The direct observation of secondary radical chain chemistry in the heterogeneous reaction of chlorine atoms with submicron squalane droplets

The reaction of Cl atoms, in the presence of Cl(2) and O(2), with sub-micron squalane particles is used as a model system to explore how surface hydrogen abstraction reactions initiate chain reactions that rapidly transform the chemical composition of an organic particle. The heterogeneous reaction is measured in a photochemical flow tube reactor in which chlorine atoms are produced by the photolysis of Cl(2) at 365 nm. By monitoring the heterogeneous reaction, using a vacuum ultraviolet photoionization aerosol mass spectrometer, the effective reactive uptake coefficient and the distributions of both oxygenated and chlorinated reaction products are measured and found to depend sensitively upon O(2), Cl(2), and Cl concentrations in the flow reactor. In the absence of O(2), the effective reactive uptake coefficient monotonically increases with Cl(2) concentration to a value of ～3, clearly indicating the presence of secondary chain chemistry occurring in the condensed phase. The effective uptake coefficient decreases with increasing O(2) approaching a diffusion corrected value of 0.65 ± 0.07, when 20% of the total nitrogen flow rate in the reactor is replaced with O(2). Using a kinetic model it is found that the amount of secondary chemistry and the product distributions in the aerosol phase are controlled by the competitive reaction rates of O(2) and Cl(2) with alkyl radicals. The role that a heterogeneous pathway might play in the reaction of alkyl radicals with O(2) and Cl(2) is investigated within a reasonable range of reaction parameters. These results show, more generally, that for heterogeneous reactions involving secondary chain chemistry, time and radical concentration are not interchangeable kinetic quantities, but rather the observed reaction rate and product formation chemistry depends sensitively upon the concentrations and time evolution of radical initiators and those species that propagate or terminate free radical chain reactions.     

Electrochemical oxidation of 2,4,5-triaryl-substituted pyrroles. II. Oxidative dimerization of 4,5-diphenyl-2-mesitylylpyrrole

2,4,5-Triaryl-substituted pyrroles lead, upon chemical or electrochemical oxidation, to an intermediate β-β'-dimer, which, in the course of the reaction, undergoes further oxidation to a tetracyclic derivative. To improve the selectivity towards the uncyclized dimer the oxidation of a triarylpyrrole in which the ortho positions of the phenyl group in position 2 are hindered by the presence of methyl groups was attempted. The cyclization was hindered, but an α-β'-dimer was obtained as the major product. An unexspected isomeric α-β'-dimer, in which the mesitylyl group is shifted into the β position of the pyrrole ring which undergoes the oxidation, was obtained in minor amounts. Electroanalytical data indicate that the process goes through the formation of a monomeric radical cation, followed by a slow chemical reaction.     

Formation of the diphenyl molecule in the crossed beam reaction of phenyl radicals with benzene

The chemical dynamics to form the D5-diphenyl molecule, C6H5C6D5, via the neutral-neutral reaction of phenyl radicals (C6H5) with D6-benzene (C6D6), was investigated in a crossed molecular beams experiment at a collision energy of 185 kJ mol(-1). The laboratory angular distribution and time-of-flight spectra of the C6H5C6D5 product were recorded at mass to charge mz of 159. Forward-convolution fitting of our data reveals that the reaction dynamics are governed by an initial addition of the phenyl radical to the pi electron density of the D6-benzene molecule yielding a short-lived C6H5C6D6 collision complex. The latter undergoes atomic deuterium elimination via a tight exit transition state located about 30 kJ mol(-1) above the separated reactants; the overall reaction to form D5-diphenyl from phenyl and D6-benzene was found to be weakly exoergic. The explicit identification of the D5-biphenyl molecules suggests that in high temperature combustion flames, a diphenyl molecule can be formed via a single collision event between a phenyl radical and a benzene molecule.     

