Relative concentrations and relaxation properties of isomeric alkyl radicals in γ-irradiated n-alkane single crystals

The relative concentrations of alkyl radicals CH₃C?HCH₂R(I) and R'CH₂C?HCH₂R''(II) were measured at low microwave power in some n-alkane single crystals γ-irradiated at 77 K to a dose of 1 Mrad. The relative concentration of radical (II) increased as the number of carbon atoms became larger. The amount of radical (I) was in agreement with a mechanism where all CH bonds in an n-alkane molecule are raptured with the same probability followed by an isomerization of primary alkyl radicals to radical (I). In n-decane for instance this mechanism predicts 45.5% of radical (I) compared to the experimental value of 45.5%. Saturation measurements of radical (I) and radical (II) under slow passage condition showed that the spin-lattice relaxation time T₁ is shorter for radical (I) (ca. 3 × 10^?4s) than for radical (II) (ca. 80 × 10^?4s), while the spin-spin relaxation times T₂ are similar (ca. 2 × 10^?8s). The relatively short relaxation time T₁ in radical (I) is thought to originate from higher mobility of the end of the alkane chain, where the unpaired electron is localized, and also a modulation of the hyperfine coupling from protons in the nearby rotating methyl group. The broad linewidth in irradiated protiated cyrstals was by comparison with results from deuterated matrices concluded to depend on slightly distorted radicals in damaged regions (spurs, short tracks, blobs) and not on electron dipole-dipole interactions. Unresolved γ-proton couplings in radical (I) are thought to cause the spin-flip transitions at high microwave power.     

Line dependence of CIDEP polarizations for the p-benzosemiquinone,radical

Time-resolved steady state experiments were performed on six prominent hf-lines of the p-benzosemiquinone radical (PBQH) dissolved in ethylene glycol using an improved experimental technique. For the same six lines T₁ and _T2 were determined experimentally. Initial and radical pair polarizations as well as the second order chemical decay constant β were obtained by simulating each of the time-resolved curves using a new procedure. It was found that both the initial polarization P_a^∞(I) and β were the same within experimental accuracy for all six lines in four different experiments. The radical pair polarization P_a*, however, varied within the experimental accuracy in the manner predicted qualitatively from the theory for a second order termination reaction. Light attenuation experiments with both pulsed and continuous light confirmed these results. The magnitude of P_a^∞ (I) may be interpreted in terms of the microscopic theory for the photo-activated triplet mechanism, whereas the magnitude of P_a* appears to be too high by a factor of four to six as compared to the microscopic theory for the radical pair mechanism.     

A calculation of thermal degradation initiated by random scission,unsteady radical concentration

Changes in molecular weight distribution and in sample volume were calculated for thermal degradation of a polymer. The thermal degradation scheme consists of random scission initiation, depropagation and disproportionation termination reactions. An unsteady radical concentration was considered. There are two parameters, normalized zip length z/x₀ and radical number per initial chain length z′x₀, describing the thermal degradation scheme with an unsteady radical concentration. The effects of the initial number-average molecular weight and order of the disproportionation termination reaction on changes in molecular weight, the sample volume and polydispersity are not significant as long as these two parameters have the same value for each polymer sample. Molecular weights of a degrading sample calculated from the steady state radical concentration tend to be over-estimated and sample volumes tend to be underestimated compared to those calculated with an unsteady radical concentration. The validity of approximations used in the calculation assuming a steady state radical concentration is examined by comparing with results calculated with an unsteady radical concentration for various values of the two parameters. An unrealistically large build-up of monomer radicals is found for both calculations based on the steady state and the unsteady radical concentrations. Two special treatments of monomer radicals can dissipate the build-up of monomer radicals: (1) their immediate vaporization, or (2) an enhanced rate of the termination reaction for the monomer radicals. As a guide, the model based on an unsteady radical concentration is preferred, if the value of z′x₀ exceeds 0.1.     

