Matrix isolation and DFT calculations of the TMM radical cation generated via the single electron oxidation of a methylenecyclopropane

A γ-irradiation of 2,2-diphenyl-1-methylenecyclopropane (3) in a degassed n-butyl chloride glassy matrix at 77 K produced an intense UV/vis absorption band with λ_ab at 432 nm. This result and calculations based on density functional theory for its radical cation 3⁺ and the corresponding trimethylenemethane radical cation (2⁺) strongly suggest that a single electron oxidation of 3 followed by ready ring opening affords 2⁺, whose molecular geometry is largely twisted (θ = 44.0°), and the positive charge and spin are localized mainly in the diphenyl methyl and allyl moieties, respectively.     

OH-radical induced degradation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 4-chloro-2-methylphenoxyacetic acid (MCPA): A pulse radiolysis and gamma-radiolysis study

The reactions of OH, H and e_aq⁻ with 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 4-chloro-2-methylphenoxyacetic acid (MCPA) were studied by pulse radiolysis. The site of OH-radicals addition to the aromatic ring of 2,4,5-T was found to be—C1: ∼18%, C2/C4/C5: total ∼28% and C3/C6: total ∼41%. The overall rate constants with OH-radicals were k(OH+2,4,5-T)=6.4 (±0.5)×10⁹ mol dm⁻³ s⁻¹ and k(OH+MCPA)=8.5 (±0.8)×10⁹ mol dm⁻³ s⁻¹. The radiation induced decomposition of the pesticides, chloride- and product formation (phenolic compounds, aliphatic acids) was studied by gamma radiolysis as a function of dose. A mechanism for acetate formation is discussed. The presence of oxygen during irradiation affected the decomposition rate only indiscernibly, however, chloride elimination, ring fragmentation (formation of aliphatic acids), TOC- and toxicity reduction were strongly enhanced. For complete removal of 500 μmol dm⁻³ herbicides a dose of ∼4 kGy was required. Using air saturation during irradiation a reduction of 37-40% of the TOC was observable at 5 kGy, detoxification (luminescence inhibition <20%) was achieved with 10 kGy.     

2π + 2π cycloaddition kinetics of some fluoro olefins and fluoro vinyl ethers

The second order 2π + 2π homo- and co-dimerization between various classes of fluorinated olefins has been investigated. The fluorinated olefins examined in this study were: (1) R_fOCFCF₂ (perfluorinated vinyl ethers); (2) R_fCFCF₂ (perfluorinated terminal olefins); (3) RCH₂CFCF₂; (4) PhOCFCF₂ (aryl perfluorinated vinyl ethers).Homo-dimerizations between vinyl ethers have an E_a between 20 and 24 kcal mol⁻¹ while homo-dimerizations between terminal olefins have an average E_a between 35 and 40 kcal mol⁻¹; vinyl groups have a second order cyclodimerization rate constant of formation between 1 × 10⁻⁷ and 1 × 10⁻⁴ M⁻¹ S⁻¹ while vinyl ethers have a second order cyclodimerization rate constant of formation = 1 × 10⁻¹ M⁻¹ s⁻¹. If there is a CH₂ group α to the terminal olefin, the E_a of cyclodimerization is about 7 kcal mol⁻¹) lower with respect to those olefins with a CF₂ α to the instauration. At 270 °C co-dimerizations have an average ΔS^≠ = −45 cal K⁻¹ mol⁻¹ and a second order rate constant of cyclodimerization ranging between 0.1 × 10⁻⁴ M⁻¹ S⁻¹ and 16 × 10⁻⁴ M⁻¹ S⁻¹ while homo-dimerizations have an average ΔS^≠ = −17 cal K⁻¹ mol⁻¹ and a second order rate constant which can span from 7 × 10⁻⁷ M⁻¹ S⁻¹ to as much as 1 × 10⁻¹ M⁻¹ S⁻¹ depending on the electronic nature of the perfluorinated terminal olefin.A good correlation between the electronegativity χ and the activation energy E_a demonstrates that “polarizing” groups, O, PhO, α to the olefin play an important role in the formation and stabilization of the cyclodimerization biradical intermediate.     

