Cyclohexadienyl Type Research Articles

Comparison of hydrogen atom and hydroxyl radical reactions with simple aromatic molecules in aqueous solution

The reactions of hydrogen atoms and hydroxyl radicals with benzene and 10 monosubstituted benzenes were investigated by measuring transient absorption spectra, calculating Gibbs free energies, absorption spectra of intermediates formed and determining the rate constants of H reactions. The basic reaction is addition to the aromatic ring forming cyclohexadienyl type radicals. Most radical exhibit absorption spectra with maxima in the 300–340 nm range, these maxima for molecules with strong electron withdrawing groups are shifted to 340–420 nm. The spectra of H and OH adducts are similar suggesting similar intermediates. In substituted benzenes the absorption band is due to several isomers. Ortho and para additions are important in reaction with all molecules studied. Both, H and OH can react with the –Cl, –Br and –NO2 substituents in exchange reaction without cyclohexadienyl radical. OH reacts with the aromatic ring of benzyl chloride in radical addition, while H eliminates Cl from the substituent.

Hydroxyl radical induced transformation of phenylurea herbicides: A theoretical study

Aromatic ring hydroxylation reactions occurring during radiolysis of aqueous solutions are studied on the example of phenylurea herbicides by Density Functional Theory calculations. The effect of the aqueous media is taken into account by using the Solvation Model Based on Density model. Hydroxyl radical adds to the ring because the activation free energies (0.4–47.2kJmol−1) are low and also the Gibbs free energies have high negative values ((−27.4) to (−5.9)kJmol−1). According to the calculations in most of cases the ortho- and para-addition is preferred in agreement with the experimental results. In these reactions hydroxycyclohexadienyl type radicals form. In a second type reaction, when loss of chlorine atom takes place, OH/Cl substitution occurs without cyclohexadienyl type intermediate.

EPR investigation of thermal decay of radiation-induced species of benzoic acid and its sodium and potassium salts.

The structural and kinetic features of the radiation-induced radicals of benzoic acid and its sodium and potassium salts were investigated using electron paramagnetic resonance (EPR) spectroscopy. Two main different radicals were found to be responsible for the measured spectra of the irradiated samples. It is concluded that these two radicals have a structure similar to that of cyclohexadienyl-type (CHD) and benzyl-type (BNZ) radicals. The relative contributions of the CHD and BNZ radicals to the measured peak-to-peak amplitude and to the total spectra were calculated. The room-temperature stability of the EPR signals and the decay kinetic features of the radiation-induced radicals derived from annealing at high temperatures were determined.

High-energy ionising radiation initiated decomposition of acetovanillone

Hydroxyl radical, hydrated electron and hydrogen atom intermediates of water radiolysis react with acetovanillone with rate coefficients of (1.05±0.1)×1010, (3.5±0.5)×109 and (1.7±0.2)×1010mol−1dm3s−1. Hydroxyl radical and hydrogen atom attach to the ring forming cyclohexadienyl type radicals. The hydroxyl–cyclohexadienyl radical formed in hydroxyl radical reaction in dissolved oxygen free solution partly transforms to phenoxyl radical. In the presence of O2 phenoxyl radical formation and ring destruction are observed. Hydrated electron in O2 free solution attaches to the carbonyl oxygen and undergoes protonation yielding benzyl type radical. In air saturated 0.5mmoldm−3 solution using 15kGy dose most part of acetovanillone is degraded, for complete mineralisation five times higher dose is required. The experiments clearly show that acetovanillone can be efficiently removed from water by applying irradiation technology.

EPR and ab-initio study on the solid state radiolysis of aliphatic and aromatic polyesters

Low temperature matrix EPR spectroscopy was employed to investigate the free radical intermediates in the solid state radiolysis of poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(ethylene naphthalate) (PEN), poly(1,4-cyclohexanedimethylene terephthalate- co-ethylene terephthalate) (PCT- co-ET) and poly(ethylene adipate) (PEA). With PEA the species identified is consistent with the formation of carbon centred radicals stemming from electron capture, C–O scissions and H abstractions, the most abundant species at room temperature being the α-ester radical, –CH 2–CH –COO–. The radiolysis of PEN is characterized by the predominance of radicals arising from the aromatic section, the preferred reaction being the H atom addition at a position adjacent to the ester unit, leading to the formation of cyclohexadienyl type radicals. In the copolymer, PET and PBT, the role played by the aliphatic and aromatic sections with respect to radical forming processes is of comparable importance. The cyclohexadienyl radicals arising from these latter polymers are reckoned with the prominent H addition at the sites adjacent to the esters groups which are also statistically favoured. The reactivity distribution in the aromatic rings with respect to the formation of cyclohexadienyls was analyzed in greater detail by comparing the experimental results with DFT B3LYP M.O. calculations. The DFT method has also been employed for assessing the EPR properties of the cyclohexadienyl and ion-radicals from PEN and PET models.

