the organic substrate to stabilize the resulting radical cation.     

Spectral properties and absolute rate constants for beta-scission of ring-substituted cumyloxyl radicals. A laser flash photolysis study

A laser flash photolysis study of the spectral properties and beta-scission reactions of a series of ring-substituted cumyloxyl radicals has been carried out. All cumyloxyl radicals display a broad absorption band in the visible region of the spectrum, which decays on the microsecond time scale, leading to a strong increase in absorption in the UV region of the spectrum, which is attributed to the corresponding acetophenone formed after beta-scission of the cumyloxyl radicals. The position of the visible absorption band is red-shifted by the presence of electron-donating ring substituents, while a blue-shift is observed in the presence of electron-withdrawing ring substituents, suggesting that + R ring substituents promote charge separation in the excited cumyloxyl radical through stabilization of the partial positive charge on the aromatic ring of an incipient radical zwitterion. Along this line, an excellent Hammett-type correlation between the experimentally measured energies at the visible absorption maxima of the cumyloxyl radicals and sigma(+) substituent constants is obtained. A red-shift is also observed on going from MeCN to MeCN/H(2)O for all cumyloxyl radicals, pointing toward a specific effect of water. The ring substitution does not influence to a significant extent the rate constants for beta-scission of the cumyloxyl radicals, which varies between 7.1 x 10(5) and 1.1 x 10(6) s(-1), a result that suggests that cumyloxyl radical beta-scission is not governed by the stability of the resulting acetophenone. Finally, k(beta) increases on going from MeCN to the more polar MeCN/H(2)O 1:1 for all cumyloxyl radicals, an observation that reflects the increased stabilization of the transition state for beta-scission through increased solvation of the incipient acetophenone product.     

SN1-type mechanism for the carbon-carbon bond cleavage of tetrakis(4-methylphenyl)ethanone cation radical. A laser flash photolysis study

On the basis of measurements of rate constants for carbon-carbon bond cleavage of tetrakis(4-methylphenyl)ethanone cation radicals generated by pulsed laser excitation with sensitizers in solution, an S_N1-type mechanism for the bond cleavage is proposed.     

Studies on reactivity of benzoyl and benzoylperoxy radicals produced by laser flash photolysis of dibenzoyldiazene in aerated solutions

Photolysis of dibenzoyldiazene gives benzoyl radicals. In aerated solutions, the benzoyl radicals react with oxygen to yield benzoylperoxy radicals. Spin trapping studies indicate that 5,5′dimethyl-1-pyrroline N-oxide reacts with the benzoylperoxy radicals to produce the adduct which exhibits ESR parameters, A_N = 13.8 G and A_Hβ = 10.1 G. Laser photolysis studies reveal that the rate constants for the reaction between the benzoyl radical and oxygen are ca. 4 × 10⁹ M^-1 s^-1 in toluene, acetone, and ethyl acetate. The benzoylperoxy radicals undergo one-electron oxidation of tetramethyl-p-phenylenediamine, TMPD, to give an ion pair. The ion pair has an absorption spectrum similar to that of the TMPD cation radical. The formation of the ion pair is detected by monitoring the absorbance change at 600 nm after laser pulsing. From the kinetic studies for the formation of the ion pair in the presence of olefins, the bimolecular rate constants for reactions between several olefins and the benzoylperoxy radical are determined. The electrophilic addition of the benzoylperoxy radicals to olefins is discussed in comparison with the addition reactions of thiyl radicals to olefins. The detection and determination of the dipole moments of both the benzoylperoxy radicals and the ion pair are carried out with the use of the time-resolved microwave dielectric absorption technique. The distance between the positive and negative ions in the ion pair is estimated as 0.20 nm.     