Determination of photoformation rates and scavenging rate constants of hydroxyl radicals in natural waters using an automatic light irradiation and injection system

Photoformation rates and scavenging rate constants of hydroxyl radicals (OH) in natural water samples were determined by an automatic determination system. After addition of benzene as a chemical probe to a water sample in a reaction cell, light irradiation and injection of irradiated water samples into an HPLC as a function of time were performed automatically. Phenol produced by the reaction between OH and the benzene added to the water sample was determined to quantify the OH formation rate. The rate constants of OH formation from the photolysis of nitrate ions, nitrite ions and hydrogen peroxide were comparable with those obtained in previous studies. The percent of expected OH photoformation rate from added nitrate ion were high in drinking water (97.4%) and river water (99.3%). On the other hand, the low percent (65.0%) was observed in seawater due to the reaction of OH with the high concentrations of chloride and bromide ions. For the automatic system, the coefficient of variance for the determination of the OH formation rate was less than 5.0%, which is smaller than that in the previous report. When the complete time sequence of analytical cycle was 40 min for one sample, the detection limit of the photoformation rate and the sample throughput were 8 × 10⁻¹³ M s⁻¹ and 20 samples per day, respectively. The automatic system successfully determined the photoformation rates and scavenging rate constants of OH in commercial drinking water and the major source and sink of OH were identified as nitrate and bicarbonate ions, respectively.     

Twisted molecular geometry and localized electronic structure of the triplet excited gem-diphenyltrimethylenemethane biradical: substituent effects on thermoluminescence and related theoretical calculations

Substituent effects on the energies of electronic transitions (ETs) between the triplet excited and ground states of gem-diphenyltrimethylenemethane biradicals (³2a) were explored by using thermoluminescence (TL) spectroscopy and density functional theory (DFT) including time-dependent (TD) DFT. Linear free energy (Hammett) analyses of TL energies of a variety of para-substituted aryl derivatives of ³2* gave reasonable correlations with the substituent constant, σ. The slope of Hammett plots of the data are nearly identical to one obtained from a similar analysis of the photoluminescence (PL) energies of the structurally-related 1,1-diarylethyl radicals (3*). The results suggest that TL of ³2* and PL of 3* derive from a common diarylmethyl radical fluorophore. This interpretation is also supported by the DFT and TDDFT calculated electronic structures and ET energies of ³2 and 3. Thermodynamic and kinetic analyses of the charge recombination (CR) process between 2⁺ and 1⁻, which generates ³2*, revealed that substituents not only alter the TL energies but also the TL intensities of ³2*. The observations made in this effort demonstrate that ³2* as well as ³2 and 2⁺ have greatly twisted molecular geometries and highly localized electronic structures.     

Artificial photosynthetic reaction centers with carotenoid antennas

Two reaction center-antenna models based on a purpurin macrocycle linked to a C₆₀ and to a carotenoid polyene have been synthesized. In these systems the C₆₀ moiety is the primary electron acceptor, the purpurin is the primary electron donor and the carotenoid moiety acts both as an antenna and secondary electron donor. Formation of the initial charge separated state, C-Pur⁺-C₆₀⁻, following excitation with light absorbed by either the purpurin or C₆₀ takes place on the 10 ps time scale. The final charge separated state, C⁺-Pur-C₆₀⁻, is formed in one of the compounds with a quantum yield of 32% based upon light absorbed by the carotenoid. In order to function as an antenna, the carotenoid pigment must be electronically coupled to the purpurin. The purpurin C ring provides an excellent framework for locating a carotenoid polyene in partial conjugation with the macrocycle, leading to a relatively strong electronic communication between the chromophores; functionalization of a meso position of the purpurin provides a site for the covalent attachment of C₆₀.     

NHC-Pd(II)-Im (NHC = N-heterocyclic carbene; Im = 1-methylimidazole) complexes as efficient catalysts for Suzuki-Miyaura coupling reactions of aryl chlorides

A new type of well-defined N-heterocyclic carbene (NHC)-palladium chloride-imidazole complexes derived from IPrHCl or IMesHCl, PdCl₂ and 1-methylimidazole exhibits high catalytic activity in the room-temperature Suzuki-Miyaura coupling reactions of aryl or heteroaryl chlorides. Moreover, the large-scale (20.0 mmol) couplings in the presence of 0.01 mol% catalyst loading can also give the corresponding coupling products in high yields.     