Ab initio study of the reaction of propionyl (C2H5CO) radical with oxygen (O2)

The reaction of propionyl radical with oxygen has been studied using the full coupled cluster theory with the complete basis set. This is the first time to gain a conclusive insight into the reaction mechanism and kinetics for this important reaction in detail. The reaction takes place via a chemical activation mechanism. The barrierless association of propionyl with oxygen produces the propionylperoxy radical, which decomposes to form the hydroxyl radical and the three-center alpha-lactone predominantly or the four-center beta-propiolactone. The oxidation of propionyl radical to carbon monoxide or carbon dioxide is not straightforward rather via the secondary decomposition of alpha-lactone and beta-propiolactone. Kinetically, the overall rate constant is almost pressure independent and it approaches the high-pressure limit around tens of torr of helium. At temperatures below 600 K, the rate constant shows negative temperature dependence. The experimental yields of the hydroxyl radical can be well reproduced, with the average energy transferred per collision -DeltaE=20-25 cm(-1) at 213 and 295 K (helium bath gas). At low pressures, together with the hydroxy radical, alpha-lactone is the major product, while beta-propiolactone only accounts for about one-fifth of alpha-lactone. At the high-pressure limit, the production of the propionylperoxy radical is dominant together with a fraction of the isomers. The infrared spectroscopy or the mass spectroscopy techniques are suggested to be employed in the future experimental study of the C2H5CO+O2 reaction.     

A theoretical investigation of the gas-phase oxidation reaction of the saturated tert-butyl radical.

The radical-radical reaction mechanisms and dynamics of ground-state atomic oxygen [O(3P)] with the saturated tert-butyl radical (t-C4H9) are investigated using the density functional method and the complete basis set model. Two distinctive reaction pathways are predicted to be in competition: addition and abstraction. The barrierless addition of O(3P) to t-C4H9 leads to the formation of an energy-rich intermediate (OC4H9) on the lowest doublet potential energy surface, which undergoes subsequent direct elimination or isomerization-elimination leading to various products: C3H6O + CH3, iso-C4H8O + H, C3H7O + CH2, and iso-C4H8 + OH. The respective microscopic reaction processes examined with the aid of statistical calculations, predict that the major addition pathway is the formation of acetone (C3H6O) + CH3 through a low-barrier, single-step cleavage. For the direct, barrierless H-atom abstraction mechanism producing iso-C4H8 (isobutene) + OH, which was recently reported in gas-phase crossed-beam investigations, the reaction is described in terms of both an abstraction process (major) and a short-lived addition dynamic complex (minor).     

Product distribution in preparative scale electrolysis: Part I. Introduction

The purpose of the present series is to establish and discuss the relationships between the distribution of products in preparative scale electrolysis and characteristic rates or ratios and magnitude of the operational parameters for a set of reaction schemes that can be encountered in a large variety of electrochemical processes. This can help in optimizing the yield of the target product for a given electrosynthetic reaction by a proper choice of the reaction conditions. Furthermore, systematic analysis of the product distribution as a function of the various parameters may in many cases be the only route to the reaction mechanism due to the time-scale limitations of the standard electrochemical kinetic techniques. The reaction sequences thus considered involve, besids electron transfer at the electrode, fast accompanying chemical reactions and competing chemical or electrochemical follow-up reactions. Reaction intermediates are thus confined within a reaction layer the thickness of which is small as compared to that of the diffusion layer in the context of the Nernst approximation. The other basic assumptions and procedures of resolution of the mathematical models are defined and discussed. Three different electrolysis regimes are considered involving potentiostatic or galvanostatic control and continuous or discontinuous electrolysis operations. A first reaction scheme is analyzed within this context involving the competition between homogeneous and heterogeneous electron transfer after a first electron transfer followed by an irreversible chemical step. The preliminary resolution of this ECE-Disp problem is indeed required for a number of reaction mechanisms of more definite chemical interest.     

Imaging the reaction dynamics of OH + CD4. 3. Isotope effects

Isotope effects in the reaction of hydroxyl radical with methane are investigated in a crossed-beam experiment. By exploiting different combinations of OH/OD + CH4/CD4/CHD3, a total of eight isotopically variant reactions are examined to decipher the dynamical consequence of both primary and secondary isotope effects. The most prominent observation is from isotopic substitution of the transferred atom (primary isotope effect), which yields more stretch-excited water product for the D atom case than H atom. The opposite is found, however, for excitations in bending and combination modes. The secondary isotope effects, from either substituted hydroxyl radical or the three H atoms that are not being abstracted, are relatively minor and manifest themselves in spreading the vibrational distribution of the water coproducts.     

