Cyclohexadienyl Type Research Articles

Optical absorption spectra and kinetics of transients of phenolic stabilizers in n-hexadecane; A pulse radiolysis study

By means of pulse radiolysis experiments the formation of transients derived from differently structured phenolic stabilizers such as 2,6-di-tert-butyl-4-methylphenol, n-octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate and bis-(2-hydroxy-3-tert-butyl-5-methylphenyl)sulfide and one of its isomers in n-hexadecane solution was analyzed. Some phenoxyl and cyclohexadienyl type radicals, anions and their subsequent radical products are identified and characterized kinetically.

Radical formation in polydimethylsiloxanes and polydimethyldiphenylsiloxanes studied by the ESR spintrap technique

The radicals formed by γ-irradiation of solid polydimethylsiloxanes CH 3 (— Si— O—) n CH 3 and polydimethylsiloxanes containing 20 mol% diphenylsiloxane units were investigated by the sprintrap technique using 2,4,6-tri-t-butylnitrosobenzene (BNB). After irradiation in vacuum at 77 or 293 K the spin adducts of the following radicals could be identified: Si ., H 3C . and Si-CH . 2. The ratio Si ./Si-CH . 2 increases with temperature. The spin adduct of the H . radical was not detected. Based on the temperature dependence and the lack of H . radicals an ionic mechanism for radical formation is proposed. In the PDMS containing diphenylsiloxane units no spin adducts originating from radicals at the phenyl group were detected. However, in irradiated samples without spintrap after the decay of the radicals produced in the PDMS fraction, small concentrations of the radical  S i-phenyl and three radicals of cyclohexadienyl type were found.

Rate constants for the reactions of cyclohexadienyl type radicals with quinones

Observation of substituted cyclohexadienyl type radicals by muon spin rotation in substituted benzene at different concentrations of added p-benzoquinone or duroquinone allows a selective determination of rate constants for the reaction of the radicals with oxidants. Methyl substitution has no effect, methoxy substitution enhances the rate constants for the ortho and para substituted isomers.

Muonium-substituted organic free radicals in liquids. Muon-electron hyperfine coupling constants and the selectivity of formation of methyl and fluorine-substituted cyclohexadienyl type radicals

Muonium-substituted free radicals are observed by muon spin rotation when positive muons are stopped in liquid methyl, and fluorine-substituted benzenes. From muon precession frequencies in high external magnetic fields the isotropic muon-electron hyperfine coupling, constants A gm are determined. They are typical of cyclohexadienyl-type radicals. The individual assignments are based on deuteration and on substituent effects. Comparison of A gm with A p of the hydrogen analogues reveals isotope effects A μμ p/ A pμ μ of 1.15–1.21. Analysis of signal amplitudes yields substituent effects on the relative rates of Mu addition to inequivalent positions in the arenes. Both CH 3 and F are ortho directing. Addition at CH sites is favored over ipso addition by a factor of ≈ 3.

Optical Studies of Hydronaphthyl Radicals Embedded in Dihydronaphthalene Crystal

Abstract Radiation-induced radicals in dihydronaphthalene have been investigated at 4.2 K by absorption, fluorescence and fluorescence excitation studies. It was shown that the radicals in dihydronaphthalene are hydronaphtyl radicals. The advantages of using the dihydro compounds in the identification of cyclohexadienyl type radicals is discussed.

Pulse radiolysis study of chlorpromazine and promazine free radicals in aqueous solution

·OH radicals may react with chlorpromazine in four different ways, viz.(i) electron transfer to produce the cation radical, (ii) addition to the sulphur atom followed by acid-catalysed OH– elimination to yield the cation radical, (iii) addition to the aromatic rings to produce cyclohexadienyl type radicals, and (iv) abstraction of hydrogen atoms from the —CH2— which is in the α position to the ring nitrogen. Electron transfer and addition to the sulphur atom each account for 40% of the ·OH radicals. Similar considerations apply to promazine.

