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Absolute rate constants for reactions of phenoxyl radicals in solution have been compiled and evaluated from the literature (172 citations). Rate constants are included for phenoxyl radicals bearing various substituents, including semiquinone radicals and radical ions, as well as aroxyl radicals derived from polycyclic aromatic compounds. The reactions tabulated include self-reactions of the radicals, reactions with other radicals, reactions with inorganic compounds, and reactions with organic compounds. A subset of the latter group includes electron transfer reactions for which the rate constants of both the forward and the reverse reaction have been measured. The radicals were generated by radiolysis, photolysis, thermolysis, or chemical reactions, and their rate constants were determined generally by kinetic spectrophotometry or electron spin resonance.

This paper reports the first quantitative ab initio prediction of the disproportionation/combination ratio of alkyl+alkyl reactions using CH3+C2H5 as an example. The reaction has been investigated by the modified Gaussian-2 method with variational transition state or Rice-Ramsperger-Kassel-Marcus calculations for several channels producing (1) CH4+CH2CH2, (2) C3H8, (3) CH4CH3CH, (4) H2+CH3CHCH2, (5) H2+CH3CCH3, and (6) C2H6+CH2 by H-abstraction and association/decomposition mechanisms through singlet and triplet potential energy paths. Significantly, the disproportionation reaction (1) producing CH4+C2H4 was found to occur primarily by the lowest energy path via a loose hydrogen-bonding singlet molecular complex, H3CHC2H4, with a 3.5 kcal/mol binding energy and a small decomposition barrier (1.9 kcal/mol), instead of a direct H-abstraction process. Bimolecular reaction rate constants for the formation of the above products have been calculated in the temperature range 300-3000 K. At 1 atm, formation of C3H8 is dominant below 1200 K. Over 1200 K, the disproportionation reaction becomes competitive. The sum of products (3)-(6) accounts for less than 0.3% below 1500 K and it reaches around 1%-4% above 2000 K. The predicted rate constant for the disproportionation reaction with multiple reflections above the complex well, k1=5.04 x T(0.41) exp(429/T) at 200-600 K and k1=1.96 x 10(-20) T(2.45) exp(1470/T) cm3 molecule(-1) s(-1) at 600-3000 K, agrees closely with experimental values. Similarly, the predicted high-pressure rate constants for the combination reaction forming C3H8 and its reverse dissociation reaction in the temperature range 300-3000 K, k2(infinity)=2.41 x 10(-10) T(-0.34) exp(259/T) cm3 molecule(-1) s(-1) and k(-2)(infinity)=8.89 x 10(22) T(-1.67)exp(-46 037/T) s(-1), respectively, are also in good agreement with available experimental data.

The reaction of O(3P) atoms with CH3 radicals is shown to produce CO (in addition to the major product CH2O) which is detected by laser induced fluorescence. The rotational and vibrational temperatures of the CO product are about 2000 K. The results are explained by the assumption that the reaction takes place mainly by an indirect mechanism in which a methoxyl radical is formed and then dissociates unimolecularly.

We analyze the transient kinetics of a mixture of two radicals (M and P), one of which (P) has very low, or zero self-reactivity (i.e., P is a “persistent” radical). For the case where the self-reactivity of the persistent radical is zero, we find a critical value for the initial concentration ratio r≡P0/M0, above which the persistent radicals are isolated at long reaction times after all of the M’s react. If r is below its critical value the surviving fraction of P is zero. An experiment employing virtually zero self-reactivity nitroxide radicals is proposed to test the predicted critical behavior. If the self-reactivity of the M is stronger than the cross-coupling reactivity, then the critical value of the ratio r is negative, and a finite fraction of the persistent P radicals are always isolated, regardless of the initial concentration ratio r. For the case where the self-reactivity of the persistent radicals is finite but small, we find similar behavior, except now the P radicals react with one anothe...