Dediazoniation of arenediazonium salts with trivalent-phosphorus compounds. Tool for examination of the reactivity of phosphorus-centered radicals

Trialkyl phosphites ( 1 ), dialkyl phenylphosphinites ( 2 ), and alkyl diphenylphosphonites ( 3 ) as well as 2-phenyl-1,3,2-dioxaphospholan ( 4b ) and 2-phenyl-1,3,2-dioxaphosphorinan ( 4b ) give rise to dediazoniation of arenediazonium salt ( 5 ) in an alcoholic solvent under an argon atmosphere at 20°C. The reaction proceeds via a radical-chain mechanism initiated by single-electron transfer (SET) from the trivalent-phosphorus compounds to 5 , as a result of which, an aryl radical Ar⋅ and a cation radical 15 are generated from the former and the latter, respectively. The aryl radical Ar⋅ participates in this chain process abstracting a hydrogen from the solvent alcohol, yielding the corresponding arene ArH. The cation radical 15 undergoes both an ionic reaction with the solvent alcohol and a radical coupling with Ar⋅, giving the phosphoranyl radical 16 and the phosphonium ion 17 , respectively, as intermediates. The phosphoranyl intermediate 16 decomposes through either the SET process to 5 or by β-scission, yielding the oxidation product (phosphate, phosphonate, or phosphinate from 1 , 2 , or 3 , respectively, or phosphonates from 4 ). The phosphonium intermediate 17 affords the arylated product (phosphonate, phosphinate, or phosphine oxide from 1 , 2 , 3 , respectively, or the phosphinate from 4 ). Among the trivalent-phosphorus compounds tested, 1 gives the arylated product in the highest yield. This observation, together with the literature data of ESR for structurally related phosphoranyl radicals, indicates that the radical coupling of 15 with Ar⋅ is facilitated by the high spin density on its central phosphorus atom.     

Tetrazole derivatives XIX. Synthesis and properties of 1-(2-methyl-5-tetrazolyl)-3,5-diphenylformazan and a verdazyl radical

Benzaldehyde 2-methyl-5-tetrazolylhydrazone was synthesized and converted to a formazan, which undergoes methylation at the exocyclic nitrogen atom on reaction with dimethyl sulfate. The reaction product undergoes cyclization to the 1-(2-methyl-5-tetrazolyl)-3,5-diphenylleucoverdazyl radical when it is heated. Tautomerism due to location of the proton at the N(₂) or N₍₄₎ atom of the tetrazine ring is characteristic for the verdazyl radical, according to the PMR spectral data. A verdazyl radical was obtained by oxidation of the leucoverdazyl radical.See [1] for communication XVIII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 991–995, July, 1978.     

The SON2 mechanism : A non-oxidative reaction that is initiated by electron transfer oxidation

Under oxidizing conditions, aromatic chloro and fluoro compounds undergo what formally are typical nucleophilic substitution reactions with surprising ease. As an example, 4-fluoroanisole is converted the 4-acetoxyanisole by anodic or metal ion oxidative initiation, and the reaction is shown to be a chain process. It is proposed that a mechanism analogous to that of the reductively initiated S_RN1 mechanism operates: The substrate is oxidized to a radical cation by the initiator system, and the radical cation then undergoes ipso attack by the nucleophile. In the third step, the leaving group leaves as a species at the same oxidation level as the nucleophile, giving the radical cation of the product to be formed. A chain transfer step involving this ion and a new substrate molecule then completes the propagation sequence.Previously reported cases of this phenomenon are discussed and the individual steps of the chain reaction are considered in terms of their thermochemistry. It is concluded that the S_ON2 mechanism should be more favoured with easily oxidizable substrates.     