ESR spectrum of the cyclopropenyl radical

The ESR spectrum of a radical produced by photolytically generated t-butoxy radicals and cyclopropene in solution is reported. Arguments are presented for an assignment of the spectrum to the cyclopropenyl radical (CH)₃ undergoing fast exchange between equivalent distorted r_e structures.     

NH2 radicals trapped in NH3 single crystals and various polycrystalline ammonia matrices

?H₂ radical trapped in various ammonia matrices has been investigated by ESR spectroscopy. From the study of the coupling tensors of the ?H₂ radical in a single crystal of NH₃, and taking into account the motions of this radical, it is shown that for all these matrices the spectra can be interpreted on the basis of coupling tensors which give for all cases the same isotropic coupling constants: a_N = 11.3 G, a_H = 24.6 G. Nitrogen and hydrogen coupling tensors for the motionless ?H₂ radical are also discussed     

Structures and reactions of radical cations of some bicycloalkanes and their related alkenes as studied by 4 k matrix esr

The σ radical cations of most typical bicycloalkanes such as norbornane and bicyclo[2,2,2]octane are radiolytically produced at 4 K in halogenocarbon matrices and are studied by ESR spectroscopy. Their electronic and geometrical structures as well as their dynamical behaviors have been elucidated from the hyperfine structures and their temperature changes. The semi occupied molecular orbital (SOMO) of the former cation is 4a₂, in which the unpaired electron delocalizes over the four exo C-H bonds giving large hyperfine coupling. The latter is a Jahn-Teller active species and exhibits static distortion from D_3h to C_2v at 4 K in CFCl₃, and the SOMO is likely to be 6b₂, in which the unpaired electron delocalizes over the four endo C-H bonds giving large proton coupling, although a dynamically averaged structure with 12 equivalent methylene protons is observed in C-C₆F₁₂ as well as in CFCl₂CF₂Cl matrices at 77 K. The unpaired electron distribution in bicycloalkane radical cations is similar to that in cycloalkane radical cations previously studied. Upon warming both the cations undergo deprotonation to give 2-yl alkyl radicals from the exo or endo C-H bond, at which the higher unpaired electron density is populated. In addition to these radical cations, the structures and reactions of the radical cations of the related bicycloalkenes such as norbornadiene, quadricyclane, and bicyclo[2,2,2]octene have also been studied. The hydride ion transfer to an olefinic radical cation to form an alkyl radical is observed for the bicyclo[2,2,2]octene radical cation as the first example observed by ESR.     

The kinetics of radical polymerization—XXVII. Investigation of the radical reactivity of 3,3′- and 4,4′-disubstituted azobenzenes in the polymerization of vinylacetate

A study was made on the inhibition by some 3,3′- and 4,4′-disubstituted azobenzenes of the polymerization of vinylacetate initiated by azoisobutyronitrile at 50°. The inhibitory effects of these substances can be attributed to their ability to engage in radical addition giving a less reactive hydrazyl type radical. The mechanism of the inhibition has been established by ESR and kinetic (stoichiometric) measurements. The value of k₅/k₂ (characteristic of the reactivity of an inhibitor) was determined for 9 substituents. The radical reactivity of the aromatic azo group was decreased by electron donor substituents and increased by electron acceptors. The substituent effect can be well interpreted by the Hammett equation; the value of the reaction constant was ? = +0.53.     

ESR study of the photo-induced decomposition of polypropylene in the presence of N,N,N′,N′-tetramethyl-p-phenylenediamine