A quantitative study of the chemically induced dynamic electron polarization (CIDEP) spectra of acetone and deuterated acetone ketyl radicals in i‐propanol over a wide temperature range has been done to understand the properties of the radical pairs in solution, e.g., the interactions between the radicals, the diffusional and rotational motions of the radicals of the geminate radical pairs, and the microscopic solvent structure. The spin polarization of the separated radical is analyzed in terms of the stochastic‐Liouville equation. It is shown that the observed polarization cannot be explained on the basis of the normal diffusion of the radicals in the pair. The modified diffusion models which presuppose slow diffusion of the radicals in a microscopic solvent structure can account for the observed polarization. The radical pair electron paramagnetic resonance (EPR) spectra observed at very low temperatures (

The photolysis of CCl4 in the presence of C2H5Cl yielded at 25°C the following reaction products: CH3Cl; C2Cl6; 1,1,1,3‐C2H3Cl4; 1,1,1,2‐C3H4Cl4; meso‐ and racemic 2,3‐C4H8Cl2, and 1,3‐C4H8Cl2. The formation of these products is explained by recombination and disproportionation reactions of the radicals produced in the reactions CCl4+hv→CCl3+Cl, Cl+CH3CH2Cl→CH3CHCl+HCl, Cl+CH3CH2Cl→CH2CH2Cl+HCl. Data regarding various ratios of rate constants for recombination and disproportionation reactions of these free radicals are reported and discussed. Data on H atom abstraction by Cl atoms from ethyl chloride are also included.

The xenon‐sensitized radiolysis of propane has been investigated in the presence and absence of small amounts of ion and free radical interceptors. From an analysis of mixtures of deuterated propanes and a knowledge of radical disproportionation yields in the scavenged and unscavenged systems it is possible to construct a complete quantitative kinetic analysis for the decomposition. Ion titration methods demonstrate that the neutralization of C3H7+ ions occurs heterogeneously in the system investigated and leads quantitatively to the production of equivalent yields of isopropyl radicals and hydrogen atoms.

It is proposed that the anomalous spin state populations observed in electron spin resonance spectra of transient radicals in liquids are produced by the following sequence of encounters between two radicals: The radicals either collide or are produced together by a chemical reaction and, instead of recombining, diffuse apart and subsequently undergo a second nonreactive collision. During the interval between the first and second encounters the radical pair wavefunction changes under the influence of singlet–triplet mixing by the magnetic interactions within the radicals. At the second encounter the singlet–triplet splitting associated with the valence bonding–antibonding interactions between the radicals produces further changes in the radical pair wavefunction. The net effect of these changes in the radical pair wavefunction is to increase somewhat the probability of finding the finally separated radicals in those spin states which correlate separationwise with the initial state of the radical pair. The...

The activation energy for abstraction of hydrogen from cyclobutane by a methyl radical is 10.3±0.15 kcal/mole, with a pre‐exponential factor relative to methyl radical abstracting D from acetone‐d6 of 1.8. At temperatures of less than 200°, the cyclobutyl radicals disappear almost exclusively by combination with methyl radicals to form methyl cyclobutane. At higher temperatures the cyclobutyl radical (a) opens to form the ĊH2CH2CH=CH2 radical with an Eact of 18.1±0.8 kcal/mole, and (b) splits out a molecule of hydrogen. The latter reaction is considerably slower than the former.The ĊH2CH2CH=CH2 radical is unusually stable, and does not decompose appreciably even at 325°C. At 425°, the highest temperature which could be reliably studied in this system, the product analysis indicates that the radical decomposes slowly to ethylene and a vinyl radical. By contrast, n‐butyl radicals and s‐butyl radicals readily decompose at 250°C in a similar reaction media. The main reaction of the ĊH2CH2CH=CH2 radical is the abstraction of H (or D) from the parent mixture. It also combines with a methyl radical to form pentene‐1. Evidence is presented to show that the ĊH2CH2CH=CH2 radical isomerizes at temperatures of 350° and above to the resonance stabilized CH3ĊHĊHĊH2 radical, with an energy of activation of 20.0±0.7 kcal/mole.

Experimental data on the atomic cracking of propyl radicals and on the deuterization of methyl radicals are compared with some theoretical calculations. With the aid of some assumptions concerning intramolecular energy transfer in the dissociating molecules involved in these and other reactions and concerning the corresponding activated complexes, data on a number of free radical reactions are correlated.