An SN2′ displacement approach to allenyl acetates

Reaction of cuprates derived from R³MgBr/CuI/LiBr (R³ = n-alkyl) with R¹CCCH(O₂CR²)₂ (R¹ = sp² hybridised substituent, R² = mainly Me, alkyl, Ph) provides access to allenyl esters R¹R³CCCH(O₂CR²) (51-88%). Such species are not accessible via rearrangement of precursor propargylic R¹R³C(O₂CR²)CCH.     

The first observation in an organic medium and DFT calculation of the TMM radical anion generated via a single electron reduction of a methylenecyclopropane

γ-Irradiation of 2,2-diphenyl-1-methylenecyclopropane (3) in a degassed 2-methyltetrahydrofuran glassy matrix at 77 K gave an intense UV/vis absorption band with λ_ab at 496 nm. This result and calculations based on density functional theory for its radical anion 3⁻ and the corresponding trimethylenemethane radical anion (2⁻) strongly suggest that single electron reduction of 3 followed by ready ring opening affords 2⁻, whose molecular geometry is largely twisted (θ = 45.5°), and the negative charge and spin are localized mainly in the diphenyl methyl and allyl moieties, respectively.     

Oxidation of polyethylene under irradiation at low temperature and low dose rate. Part II. Low temperature thermal oxidation

This paper deals with the kinetic modelling of unstabilised polyethylene thermal oxidation, particular attention being paid to the domain of low temperatures, typically below 80 °C. Experimental data show that the temperature dependence of the induction time t_i and the steady state rate of oxygen absorption r_S display a discontinuity at 80 °C. A model based on the hypothesis that this discontinuity concerns only the PO₂ bimolecular combination processes and is essentially explained by the competition between terminating and non-terminating PO₂ + PO₂ reactions, was proposed. With pertinent values of the Arrhenius parameters of the elementary reactions under consideration, the model fits well the experimental data (in the 40-200 °C temperature range) and is consistent with previously analysed results of radiochemical ageing. According to this model, 35-40% of the bimolecular PO₂ combinations would not be terminating at 45 °C and this proportion would increase with the temperature. Concerning terminations, the relative fraction of coupling processes, leading to peroxide bridges, would decrease relatively to the disproportionation processes when the temperature increases.     

Stopped-flow method for assessment of pH and timing effect on the ABTS total antioxidant capacity assay

The pH and timing effect on the relative capacity of antioxidant compounds to scavenge the ABTS [2,2′-azinobis(3-ethylbenzothiazoline-6-sulphonic acid)] radical cation (ABTS⁺) expressed as the trolox equivalent antioxidant capacity (TEAC) is assessed for a wide range of antioxidants using a modification of the original ABTS assay at different pHs (4.6, 5.4 and 7.4).To study both fast and slow reacting antioxidants, a stopped-flow method based on a low cost laboratory-made system was used. For most of the tested antioxidants, total antioxidant capacity values were found to be a function of pH and time allowed for the reaction.Structurally similar compounds have the same time and pH-dependent behaviour even if they have significant differences in total antioxidant capacity values. The ABTS⁺ scavenging reaction rate was found to depend strongly on pH for p-coumaric acid, glutathione, BHT and albumin. Quercetin, gallic acid, (+)-catechin and (−)-epicatechin showed the higher activities at all pHs.The stopped-flow method can be utilized for screening of antioxidant compounds of unknown kinetics towards ABTS⁺ at different pHs and the results can be used to predict the total antioxidant capacity of structurally related compounds.     

Different radical scavenging tests in virgin olive oil and their relation to the total phenol content

Four different antioxidant tests (ABTS⁺, DPPH, ORAC, β-carotene-linoleate model system) were used to determine the free-radical scavenging activity of 39 extra virgin olive oils (EVOO) and compare the total phenol content by the Folin-Ciocalteu method. The correlation between the total phenols and antioxidant capacities measured by the four methods was very high, and highest with ABTS⁺ (R² = 0.9905). Some of these methods of measurement were applied to olive-oil samples (OO), with approximately a 50% decrease in the value of the antioxidant capacity in comparison with values found for EVOO. In conclusion, the results show that all the methods tested were suitable for determining the antioxidant capacity of olive oil. The Picual variety of extra-virgin olive oil showed high antioxidant activity.     