The mechanism of uv (λ > 325 nm) photodegradation of polypropylene (PP) containing N,N,N′,N′-tetramethyl-p-phenylenediamine (T₄MPD) has been investigated by means of ESR spectroscopy. The observed spectra after uv irradiation of both isotactic-PP (IPP) and stereoblock-PP (SPP) samples in vacuum at 77 K consisted principally of a broad singlet which was assigned to a T₄MPD cation radical (T₄MPD^+·). On the other hand, the spectrum observed after irradiation of an atactic polypropylene (APP) sample at 77 K in vacuum was resolved into several components which decayed almost up to ca. 263 K to give rise to the broad singlet of T₄MPD^+·. One component was a sharp quartet which was assigned to a methyl radical, ·CH₃·. The other component, a singlet, was attributed to a trapped electron, e_t^?.By comparison of the ESR spectrum of deuterated T₄MPD with that of the normal compound it was found that 60 ～ 70% of the methyl radicals arose from the added T₄MPD due to β-scission, which also formed the N,N,N′-trimethyl-p-phenylenediamine radical, T₃MPD·. The T₃MPD· radical presumably captures an electron at lower temperatures to become a carbanion, T₃MPD^?, which releases the electron to reproduce the T₃MPD· radical at elevated temperatures. This production of the radical T₃MPD· due to the liberation of an electron provides an explanation for the observed increase in intensity of the decay curve in the temperature range from ? 168 K to 185 K. The remaining fraction, 30 ～ 40%, of the total methyl radicals was produced from the PP matrix by an energy transfer from the excited T₄MPD¹ to the PP matrix. The broad singlet which appeared in the temperature range near 195 K was attributed to an acyl radical ～CH₂CH(CH₃)CH₂?O from the observed g-value. By photoillumination of this sample this broad singlet was converted reversibly into the quartet which was assigned to the radical ～CH₂CH(CH₂·)CH₂CHO.     

Effect of ketones on the equilibrium between peroxy radicals carrying the chains of cyclohexanol oxidation with molecular oxygen

The influence of methyl phenyl ketone and diphenyl ketone on the kinetics of cyclohexanol oxidation initiated by cumyl peroxide at 373 K was studied. The ratio of the chain propagation rate constant involving the 1-hydroxy-1-phenylethylperoxy radical to the equilibrium constant of radical dissociation into methyl phenyl ketone and hydroperoxy radical (k _2.3/K′₂ = 900 L² mol^?2 s^?1) was determined by the selective inhibition method.     

Electron spin resonance spectra of some phosphonyl radicals and related species

The isotropic ESR spectra of a number of phosphonyl radicals (X₂$\text{P}$O), the dimethylphosphinyl radical, and the phosphoranyl radical (MeO)₃$\text{P}$OBu-t, are described, and accurate values of the phosphorus hyperfine splittings and g-factors are reported. For X₂$\text{P}$O, the value of a(P) increases and the g-factor decreases as the electronegativity of X increases. There is a linear relationship between a(P) for X₂$\text{P}$O and ¹J(PH) for X₂P(O)H, but the same relationship does not hold for Me₂P- and Me₂PH. The spectrum of the di-n-hexylphosphonyl radical shows coupling to two pairs of α-methylene protons, and this non-equivalence is attributed to the pyramidal structure of the phosphonyl radical.     

Esr and structure of sulfur-centered radicals and radical ions. γ-Irradiated dimethyl disulfide and methane thiol single crystals at 77°k

Single crystals of dimethyl disulfide and methane thiol were irradiated at 77°K. CH₃S radicals were produced in both compounds and measurement of the isotropic coupling constant from the methyl protons gave a value of 7.6 G. In the dimethyl disulfide crystal both the anion, CH₃SSCH₃, and the cation, CH₃SSCH⁺₃, radicals were observed. The disulfide anion radical exhibited an isotropic septet of lines with a = 5.0 G. Comparison with measurements on a polycrystalline sample gave g⊥ = 2.020 and g6 = 2.000 for this radical. The disulfide cation radical exhibited an evenly spaced septet of lines with a = 9.1 G and a maximum value for the g factor of 2.032.On illumination with IR radiation (λ > 590 mm) the disulfide cation radicals were easily bleached together with about 50% of the disulfide anion radicals suggesting a photoinduced neutralization process. The presence of weak ³³S satellite lines in the anion radical spectrum indicates that 12% of the unpaired spin is localized to the two sulfur 3s orbitals. The structure of the disulfide cation radical is discussed in relation to earlier studies and a dihedral angle of 180° is proposed. The mechanisms for radical formation and decay in dimethyl disulfide and methane thiol are also discussed.     