Lock-exchange experiments with an autocatalytic reaction front

A viscous lock-exchange gravity current corresponds to the reciprocal exchange of two fluids of different densities in a horizontal channel. The resulting front between the two fluids spreads as the square root of time, with a diffusion coefficient reflecting the buoyancy, viscosity, and geometrical configuration of the current. On the other hand, an autocatalytic reaction front between a reactant and a product may propagate as a solitary wave, namely, at a constant velocity and with a stationary concentration profile, resulting from the balance between molecular diffusion and chemical reaction. In most systems, the fluid left behind the front has a different density leading to a lock-exchange configuration. We revisit, with a chemical reaction, the classical situation of lock-exchange. We present an experimental analysis of buoyancy effects on the shape and the velocity of the iodate arsenous acid autocatalytic reaction fronts, propagating in horizontal rectangular channels and for a wide range of aspect ratios (1/3 to 20) and cylindrical tubes. We do observe stationary-shaped fronts, spanning the height of the cell and propagating along the cell axis. Our data support the contention that the front velocity and its extension are linked to each other and that their variations scale with a single variable involving the diffusion coefficient of the lock-exchange in the absence of chemical reaction. This analysis is supported by results obtained with lattice Bathnagar-Gross-Krook (BGK) simulations Jarrige et al. [Phys. Rev. E 81, 06631 (2010)], in other geometries (like in 2D simulations by Rongy et al. [J. Chem. Phys. 127, 114710 (2007)] and experiments in cylindrical tubes by Pojman et al. [J. Phys. Chem. 95, 1299 (1991)]), and for another chemical reaction Schuszter et al. [Phys. Rev. E 79, 016216 (2009)].     

Structural effects on the beta-scission reaction of tertiary arylcarbinyloxyl radicals. The role of alpha-cyclopropyl and alpha-cyclobutyl groups

A product and time-resolved kinetic study on the reactivity of tertiary arylcarbinyloxyl radicals bearing alpha-cyclopropyl and alpha-cyclobutyl groups has been carried out. Both the 1-cyclopropyl-1-phenylethoxyl (1.) and alpha,alpha-dicyclopropylphenylmethoxyl (2.) radicals undergo beta-scission to give cyclopropyl phenyl ketone as the major or exclusive product with rate constants higher than that measured for the cumyloxyl radical. It is proposed that in the transition state for beta-scission of 1. and 2., formation of the C=O double bond is assisted by overlap with the C-C bonding orbitals of the cyclopropane ring. With tertiary arylcarbinyloxyl radicals bearing alpha-cyclobutyl groups such as the 1-cyclobutyl-1-phenylethoxyl (4.) and 1-cyclobutyl-1-phenylpropoxyl (5.) radicals, the fragmentation regioselectivity is essentially governed by the stability of the radical formed by beta-scission. Accordingly, 4. undergoes exclusive C-cyclobutyl bond cleavage to give acetophenone, whereas with 5., competition between C-cyclobutyl and C-ethyl bond cleavage, leading to propiophenone and cyclobutylphenyl ketone in a 2:1 ratio, is observed.     

Importance of cytochromes in cyclization reactions: Quantum chemical study on a model reaction of proguanil to cycloguanil

Proguanil, an anti‐malarial prodrug, undergoes cytochrome P450 catalyzed biotransformation to the pharmacologically active triazine metabolite (cycloguanil), which inhibits plasmodial dihydrofolate reductase. This cyclization is catalyzed by CYP2C19 and many anti‐malarial lead compounds are being designed and synthesized to exploit this pathway. Quantum chemical calculations were performed using the model species (Cpd I for active species of cytochrome and N4‐isopropyl‐N6‐methylbiguanide for proguanil) to elucidate the mechanism of the cyclization pathway. The overall reaction involves the loss of a water molecule, and is exothermic by approximately 55 kcal/mol, and involves a barrier of approximately 17 kcal/mol. The plausible reaction pathway involves the initial H‐radical abstraction from the isopropyl group by Cpd I, followed by two alternative paths‐ (i) oxygen rebound to provide hydroxyl derivative and (ii) loss of additional H‐radical to yield 1,3,5‐triazatriene, which undergoes cyclization. This study helped in understanding the role of the active species of cytochromes in this important cyclization reaction. © 2014 Wiley Periodicals, Inc.     

Intermediate Q from soluble methane monooxygenase hydroxylates the mechanistic substrate probe norcarane: evidence for a stepwise reaction.

