Magnetic Field Effects on the Dynamics of Radical Pairs in Micelles: A New Approach to Understanding the "Cage Effect"

The effect of temperature on the photolysis of dibenzyl ketone and 4-methyldibenzyl ketone in sodium dodecyl sulfate micelles was studied by laser flash photolysis and product distributions derived from steady-state photolysis. At high temperatures, the product distribution and radical decay kinetics are primarily due to random encounters of radicals, and the "cage effect" cannot be rationalized by geminate recombination reactions that occur before the radicals escape from the micelles. A mechanism is proposed in which the enhancement of the crosstermination product derived from random encounters is due to the different partitioning of each radical species between the micelles and the aqueous phase, thereby leading to different rates for the self-termination reactions.     

Reactivities of dianions of pyridyl phenyl ketones

Substituted pyridylphenylcarbinols are formed in the reaction of the dianions of isomeric pyridyl phenyl ketones with alkyl halides, aldehydes, and benzonitrile. The reaction of the dianion of 3-pyridyl phenyl ketone with benzophenone, in contrast to the analogous reaction of the dianion of 2-pyridyl phenyl ketone, gives 5-benzoyl-2-diphenylhydroxymethyl-1,2-dihydropyridine. It is assumed that the observed reaction includes one-electron transfer and the formation of two anion radicals. Recombination of the anion radical of 3-pyridyl phenyl ketone leads to 5,5′-dibenzoyl-2,2′-di(1,2-dihydropyridyl).     

Product studies and laser flash photolysis on alkyl radicals containing two different beta-leaving groups are consonant with the formation of an olefin cation radical.

1-Bromo-2-methoxy-1-phenylpropan-2-yl (3) and 2-methoxy-1-phenyl-1-diphenylphosphatopropan-2-yl (4) were generated under continual photolysis from the respective PTOC precursors in a mixture of acetonitrile and methanol. The radicals undergo heterolytic fragmentation of the substituent in the beta-position to generate the olefin cation radical (5). Z-2-Methoxy-1-phenylpropene (15) is the major product formed in the presence of 1,4-cyclohexadiene, and is believed to result from hydrogen atom transfer to the oxygen of the olefin cation radical, followed by deprotonation. Laser flash photolysis experiments indicate that reaction between 5 and 1,4-cyclohexadiene occurs with a rate constant of approximately 6 x 10(5) M(-1) s(-1). 2,2-Dimethoxy-1-phenylpropane (18) is observed as a minor product. Laser flash photolysis experiments place an upper limit on methanol trapping of 5 at k <1 x 10(3) M(-1) s(-1) and do not provide any evidence for the formation of reactive intermediates other than 5. The use of two PTOC precursors containing different leaving groups to generate a common olefin cation radical enables one to utilize product analysis to probe for the intermediacy of other reactive intermediates. The ratio of 15:18 is dependent upon hydrogen atom donor concentration, but is independent of the PTOC precursor. These observations are consistent with the proposal that both products result from trapping of 5 that is formed via heterolysis of 3 and 4.     

Measurement of rate processes of free radicals in homogeneous and micellar solutions by cidnp

CIDNP is used to study rate processes of free radicals in both homogeneous and micellar solution. An estimate of the lifetime of the phenyl-acetyl radical at ambient temperature (τ__co?10^?7 sec) produced during photolysis of dibenzyl ketone is made based on quantitative CIDNP measurements and computer simulations. Observation of CIDNP in micellar solution is shown to be consistent with an isotropic medium which restricts diffusion on a short time scale, allowing for an increased tendency toward cage reaction. In the case of t-butyl/pivaloyl radical pairs, escape of the radical fragments from the micelle is shown to be competitive with decarbonylation of the pivaloyl radical Likewise, CIDNP is consistent with product yield results which show the enhanced tendency of triplet born benzyl radical pairs to undergo cage reaction when they are sequestered in a micelle.     