Spin density distribution in mononuclear Rh(0) complexes: A combined experimental and DFT study

The paramagnetic complex [Rh(trop₂dach)]2 was obtained by reduction of the almost planar 16-electron cationic precursor complex, [Rh(trop₂dach)]⁺1 and characterized by EPR spectroscopy [g₁₁ = 2.069, g₂₂ = 2.014, g₃₃ = 1.964, g_iso = 2.016; A(Rh) = (<40, 29, 30)]. The unobservable small nitrogen hyperfine coupling and DFT calculations show that most of the spin density is localized on the hydrocarbon ligand framework and only about 35% on the metal center. DFT calculations on various 17 electron rhodium complexes with carbonyl, olefine, or phosphane ligands like [Rh(CO)₄], [Rh(cod)₂], and [Rh(dppe)₂] reveal that in none of these the spin density at the metal center exceeds 45%. That is all formally Rh(0) complexes reported to date are better described as highly delocalized radicals and an assignment of the formal metal oxidation state is not meaningful.     

Novel (η-arene)-ruthenium(II) complexes containing bis(iminophosphorano)methanide and methandiide ligands

The novel bis(iminophosphorano)methanes CH₂[P{NP(S)(OR)₂}Ph₂]₂ (R = Ph (1a), Et (1b)) have been obtained by oxydation of dppm with the corresponding thiophosphorylated azides (RO)₂P(S)N₃. Deprotonation of 1a,b with KH generates the methanide species KCH[P{NP(S)(OR)₂}Ph₂]₂ (R = Ph (2a), Et (2b)). The ruthenium(II) dimer [{Ru(η⁶-p-cymene)(μ-Cl)Cl}₂] reacts with 2a,b to afford the cationic complexes [Ru(η⁶-p-cymene)(κ³-C,N,S-CH[P{NP(S)(OR)₂}Ph₂]₂)]⁺ (R = Ph (3a), Et (3b)), via selective κ³-C,N,S-coordination of the bis(iminophosphorano)methanide anions to ruthenium. The structure of [Ru(η⁶-p-cymene)(κ³-C,N,S-CH[P{NP(S)(OEt)₂}Ph₂]₂)][PF₆] (3b) has been confirmed by single-crystal X-ray crystallography. Deprotonation of complexes 3a,b with NaH leads to the neutral carbene derivatives [Ru(η⁶-p-cymene)(κ²-C,N-C[P{NP(S)(OR)₂}Ph₂]₂)] (R = Ph (4a), Et (4b)).     

Argon matrix effect on the geometry and infrared spectrum of H2CMH2 (M = Ti, Zr, Hf): A theoretical study

Within the framework of polarizable continuum model with integral equation formalism (IEF-PCM), an argon matrix effect on the geometry and infrared frequencies of the agostic H₂CMH₂ (M = Ti, Zr, Hf) methylidene complexes was investigated at B3LYP level of theory with the 6-311++G(3df,3pd) basis set for C, H, and Ti atoms and Stuttgart/Dresden ECPs MWB28 and MWB60 for the Zr and Hf atoms. At the B3LYP/IEF-PCM level of theory, H₂CTiH₂ was optimized to an energy minimum having a pyramidal structure. The calculated dipole moment of this structure is 3.06 D. The B3LYP/IEF-PCM simulations gave the three complexes’ agostic angle ∠HCM (°), distance r(H?M) (Å), and CM bond length r(CM) (Å) as follows: ∠HCTi = 87.4, r(H?Ti) = 2.079, r(CTi) = 1.803; ∠HCZr = 89.3, r(H?Zr) = 2.243, r(CZr) = 1.956; ∠HCHf = 94.7, r(H?Hf) = 2.343, r(CHf) = 1.972. As a comparison, the B3LYP simulations gave the values as follows: ∠HCTi = 91.5, r(H?Ti) = 2.150, r(CTi) = 1.811; ∠HCZr = 92.9, r(H?Zr) = 2.299, r(CZr) = 1.955; ∠HCHf = 95.6, r(H?Hf) = 2.352, r(CHf) = 1.967. As far as the MH₂ symmetric and asymmetric stretching and CH₂ wagging frequencies are concerned, the IEF-PCM calculated values are in better agreement with the experimental argon matrix ones than those calculated based on a gas phase model.     

