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.     

A synthetic approach to C-nor-D-homosteroids based on a cascade of radical cyclisations from a vinylcyclopropane-substituted acyl radical precursor

Treatment of the arylvinylcyclopropane-substituted seleno ester 5 with Bu₃SnH–AIBN, under high dilution in benzene at 80 °C, led to a 1:1 mixture of C10 methyl epimers of the C-nor-D-homosteroid ring system 24/25. The homosteroid was formed from 5 via a cascade of sequential acyl 13-endo trig radical macrocyclisation, benzyl radical 5-exo trig transannulation and alkyl radical transannulation reactions (Scheme 1). The macrocyclic dienone 23 was also isolated as an intermediate in the radical cascade between 5 and 24/25, and the dioxolanes 29 were interesting by-products. The cascade of radical cyclisations leading to the homosteroid 24/25 from the acyl radical precursor 5 is compared and contrasted with similar radical cascades from arylvinylcyclopropane-substituted alkyl radical precursors, i.e, 30→31 and 32b→38.     

Anodic cyclization reactions: probing the chemistry of N,O-ketene acetal derived radical cations

The chemical reactivity of radical cations derived from N,O-ketene acetals has been examined and compared with the reactivity of radical cations derived from both ketene dithioacetals and enol ethers. Synthetically, the N,O-ketene acetal radical cations lead to more efficient cyclization reactions than either the ketene dithioacetal or enol ether derived radical cations. Cyclic voltammetry experiments using allylsilane trapping groups show that the efficiency of these cyclizations is not due to the N,O-ketene acetal radical cations being more reactive but rather more stable to decomposition. Finally, cyclizations using chiral oxizolidinones were examined.     

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.     

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.     

Reductive polymerization of Para-cyanobenzaldehyde in dimethyl sulfoxide solutions using the rotating ring-disk electrode technique

The rotating ring-disk electrode (RRDE) technique has been employed to study the reductive polymerization mechanism of para-cyanobenzaldehyde (CBA) in dimethyl sulfoxide (DMSO) solutions. The radical anion of CBA underwent polymerization through two different reaction routes. They were found to be the successive parent molecule addition to the radical anion reactions and the dimerization of the radical anion followed by parent molecule additions. Digital simulation methods were employed to simulate the mechanism and to obtain the second-order reaction rate constants of the radical anions from collection efficiency measurements. The reaction rate constants were found to be 1.45 M^?1 s^?1 for reactions of the radical anion with the parent molecules, and 28.6 M^?1 s^?1 for dimerization of the radical anion followed by trapping a parent molecule immediately after the dimeric dianion is generated.     

Intrinsic acidity and redox properties of the adenine radical cation

The intrinsic chemical properties of the gaseous adenine radical cation were examined by using dual cell Fourier transform ion cyclotron resonance mass spectrometry. The adiabatic recombination energy of the radical cation (ionization energy of neutral adenine) was found by bracketing experiments to be 8.55 ± 0.1 eV (at 298 K; earlier literature values range from 8.3 to 8.9 eV). Based on this value, the heat of formation (ΔH_f²⁹⁸) of the adenine radical cation is estimated to be 246 ± 3 kcal/mol. The acidity (ΔH_acid²⁹⁸) of the adenine radical cation was bracketed to be 221 ± 2 kcal/mol. These thermochemical values suggest that the adenine radical cation reacts with neutral guanine by electron abstraction or proton transfer, with neutral cytosine by proton transfer, and via neither pathway with neutral thymine, molecular water or a sugar moiety of DNA (modeled by tetrahydrofuran). Experimental examination of the gas-phase reactivity of the adenine radical cation revealed a slow deuterium atom abstraction from perdeuterated tetrahydrofuran. Hence, in the absence of a nearby guanine or cytosine, the adenine radical cation may be able to abstract a hydrogen atom from a sugar moiety of DNA.     

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.     

