Formation Of Free Radical Ions Research Articles

Research on the Interaction Mechanisms between ScCO2 and Low-Rank/High-Rank Coal with the ReaxFF-MD Force Field.

CO2 geological sequestration in coal seams can be carried out to achieve the dual objectives of CO2 emission reduction and enhanced coalbed methane production, making it a highly promising carbon capture and storage technology. However, the injection of CO2 into coal reservoirs in the form of supercritical fluid (ScCO2) leads to complex physicochemical reactions with the coal seam, altering the properties of the coal reservoir and impacting the effectiveness of CO2 sequestration and methane production enhancement. In this paper, theoretical calculations based on ReaxFF-MD were conducted to study the interaction mechanism between ScCO2 and the macromolecular structures of both low-rank and high-rank coal, to address the limitations of experimental methods. The reaction of ScCO2 with low-rank coal and high-rank coal exhibited significant differences. At the swelling stage, the low-rank coal experienced a decrease in aromatic structure and aliphatic structure, and high-rank coal showed an increase in aromatic structure and a decrease in aliphatic structure, while the swelling phenomenon was more pronounced in high-rank coal. At the dissolution stage, low-rank coal was initially decomposed into two secondary molecular fragments, and then these recombined to form a new molecular structure; the aromatic structure increased and the aliphatic structure decreased. In contrast, high-rank coal showed the occurrence of stretches-breakage-movement-reconnection, a reduction in aromatic structure, and an increase in aliphatic structure. The primary reasons for these variations lie in the distinct molecular structure compositions and the properties of ScCO2, leading to different reaction pathways of the functional group and aromatic structure. The reaction pathways of functional groups and aromatic structures in coal can be summarized as follows: the breakage of the O-H bond in hydroxyl groups, the breakage of the C-OH bond in carboxyl groups, the transformation of aliphatic structures into smaller hydrocarbon compounds or the formation of long-chain alkenes, and various pathways involving the breakage, rearrangement, and recombination of aromatic structures. In low-rank coal, there is a higher abundance of oxygen-containing functional groups and aliphatic structures. The breakage of O-H and C-OH chemical bonds results in the formation of free radical ions, while some aliphatic structures detach to produce hydrocarbons. Additionally, some of these aliphatic structures combine with carbonyl groups and free radical ions to generate new aromatic structures. Conversely, in high-rank coal, a lower content of oxygen-containing functional groups and aliphatic structures, along with stronger intramolecular forces, results in fewer chemical bond breakages and makes it less conducive to the formation of new aromatic structures. These results elucidate the specific deformations of different chemical groups, offering a molecular-level understanding of the interaction between CO2 and coal.

Light‐Triggered, Spatially Localized Chemistry by Photoinduced Electron Transfer

AbstractIt is of immense interest to exert spatial and temporal control of chemical reactions. It is now demonstrated that irradiation can trigger reactions specifically at the surface of a simple colloidal construct, obtained by adsorbing polyethyleneimine on fluorescent colloidal particles. Exciting the fluorescent dye in the colloid affords photoinduced electron transfer to spatially proximal amine groups on the adsorbed polymer to form free radical ions. It is demonstrated that these can be harnessed to polymerize acrylic acid monomer at the particle surface, or to break up colloidal assemblies by cleaving a cross‐linked polymer mesh. Formation of free radical ions is not a function of the size of the colloid, neither is it restricted to a specific fluorophore. Fluorophores with redox potentials that allow photoinduced electron transfer with amine groups show formation of free radical ions.

Light-Triggered, Spatially Localized Chemistry by Photoinduced Electron Transfer.

It is of immense interest to exert spatial and temporal control of chemical reactions. It is now demonstrated that irradiation can trigger reactions specifically at the surface of a simple colloidal construct, obtained by adsorbing polyethyleneimine on fluorescent colloidal particles. Exciting the fluorescent dye in the colloid affords photoinduced electron transfer to spatially proximal amine groups on the adsorbed polymer to form free radical ions. It is demonstrated that these can be harnessed to polymerize acrylic acid monomer at the particle surface, or to break up colloidal assemblies by cleaving a cross-linked polymer mesh. Formation of free radical ions is not a function of the size of the colloid, neither is it restricted to a specific fluorophore. Fluorophores with redox potentials that allow photoinduced electron transfer with amine groups show formation of free radical ions.

Amino acid profile in patients with thalassemia major analyzed by liquid chromatography-tandem mass spectrometry

Patients with thalassemia major develop secondary iron overload due to ineffective erythropoiesis and recurrent transfusions. Excessive iron overload results in increased non-transferrin-bound iron, which induces tissue injury as it catalyzes the formation of free radical ions. Iron accumulation can impair the structural and functional integrity of the intestines and reduce amino acid availability. Iron overload can be alleviated by iron chelation. Transferrin is the body’s natural chelator that predominantly comprises amino acids glycine and aspartic acid. Research has shown lower transferrin levels in patients with iron overload. The present study aimed to assess the changes in iron status and the amino acid profile in patients with thalassemia major. Serum iron; unsaturated iron-binding capacity; total iron-binding capacity; and serum ferritin, glycine, and aspartic acid were assessed in a cohort of 21 patients (13 patients with beta thalassemia major and 8 with beta Hb E thalassemia). The results showed significant changes in iron status: elevated ferritin post-transfusion and decreased ferritin and elevated glycine levels after chelation. Future studies are required to assess the changes in other amino acids.