Electron Capture Dissociation of Hydrogen-Deficient Peptide Radical Cations

Hydrogen-deficient peptide radical cations exhibit fascinating gas phase chemistry, which is governed by radical driven dissociation and, in many cases, by a combination of radical and charge driven fragmentation. Here we examine electron capture dissociation (ECD) of doubly, [M + H]^2+?, and triply, [M + 2H]^3+?, charged hydrogen-deficient species, aiming to investigate the effect of a hydrogen-deficient radical site on the ECD outcome and characterize the dissociation pathways of hydrogen-deficient species in ECD. ECD of [M + H]^2+? and [M + 2H]^3+? precursor ions resulted in efficient electron capture by the hydrogen-deficient species. However, the intensities of c- and z-type product ions were reduced, compared with those observed for the even electron species, indicating suppression of N?CC_?? backbone bond cleavages. We postulate that radical recombination occurs after the initial electron capture event leading to a stable even electron intermediate, which does not trigger N?CC_?? bond dissociations. Although the intensities of c- and z-type product ions were reduced, the number of backbone bond cleavages remained largely unaffected between the ECD spectra of the even electron and hydrogen-deficient species. We hypothesize that a small ion population exist as a biradical, which can trigger N?CC_?? bond cleavages. Alternatively, radical recombination and N?CC_?? bond cleavages can be in competition, with radical recombination being the dominant pathway and N?CC_?? cleavages occurring to a lesser degree. Formation of b- and y-type ions observed for two of the hydrogen-deficient peptides examined is also discussed.     

Time resolved absorption and resonance Raman spectroscopic studies of the oxidation of benzidine in aqueous solution

We report a time resolved resonance Raman study of transient radicals produced in the pulse radiolytic oxidation of benzidine in aqueous solution. The intense and structured transient absorption in the 400–470 nm region, observed at microsecond times in the acidic medium, is attributed to the benzidine radical cation. The Raman spectrum, observed by excitation in resonance with this absorption, exhibits eight prominent bands which are assigned to planar phenyl vibrations. The ring breathing mode (v₁) at 844 cm^-1 is most highly resonance enhanced, indicating an overall expansion of the ring CC bonds in the excited state. The interring CC bond, with partial double bond character, is characterized by an intense (v₁₃) Raman band at 1335 cm^-1. The frequency of the in-phase v_7a CN stretching vibration is 1540 cm^-1. These frequencies and the presence of weak bands attributable to non-planar phenyl vibrations indicate the radical to be slightly non-planar. The pK_a for the proton loss from the radical cation is 10.87, four units higher than for the aniline radical cation. At high pH the observed transient has a broad and structureless absorption at ∽ 380 nm. It is identified from its resonance Raman features as the 4(4′aminophenyl)anilino radical formed by proton loss from the radical cation. The interring CC bond is characterized by a Raman band at 1292 cm^-1, indicating it to be a single bond. The structure of this neutral radical is highly nonplanar, with little conjugation between the two ring systems so that electronic excitation is primarily confined to the anilino moiety. The acidic and basic forms of the radical react rapidly in second order processes to produce products which absorb strongly at, respectively, 360 and 410 nm.     