Radical addition reactions of chiral phosphorus hydrides

Intermolecular radical addition of a (1R,2R,3S,5R)-(−)-pinanediol-derived thiophosphite leads to the diastereoselective formation of organophosphorus adducts. Addition of the intermediate phosphonothioyl radical to electron-rich alkenes or alkynes occurs with retention of configuration at phosphorus.     

An electron spin resonance study of radicals from chloramine-T—2 : Spin trapping of photolysis products of chloramine-t at alkaline pH

Irradiation of chloramine-T at alkaline pH in the presence of the spin trap 2-methyl-2-nitrosopropane gave evidence for the trapping of several sulfur-centred radicals and a carbon-centred radical. Trapping experiments with 5,5-dimethyl-pyrrolidine-1-oxide gave evidence for the production of a nitrogen- centred radical and a carbon-centred radical. The spin trap α-phenyl-t-butyl-nitrone gave evidence for a nitrogen-centred radical, a sulfur-centred radical and the H-atom adduct of the spin trap. The identity of the trapped species was confirmed by irradiation of the following chemical analogues of chloramine-T as “model [compounds” in alkaline solution; chloramine-B (sodium salt of N-chlorobenzene sulfonamide), p-toluenesulfonamide, p-toluenesulfonic acid, p-toluenesulfinic acid. To aid in the assignment of the radical adducts where mixtures of species occurred, computer simulation of the spectra was performed.     

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.     

Analysis of guest—host and intra-guest transitions for hydronaphthyl radicals in naphthalene crystals

High resolution spectroscopic studies of optical absorption and emission around 539 nm and 636 nm for 1- and 2-hydronaphthyl radicals in naphthalene crystals have been made. For each radical several lines with the same polarization as the intra-guest OO vibrational lines were assigned as the vibrational progression, the separation being explained in terms of the intramolecular vibrations of naphthalene. About 20 other lines for each radical, including completely a- or b-polarized lines, were ascribed to the guest—host charge transfer transitions. Each line was assigned to the transition to specific molecules around the radical taking the polarization into consideration. The energy of the transition to a molecule at a distance r can be described by a coulombic relation with the dielectric constant ?: E = E_∞ ?e²/?r, where E_∞ is a constant. The phonon sidebands of the absorption and emission lines of the most intense charge transfer transition in the 1-hydronaphthyl radical was also analyzed and compared with phonon singularities in naphthalene crystals.     

Radical oxyamination of vinyl azides with N-hydroxyphthalimide under the action of [bis(trifluoroacetoxy)iodo]benzene

O,O′-Bis(phthalimido)-modified 2-(hydroxyimino)ethanols containing N–O–N fragment were synthesized in high yields via the reaction of vinyl azides with N-hydroxyphthalimide under the action of hypervalent iodine-based oxidant. The reaction proceeds under mild conditions and is compatible with a wide range of vinyl azides. Presumably, the process starts with the oxidative formation of phthalimide-N-oxyl radical, followed by its addition to vinyl azide with the subsequent trapping of the generated iminyl radical with the second phthalimide-N-oxyl radical.     

A comparison of the relative energies of isomeric intermediates formed in electrophilic,radical, and nucleophilic aromatic substitution reactions

The relative energetics of isomeric σ-complex intermediates formed in electrophilic, free radical, and nucleophilic attack of Cl, H, and Me on PhCl and PhMe are compared for all positions: o, m, p, and ipso. The results are presented as ΔH, computed by the MINDO/3 method, for the appropriate isodesmic reaction with benzene. For the chlorobenzene intermediates, there is a marked increase in the relative preference for the ipso-position as the reactions change from electrophilic to free radical to nucleophilic. For toluene intermediates, the order of stability-p > o > m > ipso-is seen in all reactions except for one case, free radical methylation. In general, the free radical intermediates show the smallest range of energy differences. Comparison of predictions from these calculations with experimental results (largely partial rate factors and product ratios) shows some qualitative agreement.     