Kinetic studies on the temperature dependence of the BrO + BrO reaction using laser flash photolysis

The BrO self-reaction, BrO + BrO → products (1), has been studied using laser flash photolysis coupled with UV absorption spectroscopy over the temperature range T = 266.5-321.6 K, under atmospheric pressure. BrO radicals were generated via laser photolysis of Br(2) in the presence of excess ozone. Both BrO and O(3) were monitored via UV absorption spectroscopy using charge-coupled device (CCD) detection. Simultaneous fitting to both temporal concentration traces allowed determination of the rate constant of the two channels of , BrO + BrO → 2Br + O(2) (1a); BrO + BrO → Br(2) + O(2) (1b), hence the calculation of the overall rate of and the branching ratio, α: k(1a)/cm(3) molecule(-1) s(-1) = (1.92 ± 1.54) × 10(-12) exp[(126 ± 214)/T], k(1b)/cm(3) molecule(-1) s(-1) = (3.4 ± 0.8) × 10(-13) exp[(181 ± 70)/T], k(1)/cm(3) molecule(-1) s(-1) = (2.3 ± 1.5) × 10(-12) exp(134 ± 185 /T) and α = k(1a)/k(1) = (0.84 ± 0.09) exp[(-7 ± 32)/T]. Errors are 1σ, statistical only. Results from this work show a weaker temperature dependence of the branching ratio for channel (1a) than that found in previous work, leading to values of α at temperatures typical of the Polar Boundary Layer higher than those reported by previous studies. This implies a shift of the partitioning between the two channels of the BrO self-reaction towards the bromine atom and hence directly ozone-depleting channel (1a).     

Photophysical properties of the newly synthesized triad based on [70]fullerene studies with laser flash photolysis

N,N-Dimethylaniline-pyrazolinoC70-ferrocene has been prepared with the 1,3-dipolar cycloaddition reaction of a nitrile imine with C70. Although three regioisomers regarding the position of the pyrazolino group on the C70 were identified in the reaction products, molecular orbital calculations indicate that the stabilities and electronic properties of the three isomers are almost the same, which was confirmed by the sharp redox peaks. The photophysical properties of the triads have been investigated by measuring the time-resolved emission and transient absorption spectra showing that charge separation takes place efficiently via the photoexcited singlet state of the C70 moiety with accepting an electron from the donor moieties. It was found that the pyrazolino ring mediates a charge separation between the donor moieties and the photoexcited C70 moiety.     

The kinetics of the addition of alkyl radicals to carbonyl groups. II. The addition of methyl radicals to trifluoroacetone in the gas phase. The formation reaction of the trifluoro-t-butoxy radical

By photolyzing azomethane over the temperature range 331–491 K in the presence of trifluoroacetone the kinetics of the addition reaction (1), ?H₃ + CF₃COCH₃ → CF₃C(?)(CH₃)₂ have been studied. Detailed analyses have shown that the principal product of the adduct radical, CF₃C(?)(CH₃)₂, is CH₃COCH₃ from reaction (?2), CF₃C(?)(CH₃)₂ → CH₃COCH₃ + ?F₃. The rate constant of the addition reaction has been determined to be k₁(dm³/mol s) = (4.5 ± 1.4) × 10⁷ exp(-(3370 ± 120)/T) over the temperature range 331–491 K, based on the value k₃ = 2.2 × 10¹⁰ dm³/mol s for the reaction (3), 2?H₃ → C₂H₆. The results are discussed in relation to existing data for radical additions to groups.     

The kinetics of the addition of alkyl radicals to carbonyl groups. Part I. The addition of methyl radicals to hexafluoroacetone in the gas phase. The formation of the hexafluoro-t-butoxy radical

By pyrolyzing di-t-butyl peroxide over the temperature range of 405–450 K in the presence of hexafluoroacetone the kinetics of the addition reaction (1), CH₃ + (CF₃)₂CO→; (CF₃)₂C(?)CH₃, have been studied. Detailed analyses have shown that the principal product of the adduct radical, (CF₃)₂C(?)CH₃, is CF₃COCH₃ from reaction (2), (CF₃)₂C(?)CH₃ → CF₃COCH₃ + CF₃. The rate constant of the addition reaction was determined to be k₁(dm³/mol·s) = (1.1 ± 4.0) + 10⁹ exp(-(3680 ± 480)/T) over the temperature range 405–450 K, based on the value k₃ = 2.2 × 10¹⁰ dm³/mol·s for reaction (3), 2CH₃ → C₂H₆. The results are discussed in relation to existing data for radical additions to carbonyl groups.     