Effect of gamma radiation and accelerated electron beam on stable paramagnetic centers induction in bone mineral: influence of dose, irradiation temperature and bone defatting

Ionizing radiation has been found to induce stable defects in the crystalline lattice of bone mineral hydroxyapatite, defined as CO(2) (-) radical ions possessing spins. The purpose of our study was to evaluate CO(2) (-) radical ions induced in non-defatted or defatted human compact bone by gamma radiation (G) and accelerated electron beam (EB), applied with two doses at different temperatures. Moreover, the potential effect of free radical ion formation on mechanical parameters of compact bone, tested under compression in the previous studies, was evaluated. Bone rings from femoral shafts of six male donors (age 51 ± 3 years) were collected and assigned to sixteen experimental groups according to different processing methods (non-defatted or defatted), G and EB irradiation dose (25 or 35 kGy), and irradiation temperature [ambient temperature (AT) or dry ice (DI)]. Untreated group served as control. Following grinding under LN2 and lyophilization, CO(2) (-) radical ions in bone powder were measured by electron paramagnetic resonance spectrometry. We have found that irradiation of bone with G and EB induces formation of enormous amounts of CO(2) (-) radical ions, absent from native tissue. Free radical ion formation was dose-dependent when irradiation was performed at AT, and significantly lower in EB as compared to G-irradiated groups. In contrast, no marked effect of dose was observed when deep-frozen (DI) bone samples were irradiated with G or EB, and free radical ion numbers seemed to be slightly higher in EB-irradiated groups. Irradiation at AT induced much higher quantities of CO(2) (-) radical ions then on DI. That effect was more pronounced in G-irradiated bone specimens, probably due to longer exposure time. Similarly, bone defatting protective effect on free radical ion formation was found only in groups irradiated for several hours with gamma radiation at ambient temperature. Ambient irradiation temperature together with exposure time seem to be key parameters promoting CO(2) (-) radical ion formation in bone mineral and may mask the opposite effect of defatting and the possible effect of irradiation type. Significant weak negative correlations between CO(2) (-) radical ion number and some mechanical properties of compact bone rings (Young's modulus and ultimate stress) were found.

Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent–Solute Interactions

The dynamics of the bimolecular quenching of triplet excited benzophenone by anisole was studied by nanosecond flash photolysis. We carried out a detailed study of the solvent dependence of the reaction rates and efficiencies in a number of protic and non-protic solvents. These studies were augmented by theoretical modelling and experimental investigation of solute/solvent interactions in the triplet excited and the ground state, respectively. The triplet quenching that follows Stern-Volmer kinetics in all cases is profoundly dependent on the nature of the solvent, with the highest reactivity being consistently found in protic solvents. The results in non-protic solvents are compatible with unproductive quenching via a charge-transfer state, whereas the generally fast quenching in protic solvents is accompanied by efficient formation of free-radical products. Analysis of the solvent dependence in terms of Marcus theory reveals the impact of specific solvation of benzophenone by protic solvents on the ET driving force and kinetics. Specific solvation is found to support efficient free radical ion formation in media of moderate and low polarity as well.

Salt effects on the reactions of radical ion pairs formed by electron transfer quenching of triplet 2-methyl-1,4-naphthoquinone by amines. Optical flash photolysis and step-scan FTIR investigations.

Electron transfer quenching of triplet 2-methyl-1,4-naphthoquinone by amines in acetonitrile was investigated by means of nanosecond flash photolysis and step-scan infrared spectroscopy. With 2,5-di-tert-butylaniline as the electron donor, proton transfer following the primary electron transfer process forms neutral radicals, which were identified by comparing the resulting transient IR absorption spectra with those obtained in the presence of phenol as the quencher. Addition of lithium perchlorate promotes the formation of free radical ions. The salt effect on the reactions of the nascent radical ion pairs formed by electron transfer is discussed in terms of Eigen theory. The quinone radical anion strongly associates with lithium cations. The salt retards the diffusional recombination of the free radical ions, as predicted by the Debye-Smoluchowski equation. The observed, predictable salt effects provide a useful diagnostic tool for elucidation of the reaction mechanism.

Photoinduced Electron Transfer Between Fullerenes (C60/C70) and Ethylenedithiotetrathiafulvalene

Electron transfer from ethylenedithiotetrathiafulvalene (EDT-TTF) to photo-excited C 60 /C 70 in polar and in nonpolar solvents has been investigated by laser flash photolysis. The transient absorption band of the triplet state of C60 /C 70 ( 3 C 60 */ 3 C 70 *) was effectively quenched by EDT-TTF with the appearance of the absorption bands of C 60 .- /C 70 .- , showing that the electron transfer takes place from EDT-TTF to 3 C 60 */ 3 C 70 * in polar solvents. Even in non-polar solvents, a small amount of C 61 .- /C 70 .- disappeared by back electron transfer to (EDT - TTF) .+ with second-order kinetics, suggesting the formation of free radical ions. The radical ions disappeared with first-order kinetics in nonpolar solvent, indicating that the radical ions are present as contact ion-pair. In the presence of O 2 , the competition for the quenching of 3 C 60 */ 3 C 70 * takes place between electron transfer from EDT-TTF and energy transfer to O 2 . Protonation reaction of C 60 .- /C 70 .- was also followed in the presence of appropriate proton source.