Norcarane is a valuable mechanistic probe for enzyme-catalyzed hydrocarbon oxidation reactions because different products or product distributions result from concerted, radical, and cation based reactions. Soluble methane monooxygenase (sMMO) from Methylosinus trichosporium OB3b catalyzes the oxidation of norcarane to afford 3-hydroxymethylcyclohexene and 3-cycloheptenol, compounds characteristic of radical and cationic intermediates, respectively, in addition to 2- and 3-norcaranols. Past single turnover transient kinetic studies have identified several optically distinct intermediates from the catalytic cycle of the hydroxylase component of sMMO. Thus, the reaction between norcarane and key reaction intermediates can be directly monitored. The presence of norcarane increases the rate of decay of only one intermediate, the high-valent bis-mu-oxo Fe(IV)(2) cluster-containing species compound Q, showing that it is responsible for the majority of the oxidation chemistry. The observation of products from both radical and cationic intermediates from norcarane oxidation catalyzed by sMMO is consistent with a mechanism in which an initial substrate radical intermediate is formed by hydrogen atom abstraction. This intermediate then undergoes either oxygen rebound, intramolecular rearrangement followed by oxygen rebound, or loss of a second electron to yield a cationic intermediate to which OH(-) is transferred. The estimated lower limit of 20 ps for the lifetime of the putative radical intermediate is in accord with values determined from previous studies of sterically hindered sMMO probes.     

Site specificity of alpha-H abstraction reaction among secondary structure motif--an ab initio study

The initial step of protein oxidation is studied through alpha-H abstraction by an OH radical with various secondary structure motifs of proteins. It is found that there exist preferential alpha-Hs in this kind of abstractions. The typical abstraction mechanism involves three steps: forming a pre-reactive complex before abstraction, the abstraction reaction, and the H(2)O detachment from a post-reactive complex to form the product, C(alpha)-center radical. Using the stability of the pre-reactive complex and the reaction barrier, we provide some explanation for this site preference. The feasibility of alpha-H abstraction by OH radical depends not only on the types of secondary structure, but also on the reaction condition, such as in aqueous or in gas phase. Moreover, the reactivity of the abstraction also depends on the location of alpha-H in the secondary structure motifs. The preferential alpha-Hs to be abstracted in beta-sheet are those immediate to the amide or carbonyl group, and without involving hydrogen bonding, whereas in reverse turns, the preferential alpha-Hs are near the C-terminal of type I and near the N-terminal of type II. In general, the alpha-Hs in alpha-helix are more difficult to be abstracted than those in beta-sheet and polypeptide in linear form. It is consistent with the trend of their bond dissociation energies. Our theoretical rate constant of N-acetyldiglycin-methylamide (Ac(Gly)(2)NHCH(3)) in aqueous solution (6.75 x 10(8) M(-1) s(-1)) is close to the experimental observation of N-acetyldiglycinamide (Ac(Gly)(2)NH(2)) (8.6 x 10(8) M(-1) s(-1)).     

A crossed beams study of the reaction of carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X 1A')--investigating the formation of cyano propargyl radicals

The chemical dynamics of the reaction of ground state carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X 1A'), were examined under single collision conditions at collision energies of 29.9 and 43.9 kJ mol(-1) using the crossed molecular beams approach. The experimental studies were combined with electronic structure calculations on the triplet C4H3N potential energy surface (H. F. Su, R. I. Kaiser, A. H. H. Chang, J. Chem. Phys., 2005, 122, 074320). Our investigations suggest that the reaction follows indirect scattering dynamics via addition of the carbon atom to the carbon-carbon double bond of the vinyl cyanide molecule yielding a cyano cyclopropylidene collision complex. The latter undergoes ring opening to form cis/trans triplet cyano allene which fragments predominantly to the 1-cyano propargyl radical via tight exit transition states; the 3-cyano propargyl isomer was inferred to be formed at least a factor of two less; also, no molecular hydrogen elimination channel was observed experimentally. These results are in agreement with the computational studies predicting solely the existence of a carbon versus hydrogen atom exchange pathway and the dominance of the 1-cyano propargyl radical product. The discovery of the cyano propargyl radical in the reaction of atomic carbon with vinyl cyanide under single collision conditions implies that this molecule can be an important reaction intermediate in combustion flames and also in extraterrestrial environments (cold molecular clouds, circumstellar envelopes of carbon stars) which could lead to the formation of cyano benzene (C6H5CN) upon reaction with a propargyl radical.     