The condensed‐phase photolysis of i‐C5H12–i‐C5D12 mixtures, and CH3CD (CH3) CH2CH3 has been investigated at photon energies above and below the ionization energy of 2‐methylbutane (10.32 eV), that is, at 1470 A (8.4 eV), 1236 A (10.0 eV), and 1048–1067 A (11.6–11.8 eV). The electronically excited 2‐methylbutane molecule undergoes essentially the same primary decomposition processes in the condensed phase as in the gas phase. However, in contrast with the gas phase, the internally excited fragments formed in these primary processes are entirely deactivated.It is demonstrated that C5H12+ ions are produced at 1048–1067 A and to a lesser extent at 1236 A. A fraction of these ions transfer an H2 molecule to c‐C3D6 to form CD2HCD2CD2H as a product. In the krypton and argon sensitized radiolysis at 77° and 20°K, respectively, energy transfer from the rare‐gas matrix to 2‐methylbutane occurs with high efficiency and all products can be accounted for by decomposition of neutral excited 2‐methylbutane, formed by excitation transfer and/or neutralization of ions formed in charge‐transfer reactions with rare gas ions.In the radiolysis of pure 2‐methylbutane at 77°K, ethane and propane are almost entirely formed through molecular elimination and geminate radical disproportionation processes. Addition of an electron scavenger such as CCl4 lowers the yields of these products, as well as those of the corresponding olefins (e.g., C3H6 and C2H4). A change in the neutralization mechanism of the parent ion (C5H12+) can account for the latter observation. Apparently, fragmentation of the parent ion is unimportant in the condensed phase. Butane (G∼0.1) is the only product which can tentatively be related to reaction of fragment ions (C4H8+, C4H9+). The modes of formation of products, such as the pentenes and the hydrogen in the radiolysis of pure C5H12 remain unexplained.

The addition of HI to C2H4 yields C2H5I at low temperatures and C2H6+I2 at higher temperatures. From 289° to 331°C the rate law is mixed—second order with C2H6+I2 as products. The rate is independent of added I2 and has a specific rate constant k2 (units, liter/mole‐sec): logk2=8.52−(28900/4.575T). The pyrolysis of C2H5I from 330° to 392°C goes by a first‐order process to C2H4+C2H6+I2. The first‐order rate constant is given by (units, sec—1): logku=13.66−(50000/4.575T).If it is assumed that both processes have the same rate determining step, namely the reversible dissociation of C2H5I, they yield an equilibrium constant in good agreement with independently calculated values of Keq for this dissociation. Under these conditions k1, the specific rate constant for the unimolecular split of C2H5I into C2H4+HI is given by ku/2. Arguments are presented to show that this common path involves the four‐center molecular transition state rather than a radical disproportionation. The latter, however, may account for as much as 20% of the pyrolysis at 392°C.

Arrhenius parameters for the reactions of trifluoromethyl radicals, produced by the photolysis of hexafluoroazomethane, with SiH4 and SiD4 have been measured. It has been found that CF3 radicals are more reactive than CH3, the difference in reactivity being due to lower activation energies, as the A factors are almost identical. A theoretical value of Ea obtained by the BEBO method using a simple three‐mass‐point model is compared with the experimental. The BEBO and LEPS methods have been used to predict kinetic isotope effects with and without tunneling corrections for comparison with experimental results.

The entropies of alkyl and other simple free radicals may be estimated with sufficient accuracy to make informative calculations about the equilibria involved in the reactions of these radicals. Such calculations have been made for metathetical reactions, the decomposition of saturated molecules, and the decomposition of free radicals. The method provides a means of obtaining knowledge about reactions which are not readily susceptible to direct experimental investigation. The deviations of the frequency factors of some unimolecular decompositions from a value of 1013±1 are discussed.

The recombination and disproportionation reactions of CH2CH2Cl and CCl3 radicals have been investigated. Trichloromethyl radicals and monochloroethyl radicals were produced by photolyzing carbon tetrachloride in the presence of ethylene. The main features of the resulting reaction mechanism are given by reactions (1) to (8) CCl4+hν→CCl3+Cl, Cl+C2H4→C2H4Cl, 2C2H4Cl→(C2H4Cl)2, 2C2H4Cl→C2H4+C2H4Cl2, 2C2H4Cl→C2H5Cl+C2H3Cl, 2CCl3→C2Cl6, C2H4Cl+CCl3→CCl3C2H4Cl, C2H4Cl+CCl3→CHCl3+C2H3Cl. The following rate constants have been determined k4/k3≤0.1;k5/k3≤0.1;k7/(k312k612)=2.5±0.2,and k8/k7=0.11±0.02.

Methyl‐ and ethyl‐induced polymerizations of ethylene and propylene were carried out in static, as well as flow experiments. The latter allowed collection of enough reaction products for analysis by low temperature distillation. Efficiency of polymerization in propylene is much lower than in ethylene which is explained by the easy formation of allyl radicals. Analyses of the reaction products readily indicate that the polymer is formed by the successive addition of olefin molecules to radicals, that chain transfer through radical decomposition takes place, and that the chains are terminated by disproportionation or association of two radicals.