Initial Radical Separation after Photolysis of 2,2′‐Azobis(isobutyronitrile) (AIBN) in Solution: Modeling the Primary Cage Effect for Polar Radicals

There is a contradiction as to the initial spatial separation r_i of the two transient 2‐cyanoprop‐2‐yl radicals (Me₂ ? CN) formed by flash photolysis of 2,2′‐azobis(isobutyronitrile) (AIBN) in solvents of various viscosities. The cage effect, expressed in terms of the in‐cage termination probability of the resulting radicals, is predicted correctly by classical Langevin models assuming a decrease of r_i with increasing viscosity. However, the electron‐spin polarization of the radicals escaping the primary cage clearly indicates that the initial separation distance r_i is independent of the solution viscosity. This obvious discrepancy can be reconciled by accounting for the strong electric dipole moments of these radicals and the resulting inter‐radical dipole? dipole interaction potential. We propose a primary‐caging model for polar radicals in solution based on an attractive inter‐radical mean‐force potential. The model is applied to the flash photolysis of AIBN and shown to describe properly the viscosity dependence of both the in‐cage termination probability (cage effect) and the electron‐spin polarization of the escaping 2‐cyanoprop‐2‐yl radicals.     

Magnetic Field Effects on the Dynamics of Radical Pairs: The Partition Effect in Vesicles

A combination of product studies and laser flash photolysis (LFP) was used to study the recombination of radical pairs derived from dibenzyl ketone (DBK) and its methyl derivative. Two sizes of vesicles consisting of dioctade-cyldimethylammonium chloride (DODAC) were generated. In the product studies, irradiation of the ketone led to a substantial overall cage effect both above and below the phase-transition temperature. However, LFP results demonstrate that no geminate reactions, that correspond to the reactions of radicals generated from the same precursor molecule are occurring even at room temperature. The results are discussed in terms of the partition effect where the cage effect is determined by the differences in the solubility of the radical inside the vesicle bilayer and in the aqueous phase. In small (30 nm diameter) vesicles, most of the random recombination occurs after re-entry of the radicals into the bilayer, whereas in large (?150 nm) liposomes, a significant proportion of the recombination reactions takes place in the bulk water. This work demonstrates that magnetic fields can efficiently alter the reactivity of radicals involved in nongeminate pathways and further supports the use of the radical pair mechanism to explain possible effects of magnetic fields in biological systems.     

Compatible injection and detection systems for studying the kinetics of excess electron transfer

[reaction: see text] A design for fast kinetic studies of electron transfer in radical anions is reported. alpha-Hydroxy radicals formed by 355 nm laser flash photolysis of alpha-phenacyl alcohols are deprotonated under basic conditions to give ketyl radical anions that serve as electron injectors in inter- and intramolecular electron-transfer reactions. The 2,2-diphenylcyclopropyl group serves as a reporter. When an electron is injected and transferred such that spin character is adjacent to the reporter, cyclopropyl ring opening gives a readily detected diphenylalkyl radical.     

Studies of the photolysis of aromatic esters in solution and in polymer films

Photolysis of 2-naphthylenemethyl-1-naphthylacetate (NMNA) and 9-anthracenemethyl-9-anthrylacetate (AMAA) yields radicals of methylnaphthalene and methylanthracene, respectively. The longevity of these radicals makes them suitable probes for studying primary and secondary cage recombination processes in solution, and in polymeric matrices. 2,2-Di-(4-tert-octylphenyl-1-picrylhydrazyl) (DPPH) is utilized as a radical trap to report on radicals which escape from the solvent cage. Quantum yields for photolysis of NMNA and AMAA were determined in solution and in poly(methyl methacrylate) films. Both unimolecular and bimolecular processes are suppressed as a result of the greater effective viscosity in polymeric media.     

Effect of primary radical termination on chain length and initiator efficiency

In polymerization with primary radical termination, when reaction between primary radicals, which escape from solvent cage, is not negligible, a relation between chain length and polymerization rate is found regardless of tractable approximate procedures. Such a relation is applied to the kinetic data obtained in the polymerizations of methyl methacrylate (MMA) and vinyl acetate (VA) initiated by 2,2′-azobis(2,4-valeronitrile) at 50.0°C. Further, when the primary radical termination is high, an initiator efficiency can not be approximated to a ratio of the primary radicals escaping from the cage to the total primary radicals formed in the cage. In the polymerization of MMA, after the primary radicals escapes from the cage, they immediately react with the monomer. Thus, the reaction between the primary radicals is not significant. However, in the polymerization of VA, the rate of reaction between the primary radical and the monomer might be comparable to the rate of reaction between the primary radicals when the initiator concentration is quite high.     