Ligand exchange of metal carbonyls by chain mechanisms. Electrochemical kinetics of electron transfer catalysis

The metal carbonyl derivatives LM(CO)_n of manganese, rhenium, molybdenum and tungsten undergo facile ligand substitution by an electrode-mediated process. Phosphine, pyridine and isocyanaide substitutions are shown to be chain reactions by coulometric analysis and by cyclic voltammetry of the metal carbonyl solutions containing the added nucleophiles L. Reversible electrochemical parameters can be obtained for both LM(CO)_n and the substitution product LM(CO)_n. These allow the digital simulation of the cyclic voltammograms by Feldberg's method, which provides a detailed analysis of the kinetics of the electrocatalytic mechanism for ligand substitution. Generally, the radical cation LM(CO)_n⁺ produced initially at the anode undergoes rapid exchange with L to afford the cationic substituted species LM(CO)_n⁺. This step is followed by electron transfer with the reactant LM(CO)_n to yield the substitution product LM(CO)_n and regenerate the radical cation LM(CO)_n⁺, which completes the chain propagation sequence. Multiple repetition of this cycle is indicated by the high current efficiencies which are obtainable. The second order rate constants for the ligand exchange of the 17-electron radical cations LM(CO)_n⁺ with added L are evaluated, and found to be more than 10⁶ times larger than that for the neutral, diamagnetic precursor LM(CO)_n. The radical cations LM(CO)_n⁺ also react readily with phosphine and alkyl isocyanide nucleophiles, leading to a characteristic distortion of the CV waves. Digital simulation of such cyclic voltammograms allows the kinetics of these processes, particularly the redox catalysis in the oxidation of phosphines by LM(CO)_n⁺, to be evaluated quantitatively. The enhanced reactivity of the 17-electron radical cations LM(CO)_n⁺ is discussed in relationship to recent reports of the substitution lability in other 17- and 19-electron metal carbonyls.     

Estimation of the standard reduction potentials of some 1-arylethyl radicals in acetonitrile

The redox properties of some arylethyl radicals, which are involved in the electrochemical synthesis of important anti-inflammatory agents, have been investigated in CH₃CN by an indirect electrochemical method based on the catalytic reduction of the corresponding arylethyl halides (RX) by electrogenerated aromatic or heteroaromatic anion radicals (D⁻). The reaction between RX and D⁻ leads to a radical R, which can react with D⁻ either by radical coupling (k₃) or by electron transfer (k₄). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q=k₄/(k₃+k₄), as a function of E⁰_D/D⁻ allows estimation of the reduction potentials of the arylethyl radicals. The standard reduction potentials obtained for the radicals 1-(4-isobutylphenyl)ethyl, 1-(6-methoxy-2-naphthyl)ethyl, and 1-(4-biphenyl)ethyl are −1.64, −1.62, and −1.15 V vs. SCE, respectively.     

Electron-beam initiated crosslinking in poly(N-isopropylacrylamide) aqueous solution

The reactions between the OH, H and e_aq⁻ transients of water radiolysis and a linear poly(N-isopropylacrylamide) were identified by the spectral and kinetic properties of intermediates. The radical responsible for crosslink formation is the isopropyl-centered radical that forms mainly in OH radical attack on the polymer. Below pH 3 this radical undergoes reversible protonation with pK_a≈2.1. The radical decay is composed of fast and slow parts. The initial fast decay is attributed to intramolecular reactions of radicals on the same chain (loop formation), the following slow decay to competition between further intramolecular and intermolecular (H-type crosslinks) termination processes. The differences in the network formation during irradiation of aqueous monomer and polymer solutions are discussed.     

Remote substituent effect favoring the formation of syn-adducts in the chelation controlled radical reactions of γ-benzyloxy-α-methylenecarboxylic acid esters

The chelation controlled radical reactions of ethyl γ-benzyloxy-α-methylenecarboxylates bearing a bulky γ-substituent, such as CHMe₂, CHPh₂, c-C₆H₁₁ and CH(Ph)OTBDMS, with alkyl iodides gave the syn-adducts with high diastereoselectivities. However, the diastereoselectivity for the substrates bearing a γ-substituent CH(i-Pr)OTBDMS depended critically on the configuration of the substituent; the substrate bearing the OTBDMS group anti to the γ-benzyloxy group showed poor diastereoselectivity, but its diastereomer gave syn-adduct exclusively. The high syn-selectivitiy is referred to the H-atom transfer to the outside face of radical center in the sharply folded seven-membered chelate intermediate bearing the ethoxy group with Z-geometry. The corner flapping of the radical center atom of the global minimum energy conformer generates a local minimum conformer and the H-atom transfer to the outside face of the radical center of the newly formed structure gives the anti-adduct. The poor diastereoselectivity is due to the very small energy difference between the two conformers and consequently both the syn- and anti-adducts are yielded in nearly equal amounts.     