Absorption spectra and reaction kinetics of the radical anion, radical cation and OH adduct(s) of 3,5‐dinitroanisole studied by pulse radiolysis

AbstractPulse radiolysis of an aqueous solution of 3,5‐dinitroanisole (dNA) containing 0.1 mol dm−3 isopropanol results in the formation of the radical anion of dNA by attachment of solvated electrons with a rate constant of 4.2 × 1010 dm3 mol−1 s−1 and by reaction of dNA with the isopropanol radical, (CH3)2COH, with a rate constant of 2.5 × 109 dm3 mol−1 s−1. The absorption spectrum of dNA− is characterised by a maximum at approximately 275 nm (ϵ,≈︁14 × 103 dm3 mol−1 cm−1) with shoulders at 300 nm and 410 nm. The extinction coefficient is less than 100 dm3 mol−1 cm−1 in the wavelength region 500 nm to 600 nm. The anion decays by a second order process the rate of which decreases with increasing pH.Reaction of OH radicals with dNA in an aqueous solution occurs with a rate constant of 4 × 109 dm3 mol−1 s−1 and results in an absorption in the visible region. The absorption can be separated into two components with maxima at 470 nm and 515 nm on the basis of differing decay kinetics at different wavelengths. The absorption with γmax = 470 nm has an additional band which increases into the infra red and is assigned to a complex between the OH radical and the π‐electron system of dNA. The absorption with γmax = 515 nm is assigned to a combination of the four possible cyclohexadienyl type radicals which can result from σ‐bond formation between OH and a ring carbon atom.An absorption with γmax = 480 nm, found on pulse irradiation of a deaerated solution of dNA in cyclohexane, is attributed to the radical cation of dNA. The extinction coefficient of dNA+ is estimated to be 6.0 + 0.6 × 103 dm3 mol−1 cm−1.

Radiation chemistry studies on chemotherapeutic agents. Part 1.—Pulse radiolysis of adrenalin in aqueous solutions

Adrenalin has been studied as a model radiation protective agent by means of pulse radiolysis in aqueous solutions. The rate constants for the reactions of adrenalin with e–aq and OH were determined : k(e–aq+ adr—NH+2)= 7.5 × 108 dm3 mol–1 s–1, k(e–aq+ adr—NH)= 2.5 × 108 dm3 mol–1 s–1, and k(OH + adr)= 2.2 × 1010 dm3 mol–1 s–1(pH = 9.2). e–aq attacks the amino group by splitting off methylamine, whereas OH and O–aq lead to the formation of the corresponding adducts of the cyclohexadienyl type. OH radicals can also abstract an electron from an O– group at pH > 8.

Photochemical Addition of Benzaldehyde to 1-Alkynes the Formation of lndanone Derivatives

Irradiation of a solution of benzaldehyde and a monosubstituted acetylene(n-butyl, t-butyl, n-propyl or phenylacetylene) in benzene with high presure mercury lamp gave 3-substituted 1-indanone (3-n-butyl (yield, 9.0%), 3-t-butyl (9.0%), 3rn-propyl (9.0%) or 3-phenylindanone (1, 4%o)) The reaction of benzaldehyde with phenylacetylene gave trans-ev-phenyleinnamaldehyde (16, 6%)and trans-benzalacetophenone (O.4%) in addition to the indanone (Table 1 and Equation 2) The following mechanism of the indanone, formation (Equation 5) was prpposed from the result of the reaction using deuterated starting compounds. (C6D5CHO3, n-C4H9C CD) or using an excess amount of benzaldehyde as a hydrogen donor. Additien of benzoyl radical (formed from the photochemical decomposition of benzaldehyde) to the monosubstituted acetylene gives a vinyl radical, which cyclizes to a cyclohexadienyl type intermediate by an intramolqcular, attack to the phenyl ring of benzoyl moiety. Then the therma11y allowed [1-5]sigmatropic rearrangement of hydrogen may give the enol of the 3-substituted 1-indanone. This mechanism was, confirmed by an independeht study in which benzaldehyde was allowed to react with n-butylacetylene in the presence of a radical initiator.The photochemieal formation of trans-a-phenylcinnamaldehyde and trans-benzalacetophenone sugested the interrnediacy of the, corresponding oxetene derivatives. The differenee of the reaction pathway between phenylacetylene and the monoalkyl-substituted acetylene was explained in terms of the electron donating nature of the substitdent on the acetylenic bond. Phenylacetylene may be able to trap the electrophilic excited-state of benzalde hyde, whereas the monealkyl-substituted acetylene does not.

ESR study of irradiated single crystal ofp-toluidinium chloride

Electron spin-resonance has been observed in irradiated single crystals ofp-toluidinium chloride. The spectra obtained for different orientations of the crystal are analysed on the basis of the formation of cyclohexadienyl type of radicals. Partially deuteratedp-toluidinium-ND3 chloride sample was studied to confirm these results. A qualitative study of irradiatedo-toluidinium bromide and the related deuterium labelled compounds has supported the formation of such radicals.