Alpha- and beta-thujone radical rearrangements and isomerizations. A new radical clock

Radical clocks have been extensively used in chemical and biochemical mechanistic studies. The C4 radicals of alpha- and beta-thujone can undergo two distinct rearrangement reactions that could, in principle, serve as simultaneous but independent radical clocks. We have therefore generated these C4 radicals by photolysis of the corresponding N-hydroxypyridine-2-thione ester precursors and have investigated their fates and lifetimes. Photolysis of either alpha- or beta-thujone generates the same 6:100 mixture of alpha- and beta-thujone when the radicals are quenched by thiophenol. Hydrogen atom transfer from thiophenol to the radical thus occurs preferentially from the less sterically hindered alpha-face to give beta-thujone. The third product formed in the photolysis via opening of the cyclopropyl ring is 2-methyl-5-isopropylcyclopent-2-enone. The ratio of ring opened to unopened products gives very similar values of kralpha = 4.4 x 10(7) s(-1) and krbeta = 1.0 x 10(8) s(-1) for ring opening of the radicals generated from alpha- and beta-thujone, respectively. If the C4 cation rather than radical is generated, it is converted to carvacrol, a phenol that is not obtained in the radical reactions. Thujone therefore differentiates between radical and cation pathways and provides a measure of the radical lifetime.     

Photofragmentation of Dibenzyl Ketone in the Presence of Tetramethylpiperidine Derivatives. Studies on light stabilizer mechanisms [1]

The interaction of selected tetramethylpiperidine derivatives with radicals arising from the Norrish-type I cleavage of dibenzyl ketone under oxygen was studied. Product analyses and kinetic studies showed that the investigated sterically hindered piperidine derivatives have a pronounced effect on both the nature and distribution of the products of photolysis of dibenzyl ketone in the presence of oxygen. Observations indicated that the phenylperacetoxyl radical is formed as an intermediate during irradiation and that it interacts with the additives used. Possible mechanisms of the reactions studied are discussed. The observation that oxidation of an isolated double bond by the radicals formed in dibenzyl ketone photolysis under oxygen is strongly inhibited in the presence of the studied sterically hindered amines is discussed in the light of the results presented. The findings are considered in relation to the problem of polymer stabilization.     

Hydrogen migration and vinylidene pathway for formation of methane in the 193 nm photodissociation of propene: CH3CH=CH2 and CD3CD=CD2

Photodissociation channels and the final product yields from the 193 nm photolysis of propene-h6 (CH(2)=CHCH(3)) and propene-d6 (CD(2)=CDCD(3)) have been investigated, employing gas chromatography, mass spectroscopy, and flame ionization (GC/MS/FID) detection methods. The yields of methane as well as butadiene relative to ethane show considerable variations when propene-h6 or propene-d6 are photolyzed. This suggests significant variances in the relative importance of primary photolytic processes and/or secondary radical reactions, occurring subsequent to the photolysis. Theoretical calculations suggest the potential occurrence of an intramolecular dissociation through a mechanism involving vinylidene formation, accompanied by an ethylenic H-migration through the pi-orbitals. This process affects the final yields of methane-h4 versus methane-d4 with respect to other products. The product yields from previous studies of the 193 nm photolysis of methyl vinyl ketone-h6 and -d6 (CH(2)=CHCOCH(3), CD(2)=CDCOCD(3)), alternative precursors for generating methyl and vinyl radicals, are compared with the current results for propene.     

Photodissociation dynamics of the 2-propyl radical, C3H7

The photodissociation of 2-propyl leading to propene+H was investigated with nanosecond time resolution. A supersonic beam of isolated 2-propyl radicals was produced by pyrolysis of 2-bromopopane. The kinetic energy release of the H-atom photofragment was monitored as a function of excitation wavelength by photofragment Doppler spectroscopy via the Lyman-alpha transition. The loss of hydrogen atoms after excitation proceeds in alpha position to the radical center with a rate constant of 5.8x10(7) s-1 at 254 nm. Approximately 20% of the excess energy is deposited as translation in the H-atom photofragment. In contrast 1-propyl does not lose H atoms to a significant extent. The experimental results are compared to simple Rice-Ramsperger-Kassel-Marcus calculations. The possible reaction pathways are examined in hybrid density functional theory calculations.     