Oxidation of polyethylene under irradiation at low temperature and low dose rate. Part I. The case of “pure” radiochemical initiation

As part of a study on the kinetic modelling of polyethylene oxidation under irradiation at low temperature and low dose rate, this first part deals with the kinetic regime in which thermal initiation, linked to hydroperoxide decomposition, is negligible compared to radiochemical initiation due to polymer radiolysis. The kinetic analysis is based on results published 30 years ago by Decker, Mayo and Richardson. A small modification of their mechanistic scheme, consisting in the introduction of a non-terminating bimolecular combination of PO₂ radicals, leads to a more consistent set of radiochemical yield values. The most significant change is a decrease in the radiochemical yield of radicals G_i from 10 to 8. At 45 °C, termination of PO₂ radicals is not very efficient: 35-40% of the PO₂ + PO₂ encounters are non-terminating, 75% of the termination events lead to peroxide bridges, the rest is a disproportionation according to the Russell mechanism.     

Evaluation of chemiluminescence reagents for selective detection of reactive oxygen species

In order to evaluate the chemiluminescence (CL) reagents for selective detection of reactive oxygen species (ROS), we comprehensively measured the CL responses of 20 CL reagents (three luminol derivatives, two imidazopyrazinone derivatives, eight lophine derivatives, six acridinium ester derivatives and lucigenin) against six types of ROS (superoxide anion: O₂⁻, hydroxyl radical: OH, hydrogen peroxide: H₂O₂, hypochlorite anion: ClO⁻, singlet oxygen: ¹O₂, and nitric oxide: NO). As a result of the screening, it was found that nine CL reagents selectively detected O₂⁻ while one CL reagent selectively detected OH. However, no CL reagent had selectivity on the detection of H₂O₂, ClO⁻, ¹O₂ and NO. Our screening results could help to select the most suitable CL reagent for selective determination of different ROS.As an application study, 4-methoxyphenyl-10-methylacridinium-9-carboxylate (MMAC), one of the acridinium ester derivatives, showed high selectivity on the detection of O₂⁻, and thus was applied to the assay of superoxide dismutase (SOD) activity. The dynamic range and detection limit of the developed CL assay were 0.1-10 and 0.06 U mL⁻¹, respectively. Significant correlation (r = 0.997) was observed between the results by the CL assay using MMAC and the spectrophotometric assay using 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium monosodium salt.     

Chemiluminescence study of carbonate and peroxynitrous acid and its application to the direct determination of nitrite based on solid surface enhancement

Peroxynitrous acid (ONOOH) was produced by the on-line mixing of acidified hydrogen peroxide with nitrite in a flow system. A strong chemiluminescent (CL) emission was observed when ONOOH reacted with carbonate without any special CL reagents. When cotton was present in the CL cell, the CL emission was enhanced significantly. The method was developed to determine nitrite, which showed a key improvement that any CL reagents and sensitizers were not used, resulting in better selectivity. The applicability of the present CL system was demonstrated for the sensitive and selective determination of nitrite in natural water samples without any special pretreatment. Good agreements were obtained for the determination of nitrite in tap and well waters between the present approach and a standard spectrophotometric method. The average precision was 4.6% (n=7) and detection limit (S/N=3) was 1.0×10⁻⁷ M. Based on the CL spectrum, UV spectra, and dissolved oxygen measurement, a possible CL mechanism was proposed. ONOOH was an unstable compound in acidic solution and could be quenched into peroxynitrite (ONOO⁻) in basic solution. ONOO⁻ reacted with CO₂ to produce ONOOCO₂⁻, which can rapidly decompose into NO₂ and CO₃⁻ radicals. In the presence of H⁺, CO₃⁻ radicals can protonate to bicarbonate radical (HCO₃). The recombination of HCO₃ radicals and decomposition can lead to light emission.