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.     

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.     

OH-Induced shift from carbon to oxygen acidity in the side-chain deprotonation of 2-, 3- and 4-methoxybenzyl alcohol radical cations in aqueous solution: results from pulse radiolysis and DFT calculations

DFT calculations have been carried out for 2-, 3- and 4-methoxybenzyl alcohol radical cations (1⁺, 3⁺ and 4⁺, respectively) and the α-methyl derivatives 2⁺ and 5⁺ using the UB3LYP/6-31G(d) method. The theoretical results have been compared with the experimental rate constants for deprotonation of 1⁺-5⁺ under acidic and basic conditions. In acidic solution, the decay of 1⁺-5⁺ proceeds by cleavage of the C-H bond, while in the presence of ⁻OH all the radical cations undergo deprotonation from the α-OH group. This pH-dependent change in mechanism has been interpreted qualitatively in terms of simple frontier molecular orbital theory. The ⁻OH induced α-O-H deprotonation is consistent with a charge controlled reaction, whereas the C-H deprotonation, observed when the base is H₂O, appears to be affected by frontier orbital interactions.     

Structures and EPR spectra of binary sulfur-nitrogen radicals from DFT calculations

The scattered electron paramagnetic resonance (EPR) spectroscopic data for binary sulfur-nitrogen (S,N) radicals have been compiled and critically assessed.Many of these are inorganic rings or cages.For each species, possible equilibrium structures in the gas phase and the EPR hyperfine coupling (hfc) constants have been calculated with DFT using the B3LYP functional and basis sets of triple-ζ (or better) quality.Good agreement is obtained between calculated and measured values for the well characterized [S₃N₂]⁺, a planar π-radical for which the s-component of the orbitals is likely to be reasonably independent of minor geometrical changes between gas-phase and condensed-phase states.The cage compounds [S₄N₄]⁻ and [S₄N₅]⁻², for which reliable experimental EPR spectra have been reported, show larger variation between calculated and measured hfc, as a consequence of the dependence of the s orbital content of the molecular orbitals on small structural changes.The very large disagreements between the DFT calculated and experimentally claimed hfc constants for [NS], [SNS] and [S₄N₄]⁻³ in condensed phases lead us to question their assignment.Among binary S,N radicals, ³³S hfc data has only been reported for [S₃N₂]⁺ (through isotopic enrichment).These values were essential for the correct identification of the EPR spectra of this important radical, which previously was misassigned to other species.Our results suggest that ³³S data will be equally important for the correct identification of the EPR spectra of other binary S,N species, many of which are cyclic systems, e.g.[S₃N₃], [S₄N₃] and[S₄N₅].     

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.     

Syntheses, crystal structure and properties of two novel coordination polymers with the flexible tetrazole-1-acetic acid (Htza)

Two new coordination polymers, [Ag(tza)]_∞ (1) (Htza=tetrazole-1-acetic acid) and [Cu(tza)₂]_∞ (2) have been prepared at room temperature and characterized by X-ray crystallography, IR, UV-vis, fluorescence spectra and magnetism analysis. Compound 1 exhibits extended helical chains through bridging ligand tza. The AgAg interactions between the adjacent chains form a 3-D framework featuring the extended tza-connected Ag chains that obviously affect the photoluminescent property. Compound 2 features undulated layered structure with hourglass-shaped [Cu₄(tza)₄] as subunits with the weak ferromagnetic interactions between Cu(II) ions, which are further stabilized by inter-lamellar CHO hydrogen bonds in the resulting 3-D supramolecular framework.     

Light-emitting persistent radicals for efficient sensor devices of solvent polarity

Radical adduct 1 and the novel [4-(N-indolyl)-2,6-dichlorophenyl] bis(2,4,6-trichlorophenyl)methyl radical (2) exhibit intense fluorescence emission in cyclohexane in the red wavelength region. The intensity and the wavelength of the emission band are drastically dependent on the polarity of the solvent. Besides, radical adduct 2 has been fully characterized by differential scanning calorimeter, showing a high thermal stability and a clear glass nature to form excellent films.     