Effects of oxygen in the photolysis of 2,6-di-tert-butyl-1,4-benzoquinonediazide in CCl4

Conclusions The photolysis of 2,6-di-tert-butyl-1,4-benzoquinonediazide in CCl₄, gives 2,6-di-tert-butyl-1,4-benzoquinone in the presence of O₂ in addition to the cage product, 4-chloro-4-trichloromethyl-2,6-di-tert-butylcyclohexadienone. A mechanism has been proposed for the photolysis of the quinonediazide in the presence and absence of oxygen.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2325–2327, October, 1987.     

Cage reactions in the photolysis of 2,2,4,4-tetramethyl-3-pentanone

The product quantum yields in the photolysis of 2,2,4,4-tetramethyl-3-pentanone have been measured in homogeneous solvents of different viscosities, in micellar solutions of cetyltrimethylammonium chloride and sodium dodecyl sulfate, and in dioctadecyl ammonium chloride vesicles. The product quantum yield in n-heptane was found to be 1. This value decreases to 0.5 in paraffin oil as a consequence of geminate recombination. In the presence of free radical scavengers, the extent of geminate disproportionation can be evaluated from the yields of isobutene and 2,2-dimethyl propionaldehyde. From these yields and the geminate recombination yields the total amount of geminate processes and the disproportionation-to-combination ratio for caged radicals are estimated. It is found that micelles provide the most efficient cages. In these media only about 10% of the radicals avoid cage processes. The disproportionation-to-combination ratio of tert-butyl and pivaloyl radicals was found to be extremely media dependent. The measured values ranged from about 0.2 in paraffin oil to 0.8 in cetyltrimethylammonium chloride micelles.     

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.     

Aging and degradation of polyolefins. I. Peroxide-initiated oxidations of atactic polypropylene

Efficiencies of polymer radical production by thermal decomposition of di-tert-butylperoxy oxalate (DBPO) have been measured in bulk atactic polypropylene (PP) at 25–55°C; they range from 1 to 26%, depending on [DBPO], temperature, and presence of oxygen. Most of the polymer radicals thus produced disproportionate in the absence of oxygen but form peroxy radicals in its presence. Most of the pairs of peroxy radicals interact by a first-order reaction in the polymer cage. The fraction that escapes gives hydroperoxide in a reaction that is half order in rate of initiation. In interactions of polymer peroxy radicals, in or out of the cage, about one-third give dialkyl peroxides and immediate chain termination, two-thirds give alkoxy radicals. About one-third of the later cleave at 45°C; the rest abstract hydrogen to give hydroxy groups and new polymer and polymer peroxy radicals. The primary peroxy radicals from cleavage account for the rest of the chain termination. Cleavage of alkoxy radicals and crosslinking of PP through dialkyl peroxides nearly compensate. Up to 70% of the oxygen absorbed has been found in hydroperoxides. The formation of these can be completely inhibited, but cage reactions are unaffected by inhibitors. Concentrations of free polymer peroxy radicals have been measured by electron spin resonance and found to be very high, about 10^?3M at 58–63°C. Comparison with results on 2,4-dimethylpentane indicate that rate constants for both chain propagation and termination in the polymer are much smaller than those for the model hydrocarbon but that the ratio, k_p/(2k_t)^½, is about the same.     

Evidence for a lack of reactivity of carotenoid addition radicals towards oxygen: a laser flash photolysis study of the reactions of carotenoids with acylperoxyl radicals in polar and non-polar solvents