Non‐catalytic,gas‐solid reversible reaction mechanism implementing diffuse interface model

Gas‐solid reactions in chemical and metallurgical industries often involve solid pellets and a gaseous reactant. The progress of chemical reaction is measured by the movement of zones within the pellet and has been explained in terms of diffusion and chemical reaction processes. Earlier models identified a single reaction zone, in addition to product layer and unreacted core. In the present article, two reaction zones are envisaged as a more plausible explanation of the movement of the zones as the reaction proceeds. Earlier models for reversible reactions have assumed that conditions at the interface between the reaction zone and the unreacted core correspond to equilibrium at the prevailing temperature. The gaseous concentrations were assumed to permeate the core at the interfacial values so that no reaction occured in the core. More realistically, the present article envisages an additional zone within which the gaseous concentrations fall from the equilibrium values to zero. It is assumed that in the reaction zone proper, referred to as zone I, having thickness z_I, the concentration profile is sigmoidal. This agrees with the earlier work of Khan and Bowen [1] and Prasannan and Doraswamy. [2] In zone I and the concentration of the reactant gas varies between [A_i] and [A*]. In the zone II, having thickness z₂, concentration varies linearly between [A*] and zero. This model has been applied successfully to the data of the reduction of hematite [3] at different temperatures. The contribution of different forms of resistance, diffusion in product layer, chemical reaction and diffusion in the reactant core, is assessed as function of time (start to the end of reaction). The thickness of the zones remain almost constant as the reaction progresses. In particular, the influences of the product and core diffusion coefficients and chemical equilibrium constant on the extant reaction are evaluated. The dependence of concentration profile and zone thickness on equilibrium constant, K, velocity constant, k, diffusional coefficients D_C and D_P has been investigated thoroughly. The thickness of both zones has been evaluated for leading variables. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 559–570, 1999     

Carbon nucleophiles in the Mitsunobu reaction. Mono- and dialkylation of bis(2,2,2-trifluoroethyl) malonates

[formula: see text] Simple dialkyl malonate esters, for example diethyl malonate, exhibit relatively limited scope as carbon nucleophiles in the Mitsunobu dehydrative alkylation reaction. In contrast, bis(2,2,2-trifluoroethyl) malonate readily undergoes dehydrative alkylation with primary alcohols, and using only a slight excess of malonate gives monoalkylated product in good yield. Some secondary alcohols can also be employed, and bis(2,2,2-trifluoroethyl) malonates can be used in a second dehydrative alkylation to give dialkylated products in good to excellent yield.     

Photochemistry of the three carboxypyridines in water: a pH dependent reaction

The photochemistry of ortho, meta and para-carboxypyridines (pK(a)(1)= 1.0-2.1 and pK(a)(2)= 4.7-5.3) in aqueous medium was studied by laser-flash photolysis and product studies. At pH < pK(a)(1), hydroxylated compounds are produced with low quantum yields. Within the pH range 4-7, ortho and meta isomers undergo dimerization together with decarboxylation with a quantum yield showing a very sharp maximum around pK(a)(2)([small phi](max)= 0.09 and 0.01, respectively) while the para isomer is photostable. End-of-pulse transients assigned to triplet states were detected by laser-flash photolysis at pH < pK(a)(1) and pH > 4. Additionally, the carboxypyridinyl radicals were detected as secondary intermediates at pH < pK(a)(1) and 4 < pH < 7 and the OH-adduct radicals at pH < pK(a)(1). This is in favour of an electron transfer reaction between triplet and starting compound producing a charge transfer species. The radical anion would escape as carboxypyridinyl radical while the radical cation may add water at pH < pK(a)(1) yielding the OH-adduct radical or may undergo decarboxylation at pH > 4. The high quantum yield of phototransformation of the ortho isomer at pH > 4 is due to an easy decarboxylation process. A reaction scheme is proposed accounting for the dependences of [small phi] on both the pH and the carboxypyridines concentration. This study points out the distinct pattern of reactivity of carboxypyridines depending on the ionisation state of starting compounds and isomeric substitution.     

Elementary reaction kinetics in molecular solids. Free volume model of the diffusion limited processes

The main peculiarities of free radical reaction kinetics in organic solids are discussed from the point of view of a slightly modified free volume model of molecular mobility. The reaction and diffusion acts are supposed to occur predominantly in “diluted” microzones, the probability of “dilution” depending on the minimal volume needed for reaction or diffusion, and on the energy of a local molecular displacement. It is shown that even far separated radical pairs may react in a “static” way via the long range microdilution of the structure. The model explains the separation-dependent annealing of radical pairs as well as the stepwise annealing of large fractions of quasi-isolated centres and large capture radii for the diffusion limited reaction of the remaining species. Preliminary results concerning the pressure and plastification dependence of the rate constant are also in reasonable agreement with the proposed concept.