A study on the reaction between N-substituted p-phenylenediamines and ozone: experimental results and theoretical aspects in relation to their antiozonant activity

The following p-phenylenediamines (PPD): N,N,N^′,N^′-tetramethyl-p-phenylenediamine (TMPPD), N,N^′-dimethylbutyl-p-phenylenediamine (6PPD), N,N^′-diaryl-p-phenylenediamine (DPPD), tris-(N-dimethylpentyl-p-phenylenediamine)-N^′,N^′,N^′-1,3,5-triazine (6PPDTZ), have been oxidized under the action of O₃ in diluted solutions. In all cases the radical cation or semiquinone radical was the first derivative formed by monoelectronic oxidation of the substrate. The radical cation has been studied by electronic spectroscopy and the electronic spectral changes of all mentioned PPD has been followed as function of the ozonation time. The results have been discussed in the frame of the antiozonant properties of these PPD which are used as antiozonant agents in diene rubber protection. It is shown that the antiozonant activity of each PPD considered correlates with the free enthalpy of formation of the respective radical cation. The lowest is the free energy of formation of a PPD radical cation and the highest is the antiozonant activity in a diene rubber compound.     

Exploring Radical Migration Pathways in Peptides with Positional Isomers, Deuterium Labeling, and Molecular Dynamics Simulations

One of the keys for understanding radical directed dissociation in peptides is a detailed knowledge of the factors that mediate radical migration. Peptide radicals can be created by a variety of means; however, in most circumstances, the originally created radicals must migrate to alternate locations in order to facilitate fragmentation such as backbone cleavage or side chain loss. The kinetics of radical migration are examined herein by comparing results from ortho-, meta-, and para-benzoyl radical positional isomers for several peptides. Isomers of a constrained cyclic peptide generated by several orthogonal radical initiators are also probed as a function of charge state. Cumulatively, the results suggest that small changes in radical position can significantly impact radical migration, and overall structural flexibility of the peptide is also an important controlling factor. A particularly interesting pathway for the peptide RGYALG that is sensitive to ortho versus meta or para substitution was fully mapped out by a suite of deuterium labeled peptides. This data was then used to optimize parameters in molecular dynamics-based simulations, which were subsequently used to obtain further insight into the structural underpinnings that most strongly influence the kinetics of radical migration.      

Radical cations of cyclopentadiene dimers—facets of an intriguing energy surface

The radical cations of various cyclopentadiene dimers can be generated by photoinduced electron transfer to strong electron acceptors. Nuclear spin polarization effects observed during these reactions provide insight into the radical cation structures. Three of the systems studied, endo-[4 + 2]-(1), anti-[2+2]-(2), and exo-(4+2]-dicyclopentadiene (5) give rise to unusual singly linked, delocalized radical cations, whereas the anti-[4 + 4]-dimer (3) and 1,3-bishomocubane (4) give rise to more conventional radical cations. The reactions of spiroheptadiene (9) and di(spiroheptadiene) (10) provide evidence for two different dimer radical cations, a doubly linked (D) and a singly linked (S) species. This finding supports a stepwise mechanism for the radical cation Diels-Alder reaction of 9.     

CuCl-catalyzed radical cyclisation of N-α-perchloroacyl-ketene-N,S-acetals: a new way to prepare disubstituted maleic anhydrides

The copper-catalyzed radical cyclization (RC) of N-α-perchloroacyl cyclic ketene-N,X(X=O, NR, S)-acetals was studied. While the RC of N-acyl ketene-N,O-acetals was unsuccessful, the 5-endo cyclization of the other ketene acetals provided much better results, with the following order of cyclization efficiency: hexa-atomic cyclic ketene-N,NR-acetals<penta-atomic cyclic ketene-N,S-acetals<hexa-atomic cyclic ketene-N,S-acetals. Invariably the catalytic cycle begins with the formation of a carbamoyl methyl radical. This leads to a cascade of reactions, including a radical polar crossover step, which ends with the formation of the maleimide nucleus, or precursors of this. Products from the RC of the hexa-atomic cyclic ketene-N,S-acetals, were efficiently transformed into disubstituted maleic anhydrides.