Infrared and Electronic Spectroscopy of a Model System for the Nucleophilic Substitution Intermediate in the Gas Phase: The C−N Valence Bond Formation in the Benzene−Ammonia Cluster Cation

Infrared and electronic spectroscopy was applied to the benzene-ammonia cluster cation in the gas phase, and the observed spectra revealed the formation of a new C-N valence bond between the benzene and ammonia moieties, which has been predicted by the quantum chemical calculations (Tachikawa, H. Phys. Chem. Chem.Phys. 2002, 4, 6018). This cluster cation is regarded as a model for the cyclohexadienyl type intermediate in nucleophilic substitution reactions.

Intramolecular addition of cysteine thiyl radicals to phenylalanine in peptides: formation of cyclohexadienyl type radicals

The intra-molecular addition of peptide cysteine thiyl radicals to phenylalanine yields alkylthio-substituted cyclohexadienyl radicals for the peptides Phe-Cys and Phe-Gly-Cys-Gly, i.e. even when Phe and Cys are separated by a Gly residue, and presents a possible free radical pathway to thioether-containing peptide and protein cross-links.

Mechanism of OH radical-induced oxidation of p-cresol to p-methylphenoxyl radical

The OH radical-induced oxidation of p-cresol to p-methylphenoxyl radical was studied in aqueous solution in a wide pH range by means of pulse radiolysis combined with optical spectroscopy. OH-adduct cyclohexadienyl type radicals were identified as intermediates of the reaction. In the acidic pH range the first-order rate coefficient of phenoxyl radical formation was found linearly dependent on the H3O+ concentration yielding a bimolecular rate coefficient of 1.8 × 108 mol–1 dm3 s–1. In the alkaline range a linear dependence was found on the OH– concentration with rate coefficient of 4.9 × 1010 mol–1 dm3 s–1. These findings were interpreted in terms of acid-base catalysis of the H2O elimination from the OH-adduct. With the time resolution applied, 30 ns, the radical cation p-CH3C6H4OH+. was not observed as intermediate.

Preparation of 1-phenylcyclohexa-2,5-diene-1-carboxylates and their use in free-radical mediated synthesesElectronic supplementary information (ESI) available: experimental procedures for the preparation of 1,4-dihydrobiphenyl,9 1-deuterio-1,4-dihydrobiphenyl, 1-phenylcyclohexa-2,5-diene-1-carboxylic acid with 3,4-dihydrobiphenyl-3-carboxylic acid, 2-(cyclohex-2-enyloxy)ethyl 1-phenylcyclohexa-2,5-diene-1-carboxylate and non-1-en-6-ol. Experimental details of the reactions of cyclopentyl 1-phenylcyclohexa-2,5-diene-1-carboxylate with cyclohexenone, methyl acrylate, methyl methacrylate, acrylonitrile and cyclohexene are included. See http://www.rsc.org/suppdata/p1/b1/b110527m/

Synthetic routes to pure 1-phenylcyclohexa-2,5-diene-1-carboxylic acid and derived esters were developed. Esters containing appropriately unsaturated side chains generated the corresponding alkenyl radicals and hence gave good yields of 5-exo ring closure products in organotin-free reactions. Extrusion of phenyl radicals from the intermediate cyclohexadienyl type radicals was not observed, and this alternative β-scission did not compete under any conditions. Yields from alkylations of olefins in analogous intermolecular processes were, however, poor. As a spin-off from the research, it was found that 1-phenylcyclohexa-2,5-diene-1-carboxylic acid (6) was a useful source of hydroxyformyl (formate) radicals in organic solvents.