Electron paramagnetic resonance of γ-irradiated [(CH3)4N]2ZnF4 and [(CH3)4N]2CdF4 single crystals and vibrational spectra of their unirradiated forms

γ-Irradiated [(CH₃)₄N]₂ZnF₄ and [(CH₃)₄N]₂CdF₄ single crystals were investigated by electron paramagnetic resonance (EPR) and vibrational spectroscopy of their unirradiated forms at ambient temperature. It has been found that both compounds indicate the existence of (CH₃)₃ N˙⁺ radicals. The g factors were found to be isotropic and the hyperfine constant for protons of the methyl groups was measured as 28.9 G for this radical in these substances. Each methyl group rotates around its C_3ν-axis and also rotates around the C_3ν-axis of the (CH₃)₃ N˙⁺ radical. In addition to these motions, the C_3ν-axis of the (CH₃)₃ N˙⁺ radical seems to rotate around the z-axis. These results were compared with the earlier studies in (CH₃)₃ N˙⁺ radical and discussed. The vibrational spectra of these two compounds were discussed in relation to other compounds containing tetramethylammonium salts. The assignments of the observed bands were discussed.     

Development of radical addition-cyclization-elimination reaction of oxime ether and its application to formal synthesis of (±)-martinelline

Radical addition-cyclization-elimination (RACE) reaction of oxime ether carrying unsaturated ester provides a novel method for the construction of pyrroloquinoline. Treatment of oxime ethers with Bu₃SnH and AIBN gave N-norpyrroloquinoline as a major product, which was also obtained by the radical reaction of the corresponding hydrazone and imine. The radical reaction of aldehyde and ketone carrying unsaturated ester proceeded stereoselectively to give cis-furoquinolines and cis-hydroxyesters. The RACE reactions by using each of Bu₃SnNMe₂, Bu₃SnD, and/or D₂O were also examined in order to propose a reaction pathway to N-norpyrroloquinoline. Furthermore, the synthetic utility of RACE reaction is demonstrated by preparation of a key intermediate for the synthesis of (±)-martinelline.     

Chemical exchange dynamics and structure of intramolecular OHO bridges: an ESR and indo study of 2-hydroxy- and 2,6-dihydroxy-phenoxyl

ESR spectra of phenoxyl-type radicals of 1,2-dihydroxy benzene (pyrocatechol), 1,2,3-trihydroxy benzene (pyrogallol) and their deuteroxy modifications have been studied in CCl₄ and CCl₃F solution over a temperature range ?100 ÷ +70°C. The spectrum of 2-hydroxy (deuteroxy)-phenoxy radical shows a pronounced temperature dependence, which will be analyzed by a chemical exchange model involving the proton (deuteron) of the internal H-bridge. The exchange process is described by a double minimum potential. The barrier found for this process is 8.2 kcal/mol for the proton exchange and 9.5 kcal/mol for the deuteron exchange. In contrast the ESR spectrum of the radical produced from pyrogallol is found to be temperature independent in the temperature range ?100 ÷ +25°C. Analysis of the spectrum of this radical will be shown to result from the 2,6-dihydroxyphenoxy radical. The kinetic isotope effect in the chemical exchange process of the 2-hydroxyphenoxy radical is found to be extremely large, with k_H/k_D ≈ 100 at room temperature. An INDO study is presented for the proton hyperfine coupling constants and for the potential function of the proton in the hydrogen bridge. Some discussion is devoted to the line width effects of the deuterated radical.