Factors contributing to one-electron metalloradical activation of ethene and carbon monoxide illustrated by reactions of Co(II), Rh(II), and Ir(II) porphyrins

Metallo-porphyrin complexes of Co(II), Rh(II), and Ir(II) are used as prototype metal-centered radicals in examining the factors that contribute to obtaining one-electron activated ethene and CO substrate adducts [M(CH₂CH₂)] and [M(CO)] that subsequently react on to produce complexes with reduced substrate units including M-CH₂CH₂-M, M-(CH₂)₄-M, M-C(O)-M, M-C(O)-C(O)-M, and M-C(O)H. Cobalt(II) and rhodium(II) complexes of the form [(por)M(CH₂CH₂)] and [(por)M(CO)] occur as primarily metal-centered radicals and the iridium analogs are porphyrin anion radical complexes ((por)⁻Ir^III(CH₂CH₂), (por)⁻Ir^III(CO)). Relatively small (por)Co-C bond dissociation enthalpies preclude forming any reduced substrate species. Rhodium porphyrins produce a complete set of reduced and coupled ethene and CO complexes, but iridium porphyrins only give ethene reduction and coupling products (por)Ir-CH₂CH₂-Ir(por) and (por)Ir-(CH₂)₄-Ir(por). Thermodynamic criteria and analysis of substrate reactions are used to guide interpretations of the observed reactivity.     

The role of oxygen acidity on the side-chain fragmentation of ring methoxylated benzocycloalkenol radical cations

The reactivity of 2,2-dimethyl-5-methoxyindan-1-ol (1) and 2,2-dimethyl-6-methoxytetral-1-ol (2) radical cations has been studied both in acidic and basic solution. At pH≤4 both 1⁺ and 2⁺ undergo C_αH deprotonation as the exclusive reaction with k=4.6×10⁴ and 3.2×10⁴ s⁻¹, respectively. In basic solution 1⁺ and 2⁺ behave as oxygen acids undergoing ⁻OH-induced αOH deprotonation in a diffusion controlled process (k_−OH≈10¹⁰ M⁻¹ s⁻¹). An intermediate alkoxyl radical is formed which undergoes a 1,2-hydrogen atom shift in competition with CC β-scission (with 1⁺) or as the exclusive pathway (with 2⁺). A behavior which is interpreted in terms of the greater ease of ring-opening of a five membered ring as compared to a six-membered one.     

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.     

C-S bond cleavage in the sensitized photooxygenation of tert-alkyl phenyl sulfides. The role of superoxide anion

The N-methylquinolinium tetrafluoroborate (NMQ⁺)-photosensitized oxidation of tert-alkyl phenyl sulfides 1a-c (1a, tert-alkyl=tert-butyl; 1b, tert-alkyl=2-phenyl-2-propyl; 1c, tert-alkyl=1,1-diphenylethyl) and benzyl phenyl sulfide (2) were investigated in CH₃CN by nanosecond laser flash photolysis (LFP) and steady-state irradiation either under nitrogen or in the presence of O₂. By laser irradiation, the formation of sulfide radical cations 1a⁺-c⁺ in the monomeric form (λ_max=520 nm) and of 2⁺ in both the monomeric (λ_max=520 nm) and dimeric form (λ_max=780 nm) were observed within the laser pulse. In both cases, the radical cations decayed by second-order kinetics without any apparent formation of transients attributable to C-S bond rupture. In line with these results, very small amounts of photoproducts were obtained under nitrogen thus suggesting that the sulfide radical cations mainly undergo a back electron transfer process with the reduced N-methylquinolinium (NMQ). A different situation was found in the presence of O₂ since steady-state photolysis produced substantial amounts of C-S bond cleavage products (alcohols, alkenes, and ketones from 1a-c and benzaldehyde from 2), in contrast with LFP experiments. Formation of products was, however, significantly reduced in the presence of benzoquinone, a trap for O₂⁻ generated by NMQ and O₂. For the tert-alkyl phenyl sulfides, 1a-c, these results have been interpreted by suggesting that C-S bond cleavage products in the presence of oxygen mostly derive from the decomposition of a thiadioxirane 6 formed by the reaction of the sulfide radical cation with O₂⁻. In this cleavage a sulfinate and a carbocation formed. The former is oxidized to sulfonate, whereas the carbocation can react with adventitious water to form the alcohol (and the alkene therefrom) and with O₂⁻ to produce the ketone. For 2 (a sulfide with α-CH bonds) probably a different mechanism holds, benzaldehyde coming from the α-phenylthio carbon radical formed from deprotonation by O₂⁻ of 2ⁿ⁺.     

Hole transfer in DNA: DNA as a scaffold for hole transfer between two organic molecules

Hole transfer process in ODNs conjugated with two organic molecules, pyrene (Py) and phenothiazine (Ptz) was investigated with the pulse radiolysis measurements. Monitoring the transient absorption of Py⁺ and Ptz⁺, it was shown that the hole transfer rate was dependent on the distance and sequence between Py and Ptz.