In this paper, we report the results of a laser flash photolysis study of the reactions of a range of carotenoids with acylperoxyl radicals in polar and nonpolar solvents. The results show, for the first time, that carotenoid addition radicals do not react with oxygen to form carotenoid peroxyl radicals; an observation which is of significance in relation to antioxidant/pro-oxidant properties of carotenoids. Acylperoxyl radicals, generated by photolysis of ketone precursors in oxygenated solvents, display high reactivity toward carotenoids in both polar and nonpolar solvents, but the nature of the carotenoid radicals formed is dependent on solvent polarity. In hexane, acylperoxyl radicals react with carotenoids with rate constants in the region of 10(9) M(-1) s(-1) and give rise to transient absorption changes in the visible region that are attributed to the formation of addition radicals. All of the carotenoids show bleaching in the region of ground-state absorption and, with the exception of 7,7'-dihydro-beta-carotene (77DH), no distinct absorption features due to addition radicals are observed beyond the ground state absorption region. For 77DH, the addition radical displays an absorption band that is spectrally resolved from the parent carotenoid absorption. The rate of decay of the 77DH addition radical is unaffected by oxygen in the concentration range 10(-4)-10(-2) M, suggesting that these resonance-stabilized carbon-centered radicals are not scavenged by oxygen. At low incident laser intensities, the 77DH addition radical decay kinetics are 1st order with k(1) approximately 4 x 10(3) s(-1) at room temperature. The 1st order decay is attributed to an intramolecular cyclization process, which is supported by the substantial negative entropies of activation obtained from measurements of the decay rate constants for different 77DH addition radicals as a function of temperature. No transient absorption features are observed in the red or near-infrared regions in hexane for any of the carotenoids studied. In polar solvents such as methanol, acylperoxyl radicals also react with carotenoids with rate constants in the region of 10(9) M(-1) s(-1), but give rise to transient absorption changes in both the visible and the red/near-infrared regions, where it is evident that there are two distinct species. For 77DH, the addition radical absorption around 450 nm is still evident, although its kinetic behavior differs from its behavior in hexane. For 77DH and zeta-carotene (zeta-CAR) the spectral and kinetic resolution of the various absorption bands simplifies kinetic analysis. The kinetic evidence suggests that addition radical formation precedes formation of the two near-infrared absorbing species, and that the kinetics of the addition radical decay match the kinetics of formation of the first of these species (NIR1, absorbing at shorter wavelengths). The decay of NIR1 leads to NIR2, which is attributed to the carotenoid radical cation. The solvent dielectric constant dependence of the relative amounts of NIR1 and NIR2 formed leads us to speculate that NIR1 is an ion-pair. However, an alternative assignment for NIR1 is an isomer of the radical cation. The results, in terms of the pattern of reactivity the carotenoids display and of the properties of the carotenoid radicals formed, are discussed in relation to the antioxidant/pro-oxidant properties of carotenoids.     

Quinone sensitized electron transfer photooxidation of nucleic acids: chemistry of thymine and thymidine radical cations in aqueous solution

The 2-methyl-1,4-naphthoquinone (MQ) sensitized photooxidation of nucleic acid derivatives has been studied by laser flash photolysis and steady state methods. Thymine and thymidine, as well as other DNA model compounds, quench triplet MQ by electron transfer to give MQ radical anions and pyrimidine or purine radical cations. Although the pyrimidine radical cations cannot be directly observed by flash photolysis, the addition of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) results in the formation of the TMPD radical cation via scavenging of the pyrimidine radical cation. The photooxidation products for thymine and thymidine are shown to result from subsequent chemical reactions of the radical cations in oxygenated aqueous solution. The quantum yield for substrate loss at limiting substrate concentrations is 0.38 for thymine and 0.66 for thymidine. The chemistry of the radical cations involves hydration by water leading to C(6)-OH adduct radicals of the pyrimidine and deprotonation from the N(1) position in thymine and the C(5) methyl group for thymidine. Superoxide ions produced via quenching of the quinone radical anion with oxygen appear to be involved in the formation of thymine and thymidine hydroperoxides and in the reaction with N(1)-thyminyl radicals to regenerate thymine. The effects of pH were examined in the range pH 5-8 in both the presence and absence of superoxide dismutase. Initial C(6)-OH thymine adducts are suggested to dehydrate to give N(1)-thyminyl radicals.     

The photolysis of some α-iodo-nitroalkanes in solution

α-Iodo-nitroalkanes decompose photochemically into α-nitroalkyl radicals and iodine. The α-nitroalkyl radical is detected by trapping with 2-methyl-2-nitrosopropane and identified by the hfs-constants of the resulting β-nitronitroxide observed with ESR. In the absence of nitroso compounds the α-nitroalkyl radicals can lose hydrogen to give nitro-olefins (70–80%). Minor products are nitroalkane (<5%) and ketone (10%), the latter via rearrangement of the α-nitroalkyl radical and loss of NO. When the HI elimination is blocked, as is the case with 2-iodo-2-nitroadamantane, the yield of ketone increases to 80%. Upon stabilization of the α-nitroalkyl radical by a phenyl group, dimerization becomes important. In the case of 1-cyclopropyl-1-iodo-1-nitroethane a considerable amount of ring opened product is isolated.