Pulse radiolysis study of diphenylcarbazide in aqueous solutions

The semi-reduced species formed by the reaction of e − aq with DPC at pH 6.8 was found to have three absorption bands with λ max values at 320, 360 and 480 nm. It was found to be reducing in nature. H-atom reaction with DPC at pH 2 initially gave a transient species with absorption maximum at 340 nm which can be assigned to the cyclohexadienyl type of radical. This species decayed by first order kinetics to give another transient species with spectrum similar to that obtained by e − aq reaction with DPC. Reaction of acetone ketyl radicals with DPC at pH 2 and 6.8 also gave the same spectrum as obtained by e − aq reaction. The initial OH adduct of DPC at pH 6.8 has absorption maximum at 340 nm (due to hydroxycyclohexadienyl type radical). This species also decayed by first order kinetics to give a semi-oxidised species having an absorption maximum at 480 nm. One-electron oxidants like N 3 radicals also reacted with DPC to give the same species with λ max at 480 nm.

Free radicals formed by H(Mu) addition to pyrene

Muonium addition to pyrene has been investigated by means of transverse field muon spin rotation and muon level-crossing spectroscopy. Two hydropyrenyl radicals are produced and easily detected while a third is formed in low abundance. Muon and proton hyperfine constants were experimentally determined and compared with semiempirical and ab initio calculations. It is concluded that the radicals are of cyclohexadienyl type, i.e., the Mu adds only to carbon atoms that can assume a tetrahedral -CHMu- configuration without distorting the planar carbon skeleton of pyrene.Key words: muonium, pyrene, radical, hyperfine.

Free radicals formed by H(Mu) addition to pyrene

Muonium addition to pyrene has been investigated by means of transverse field muon spin rotation and muon level-crossing spectroscopy. Two hydropyrenyl radicals are produced and easily detected while a third is formed in low abundance. Muon and proton hyperfine constants were experimentally determined and compared with semiempirical and ab initio calculations. It is concluded that the radicals are of cyclohexadienyl type, i.e., the Mu adds only to carbon atoms that can assume a tetrahedral -CHMu- configuration without distorting the planar carbon skeleton of pyrene.

Redox Reactions of 8-Hydroxyquinoline. A Pulse Radiolysis Study

Pulse radiolysis technique was used to study the reactions of e aq - , H atoms, and other reducing radicals such as COO - /COOH, (CH3)2COH, CH3CHOH, and CH2OH radicals with 8-hydroxyquinoline (8HQ) at various pH’s. e aq - reacts with 8HQ with high rate constants (k2 ) 3 10 10 at pH 7 and 2.6 10 10 at pH 12). Absorption spectra of the transient species were recorded in the wavelength region 300-800 nm. Semireduced species formed by the reaction of eaq - with 8HQ as well as that formed by H atom reaction were found to be strong reductants and were found to transfer an electron to methylviologen. COO - /COOH and alcohol radicals were found to react with 8HQ by addition reactions. The rate constants for the reactions were much higher at acidic pH’s (10 9 dm 3 mol -1 s -1 ), where 8HQ is in the protonated form, as compared to those at neutral pH (10 8 dm 3 mol -1 s -1 ), where it is present in the neutral form. These adduct radicals were also found to be reducing in nature. The rate constant for OH radical reaction with 8HQ was estimated to be 7.6 10 9 dm 3 mol -1 s -1 at pH 7 and 1.3 10 10 dm 3 mol -1 s -1 at pH 0; a cyclohexadienyl type of radical is formed in both the cases. These adducts react with oxygen (k ) 3.2 10 9 dm 3 mol -1 s -1 at pH 7) to give peroxyl radicals. Br2 - and N3 radicals react with 8HQ by electron transfer at pH 13 giving a phenoxyl-type radical. At pH 7, N3 and SO4 - radicals react with 8HQ by electron transfer whereas the reaction of the Br 2 - radical is quite slow. Only SO4 - is able to oxidize 8HQ in its protonated form existing at acidic pH.

Radiation-induced processes in poly(alkylene terephthalates) and diethyl terephthalate: evidence for formation of cyclohexadienyl-type radicals

The low-temperature radiolysis of poly(butylene terephthalate), poly(ethylene terephthalate), and diethyl terephthalate was studied by electron spin resonance spectroscopy. The radiation-induced formation of radicals of the cyclohexadienyl type was observed for all the samples studied. It was shown that the radicals of the cyclohexadienyl type represented a considerable fraction of neutral radicals produced in the radiolysis of phthalates under consideration. The identification of the signals of various alkyl-type radicals is discussed. The kinetics of radical decay at room temperature was investigated.