Steady-state and laser flash photolysis study of the carbon-carbon bond fragmentation reactions of 2-arylsulfanyl alcohol radical cations

The N-methylquinolinium tetrafluoroborate (NMQ(+))-sensitized photolysis of the erythro-1,2-diphenyl-2-arylsulfanylethanols 1-3 (1, aryl = phenyl; 2, aryl = 4-methylphenyl; 3, aryl = 3-chlorophenyl) has been investigated in MeCN, under laser flash and steady-state photolysis. Under laser irradiation, the formation of sulfide radical cations of 1-3, in the monomeric (lambda(max) = 520-540 nm) and dimeric form (lambda(max) = 720-->800 nm), was observed within the laser pulse. The radical cations decayed by first-order kinetics, and under nitrogen, the formation of ArSCH(*)Ph (lambda(max) = 350-360 nm) was clearly observed. This indicates that the decay of the radical cation is due to a fragmentation process involving the heterolytic C-C bond cleavage, a conclusion fully confirmed by steady-state photolysis experiments (formation of benzaldehyde and the dimer of the alpha-arylsulfanyl carbon radical). Whereas the fragmentation rate decreases as the C-C bond dissociation energy (BDE) increases, no rate change was observed by the replacement of OH by OD in the sulfide radical cation (k(OH)/k(OD) = 1). This suggests a transition state structure with partial C-C bond cleavage where the main effect of the OH group is the stabilization of the transition state by hydrogen bonding with the solvent. The fragmentation rate of 2-hydroxy sulfanyl radical cations turned out to be significantly slower than that of nitrogen analogues of comparable reduction potential, probably due to a more efficient overlap between the SOMO in the heteroatom and the C-C bond sigma-orbital in the second case. The fragmentation rates of 1(+*)-3(+*) were found to increase by addition of a pyridine, and plots of k(base) against base strength were linear, allowing calculation of the beta Bronsted values, which were found to increase as the reduction potential of the radical cation decreases, beta = 0.21 (3(+*)), 0.34 (1(+*)), and 0.48 (2(+*)). The reactions of 1(+*) exhibit a deuterium kinetic isotope effect with values that increase as the base strength increases: k(OH)/k(OD) = 1.3 (pyridine), 1.9 (4-ethylpyridine), and 2.3 (4-methoxypyridine). This finding and the observation that with the above three bases the rate decreases in the order 3(+*) > 1(+*) > 2(+*), i.e., as the C-C BDE increases, suggest that C-C and O-H bond cleavages are concerted but not synchronous, with the role of OH bond breaking increasing as the base becomes stronger (variable transition state). It is probable that, with the much stronger base, 4-(dimethylamino)pyridine, a change to a stepwise mechanism may occur where the slow step is the formation of a radical zwitterion that then rapidly fragmentates to products.     

Inter- and intramolecular effects of tyrosyl residues on flavin triplets and radicals as investigated by flash photolysis.

Abstract— Addition of tyrosine or derivatives to aqueous solutions of flavins does not significantly impede either formation of the flavin triplet or the rate of O₂ oxidation of the flavin radical generated by reaction of triplet with the phenol. However, the rate of radical decay is decreased. There is only a modest effect that results from altering the nature of the group on alkyl side chains of the flavin when the substituent, e.g. phenylalanine, does not complex avidly with the isoalloxazine system. However, when a tyrosyl or O-methyltyrosyl residue is covalently attached to an alkyl side chain at the N¹⁰-position of the flavin, the considerable intramolecular complexing that results markedly decreases the formation of flavin triplet and, therefore, the radical yield. The rate of triplet decay is not much different than for noninternally complexed flavins, but extensive intramolecular radical decay occurs, and the rate of 0₂ oxidation of radical is decreased. A shorter alkyl chain is more effective than a longer one for decreasing triplet production, but the greater proximity of a photooxidiz-able tyrosyl residue to the flavin nucleus within the former allows a slightly higher intramolecular radical yield. Attachment of a tyrosyl residue by a short chain from the N³-position of the flavin has only a modest effect on the production of flavin triplet and its decay. There is less radical production from internal than from external tyrosyl residues, and the rate of O₂ oxidation of the flavin radical generated by such intermolecular photoreductants as N-acetyl tyrosine ethyl ester or EDTA is somewhat decreased. The tyrosyl residue within the active-site peptide of mitochondrial monoamine oxidase is not so susceptible to photooxidation by the 8α-(S-L-cysteinyl)flavin involved, since the thioether linkage at this position severely reduces triplet production. Upon oxidation of the thioether to sulfone, however, the triplet yield is partially restored. Some flavin radical can then be generated from either the intra- or an intermolecular tyrosyl residue. Taken together, these results demonstrate that tyrosyl residues near the flavin-binding sites of flavo-proteins can become oxidized by the flavin triplet that is light-generated unless the proximity and steric disposition of the interactants is such as to allow dissipation of much of the energy as radiationless decay within a tight complex or unless an 8α-thioether linkage to the flavin coenzyme is involved. Also, flavin radicals, whether generated photochemically or by biochemical oxidation of substrate, are readily oxidized by O₂ in the presence of tyrosyl functions unless tight complexing occurs. More remarkable, though, is the decreased rate of radical decay conferred by the association with a tyrosyl residue. This stabilization of reactive flavin radicals may have considerable consequence in the catalytic mechanism of such enzymes.     