Radiolysis of ortho- and meta-terphenyl

A summary is presented of a series of investigations of the radiolytic decomposition of ortho- and meta-terphenyl by reactor radiation with varying proportions of fast neutrons and gamma rays and of meta-terphenyl by 1.3 MeV electrons in the temperature range 100 to 450 °C with radiation intensities from 0.01 to 2 Mrads/s. The observations indicate two regions of radiolytic decomposition with a transition zone at 350 to 400 °C. In the lower temperature region, G(decomposition) varied little with temperature, increased with linear energy transfer (l.e.t.), was independent of intensity, and yielded mainly high boiling products. Particular attention was given to the high temperature region, where G(decomposition) increased rapidly with temperature, was independent of l.e.t., was greater at low intensities than at high intensities, increased with increased pulse frequency for intermittent irradiation, and yielded a greater proportion of benzene and biphenyl. Thermally initiated reaction appeared unimportant except in the absence of radiation. A reaction sequence is given which accounts for these observations in terms of reactions of radical precursors and radicals of the cyclohexadienyl type. A chain reaction occurs in the higher temperature region.

Effets des Rayons Gamma sur les Poly-2- et −4-Vinylpyridmes

γ-Irradiated poly-2-vinylpyridine (2-PVP) and poly-4-vinylpyridine (4-PVP) become crosslinked and partially insoluble. The G value for crosslinking has been determined for atactic (0.27) and isotactic (0.65) 2-PVP and for atactic 4-PVP (0.33). The results are compared with published data on polystyrene. ESR spectra of irradiated 2-PVP and 4-PVP suggest that a cyclohexadienyl type radical is formed, as in polystyrene. A chain reaction involving direct intramolecular hydrogen transfer is proposed to explain the experimental results.

ESR study of free radicals formed by γ‐irradiation of poly(ethylene terephthalate)

AbstractFree radicals formed by γ‐irradiation of oriented samples of poly(ethylene terephthalate) (PET) were studied at 25°C. by ESR spectroscopy. The G value for trapped free radicals is 0.02. The predominant radical is identified as $\text{O} - \text{\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{C} H} - \text{CH}_\text{2} - \text{O} -$, and a minor component of the spectrum is tentatively assigned to the radical COĊ6H3CO. The absence of cyclohexadienyl type radicals is discussed. Previously reported dose rate effects in PET are explained by reference to a chain reaction involving β‐bond scission of the radical $\text{O} - \text{\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{C} H} - \text{CH}_\text{2} - \text{O} -$.

Mechanism of protective action of benzene rings in radiolysis of polystyrene

AbstractThe crosslinking energy in the radiolysis of polystyrene is 100‐fold that of polyethylene, whereas the free radical yield in the radiation of linear paraffins is only 5–6 times as much as that from alkylaromatic compounds. Therefore, the high radiation stability of polystyrene cannot be explained by the conventional scheme for the action of the benzene rings. In order to explain the high radiation stability of polystyrene the mechanism of the reactions taking place during radiolysis of the polymer must be examined. A hydrogen atom liberated as the result of rupture of a C–H bond under the action of radiation may react in two ways: (1) abstraction of a hydrogen atom from the aliphatic chain, and (2) addition to the double bond of the benzene ring with the formation of a free radical of the cyclohexadienyl type. Owing to the low rate of diffusion in the solid polymers and the high rate of reaction with atomic hydrogen, the radicals formed in this way should be in the immediate vicinity of the primary radical. Interaction of the latter with the radical formed by removal of a hydrogen atom from the aliphatic chain will lead to the formation of crosslinks. The interaction between the primary radical and the radical of the cyclohexadienyl type may take place in the following two ways. The radicals may recombine with each other to form crosslinks or they may engage in a disproportionation reaction. Disproportionation will lead to restoration of the initial state and hence to the dissipation of energy without crosslink formation. This will result in a drop in efficiency of the crosslinking process, the energy for the formation of one crosslink increasing. In order to obtain experimental verification of the above conceptions as to the mechanism of interaction of the primary and cyclohexadienyl radicals, the radiolysis of deuterated toluene C4H5CD3 was studied at the temperature of liquid nitrogen. The addition to the benzene ring of a deuterium atom formed on rupture of a C–D bond under the action of radiation and the subsequent disproportionation of the cyclohexadienyl and primary radicals should, owing to the isotope effect, lead to transition of deuterium atoms from the aliphatic chain to the ring. The results obtained showed that the rate of these processes should exceed by at least 5–6 fold the rate of the processes leading to crosslinkage. It thus follows that the high radiation stability of polystyrene is to a considerable extent associated with disproportionation of the primary and cyclohexadienyl radicals.