The relative contributions of direct molecular elimination and radical disproportionation reactions to the formation of nonrandomized nitrogen in the photolysis of hydrazine and hydrazine–15N have been determined by examining the production of 30N2 in the photolysis of hydrazine–15N at 1470 A in the presence of 14NO. The limiting value of the ratio H2/30N2 in close to two and suggests that the mechanism of the molecular elimination processes is best represented by the sequence N2H4→N2H2 + H2, N2H2→N2 + H2. The ratio of disproportionation to combination of radicals in the system (N2H3 and NH2) is approximately five. The formation of 29N2 is attributed to the reaction of 15NH2 with 14NO while the formation of 28N2 is discussed in terms of the reaction of atomic hydrogen with 14NO.

The primary reactions between radicals and organic molecules are discussed from the point of view of a principle which states that there shall be least change in atomic position and least change in electronic configuration during an elementary reaction. We have concluded that alkyl radicals will attack most easily exposed positive atoms, i.e., in organic molecules, the hydrogen atoms. The attack on exposed negative atoms such as N, O or Cl is less probable whereas a completely shielded carbon atom should certainly not be attacked in the primary step. A doubly or triply bonded carbon atom can, however, react with a radical. These statements are discussed from the experimental point of view, together with the question as to when it is necessary to assume that organic reactions proceed through radical chains.

The hydrazyl radical (N2H3) has been generated in the absence of direct decomposition of hydrazine by the reaction H+H2H4→N2+H2H3. Photolysis of CH3SH at 2537 A was used as a source of atomic hydrogen. Using various mixtures of 28N2H4 and 30N2H4, the distribution of isotopic nitrogens formed in this system leads to (29N2)2 / (28N2)(30N2) = 1.3 × 10−2. Identifying radomization of isotopic nitrogen with combination N2H3 and nonradomization with disproportionation of N2H3, the ratio of disproportionation to combination is approximately 4. This is an unexpectedly large value, being nearly 30 times larger than that for the analogous hydrocarbon radical C2H5.

The chemical stabilization of coumarin 1, 7‐diethylamino‐4‐methylcoumarin, in a nitrogen laser pumped dye laser and coumarin 311, 7‐dimethylamino‐4‐methylcoumarin, under cw conditions with sulfur‐free radical chain transfer agents, are described. The mechanism for stabilization involves encounter of triplet coumarin and ground state coumarin with subsequent radical formation and radical disproportionation catalyzed by the chain transfer agents. The output of the coumarin 1 dye laser decreased 10% when the dye solution was stabilized with cysteine hydrochloride over a 12‐h period. The output of the unstabilized dye laser decreased 50% during a similar period of operation.

In order to obtain information regarding disproportionation and recombination reactions of ethyl radicals and iodine atoms we have investigated the photolysis of various mixtures of diethyl ketone and methyl iodide.Ethyl radicals, produced by photolysis of diethyl ketone, react with iodine atoms, produced by photolysis of methyl iodide, according to reactions (13) and (14) C2H5+I→C2H4+HI, C2H5+I→C2H5I. The results indicate k13/k14=0.33±0.03. The influence of added gas and the effect of temperature are discussed.

Pulsed 6‐Mev electrons from a Lineac have been used to initiate free radical reactions in the cyclohexane‐oxygen system. The primary radical formed, cyclohexyl, is identified through EPR analysis of the irradiated solid sample at —100°C. In the liquid phase the cyclohexyl‐peroxy radical is formed; its decay is observed spectroscopically over a period of 10 msec, and the second‐order rate constants calculated. The principal reaction involving the disproportionation of two cyclohexyl‐peroxy radicals requires zero (

Absolute rate constants, for the reactions of hydroxyl radicals with C3H6, 1‐C4H8 and 1‐C5H10 at 297±2 K, have been measured in the gas phase, using the flash‐photolysis resonance‐absorption technique and OH from the flash‐photolysis of N2O/H2 mixtures, λ≳200 nm. The rate constants in units of cm3 mol−1 s−1 were found to be: kC3H6=1.48±0.17×1013, k1‐C4H8=2.01±0.15×1013, and k1‐C5H10=2.39±0.23×1013. The quoted errors are the 95% confidence limits. The approximate concentration of free radicals formed in the system has been determined in separate experiments. The maximum effect of secondary reactions of these radicals on the measured rates has been discussed and assessed to be less than 10%.