The flash photolysis of azomethane. Minor products from the photolysis of methyl radicals and a rate constant for CH3 + NO

The flash photolysis of azomethane in a quartz reaction vessel produces mainly ethane (>75%) plus smaller quantities of methane, ethylene, and acetylene. The minor products are interpreted quantitatively in terms of methyl radical photolysis at 216 nm to give CH₂ and H. This interpretation is substantiated by the dependence of the minor products on flash intensity. The reduction of the ethane yield on adding NO is employed to obtain a rate constant for CH₃ + NO as a function of total pressure, based on a value for methyl radical recombination of 4.2 × 10^?11 cm³/molec · sec. An RRKM analysis is used to extrapolate the data to give a limiting high-pressure rate constant for CH₃ + NO of (1.2 ± 0.1) × 10^?11 cm³/molec · sec at 298°K.     

Reactions of N-methyl-N-(4-biphenylyl)nitrenium ion with electron-rich arenes: laser flash photolysis and product studies

An arylnitrenium ion, N-methyl-N-(4-biphenylyl)nitrenium ion, was generated through photolysis of 1-(N-methyl-N-4-biphenylyl)amino-2,4,6-trimethylpyridinium tetrafluoroborate, and its reactions with various donor-substituted arenes (e.g., 1,3,5-trimethoxybenzene, mesitylene, 1,4-dimethoxybenzene, hexamethylbenzene, etc.) were examined using product analysis and laser flash photolysis. In general, trapping of the short-lived nitrenium ion by the arenes leads to three types of products: (1) the parent amine, N-methyl-N-4-biphenylylamine; (2) an ortho-adduct, where the ring position ortho to the nitrenium ion center is bonded to the arene ring; and (3) an N-adduct, where the nitrenium ion nitrogen is bonded to the trap. Laser flash photolysis studies show that the rates of these trapping reactions vary from 10(4) to 10(9) M(-1) s(-1), depending on the structure of the arene trap. These trapping rate constants do not correlate with the one-electron oxidation potential of the arene, nor with the expected stability of a sigma-complex derived from direct electrophilic aromatic substitution. It is argued that the observed rate constants correspond to initial formation of a pi-complex between the arylnitrenium ion and the arene trap. This complex then forms the observed products.     

The formation and properties of the melatonin radical: a photolysis study of melatonin with 248 nm laser light

The photolysis of melatonin in aqueous solution has been studied spectrometrically with a 248 nm laser. The formation of hydrated electrons in a monophotonic process has been confirmed in neutral solution with a quantum yield of 0.22. Two main absorption bands at 340 and 460 nm plus an absorption shoulder resulted from the counterpart of the ejected electron, a melatonin radical, in solution. The big difference for the relative intensity of the absorption peaks under various pH conditions reveals that the melatonin radical exists in the solution through an acid-base equilibrium. In support from the pH dependence of the spectrum of the intermediate, the pKa1 for the doubly-protonated melatonin radical against the mono-protonated melatonin cation radical was estimated to be -0.95 and the pKa2 for the mono-protonated melatonin cation and melatonin neutral radical was 4.5 +/- 0.5. This work will benefit the basic understanding about melatonin as a UV-light protector, as a light receptor and the antioxidation functions of melatonin.