Pulse radiolysis of vinyl monomers, in the pure state and with added pyrene

Pulse radiolysis studies have been used to investigate the early phenomena in the radiolysis of acrylic acid, methyl acrylate, butyl vinyl ether, propionic acid, methyl acetate and butyl ether; the latter three solvents were used as model compounds for these vinyl monomers. The triplet state, radical cation, radical anion, and free radical of pyrene (cyclohexadienyl type) were observed to various degrees in the radiolysis of pyrene in these monomers. In acrylic acid, where the free radical and the cation dominate, the monomer polymerizes efficiently, whereas in butyl vinyl ether, where the anion dominates, polymerization does not occur. The behavior of methyl acrylate lies between that of acrylic acid and butyl vinyl ether. However, the high intensity of the electron pulses creates a high concentration of radicals leading to a short lifetime of the radical which in turn leads to a much smaller yield of polymerization. The mechanism of polymerization under high energy radiation is found to be free radical in nature.

Pulse radiolysis study of reactions OH ., H . and e -aq with spin trap C-phenyl- N-tertiary-butylnitrone

The primary products of water radiolysis OH ., H . and e - aq react with C-phenyl- N-tert-butyl-nitrone(PBN) but not in a simple spin trapping manner. OH . adds mainly to the aromatic ring yielding cyclohexadienyl type radicals, whereas e - aq in pure water forms the PBN anion via the proposed intermediate O .- and alcohol radicals and an imine in the presence of alcohols.

Are positive ion radicals formed in pulse radiolysis of alkylbenzenesulfonates?

Oxidation of alkylbenzenesulfonates by OH radicals proceeds via two routes: 75-85% of the OH radicals react via addition to the benzene ring, while the rest abstract an H atom from the alkyl group. The relative distribution between the two paths of reaction depends on the nature of the alkyl group. No evidence for the formation of cation radicals from OH adducts was found. H radicals add to the benzene ring to form cyclohexadienyl type radicals, but when reacted with isopropylbenzenesulfonate about 15% of the H radicals abstract hydrogen from the alkyl to form the benzyl type radical. The reaction of SO{sub 4}{sup {sm bullet}{minus}} with alkylbenzenesulfonates produces 50-70% OH adducts and the rest are the benzyl type radicals. At high concentrations of solute and persulfate a short-lived precursor to the benzyl radicals has been observed. The precursors observed with p-toluenesulfonate, isopropylbenzenesulfonate, and m-toluenesulfonate decay in a first-order process with the rates 1.4 {times} 10{sup 6}, 9.4 {times} 10{sup 5}, and 2.5 {times} 10{sup 5} s{sup {minus}1}, respectively. The short-lived precursor is identified as an unstable OH adduct to the benzene ring.

Hyperfine anisotropy and crystal field effects in Mu adducts to naphthalene

Muonated cyclohexadienyl type radicals have been studied in a naphthalene single crystal using the standard transverse-field μSR technique. Muonium addition occurs at both the α and the β positions. Because of the multiplicity and weakness of the lines in the Fourier spectrum, special techniques were developed to allow the signals to be found and identified. The isotropic hyperfine coupling constants show large shifts from the solution values, with significant anisotropy in the hyperfine tensors. The results for the α radicals are similar to those observed for protonated α-hydronaphthyl radicals, but isotope effects are evident. Based on the hyperfine tensor directions and geometrical considerations, each radical has been assigned to a specific muon site within the crystal.

トリフェニル置換４Ｂ族元素（ケイ素，ゲルマニウム，スズ）中心遊離基の低温マトリックス中でのＵＶおよびＥＳＲ研究

Triphenyl-substituted group 4 B element-centered radicals, generated from the corresponding hydrides by irradiation with a low-pressure Hg arc lamp, were examined by UV and ESR at 77 K. The group 4 B element-centered radicals showed absorption peaks at 325-330 nm and molar extinction coefficients of ca.103-104. The group 4 B element substituted cyclohexadienyl type radicals were also generated by irradiation of the hydrides.

Über die Thermische Umsetzung von Mono‐n‐alkylbenzenen

Thermal Reaction of Mono‐n‐alkyl‐benzenesThe pyrolysis of ethyl‐(1), n‐propyl‐(2), n‐hexyl‐(3) and n‐octylbenzene (4) has been studied under conditions of steam cracking (600 to 800°C) in a laboratory scale tubular reactor. The overall activation energies determined for (1) to (4) were found to be nearly identical (221 to 227 kJ · mol−1) obviously caused by similar initial steps. The main liquid product observed was styren accompanied by ω‐phenyl‐1‐alkenes and α‐olefins. Benzene is an important side product of 1. It might be formed mainly by a hydrogen assisted dealkylation via a cyclohexadienyl type radical as reactive intermediate.