Photoinduced electron transfer chemistry of 2,3-dicyanonaphthalene

The electron transfer photochemistry of 2,3-dicyanonaphthalene has been studied by a combination of fluorescence and transient absorption spectroscopy. The singlet excited state has a lifetime of 26 ns in acetonitrile and reacts with aromatic and alkene donors with oxidation potentials less than ~1.8 V with rate constants that are close to the diffusion-controlled limit. Transient absorption measurements demonstrate that the fluorescence quenching leads to efficient formation of free-radical ions. The radical ion yields have been measured for several donors and are compared to those for the more commonly used sensitizer, 1.4-dicyanonaphthalene. In the absence of added donors, direct excitation of 2,3-dicyanonaphthalene provides evidence for photoionization at high laser energy, in addition to triplet formation. The results illustrate the utility of this sensitizer for photoinduced electron transfer reactions.

Salt‐induced Charge Separation in Photoinduced Electron Transfer Reactions. The Effect of Ion Size*

AbstractThe effect of added salts on the efficiency for photoinduced charge separation in two typical electron acceptor (A)/electron donor (D) systems was studied by the technique of laser flash photolysis. We investigate the exciplex‐forming pyrene/p‐dicyanobenzene (Py/DCB) and pyrene/N,N‐dimethylaniline (Py/DMA) systems in ethyl acetate. The salts selected for this study are tetrabutylammonium chloride, tetrahexylammonium chloride, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, sodium tetraphenylborate and tetrabutylammonium tetraphenyl‐borate. In most cases, the salts quench the emission of the exciplexes with rate constants near the diffusional rate limit in ethyl acetate. An apparent red shift of the fluorescence maximum of the exciplexes with increasing salt concentration is also generally observed. Laser flash photolysis experiments showed that in the absence of the salts both A/D systems yield exclusively the triplet excited state of the polyaromatic. However, in the presence of many of the electrolytes studied, induced free radical ion formation is observed. The experimental efficiencies for induced charge separation (n) depend on the A/D system and on the nature of the salt. The measured n values vary between 0 and 0.5. The most striking variation corresponds to the lithium and sodium perchlorates. These salts are almost totally inefficient in quenching the Py/DCB exciplex, while they quench and induce charge separation from the Py/DMA exciplex with a high yield. The effect of the different salts on both exciplexes may be rationalized by using the concept of the soft/hard character of the interacting ions.

Cage escape yields for photoinduced bimolecular electron transfer reactions of Re(I) complexes

The quantum efficiencies for formation of free radical ions (Φ sep) following electron transfer quenching of the metal-to-ligand charge transfer (MLCT) excited state of a series of [(diimine)Re I(CO) 3L] + complexes by diaza[2.2.2]octane (DABCO) have been determined. The observed correlation between Φ sep and the driving force for back electron transfer in the contact radical pair ( ΔG BET) is consistent with a Marcus inverted region dependence of the rate of back electron transfer on Δ G BET. Comparison of the data for the present study with that for related systems suggests that: (i) the contact radical ion pair formed by electron transfer quenching has triplet spin multiplicity; (ii) the observed rate of charge recombination in the contact radical ion pair is controlled by the rate of triplet—singlet intersystem crossing.

Electron-transfer-induced photo-oxygenation reactions and the special salt effect: determination of efficiencies of free radical ion formation from solvent-separated radical ion pairs in oxygen-saturated acetonitrile solutions

The special salt effect exerted by LiClO 4 (LP) on the rate of dicyanoanthracene (DCA)-photo-induced electron transfer oxygenation of tetraarylcyclopropane ( 1) and 1,1-diarylethylenes ( 3a–d) in acetonitrile, was used to determine several important parameters of the solvent-separated radical ion pairs (SSIP): (1) the efficiency α o of dissociation into solvated free radical ions, (2) the ratio of rate constants for the separation of SSIP and the back electron transfer within the SSIP ( k sep/ k back) and (3) the ratio of rate constants for scavenging DCA − from SSIP by LP (or Li +) and the back electron transfer ( k L/ k back). The efficiencies α o turned out to be relatively large, i.e. 0.66 for 1, and, increasing with increasing oxidation potentials, between 0.38 for 3a and 0.8 for 3d. Based on the assumption that k L equals the diffusion-controlled rate constant k diff in MeCN, k sep = 1.7 × 10 7 s −1 and k back = 8 × 10 6 s −1 were estimated for 1. Within the limits of error, k sep = (8±2) × 10 7 s −1 was obtained for substrates 3a–d, whereas k back was found to decrease from 1.64 × 10 8 s −1 for 3a to 2 × 10 7 s −1 for 3d, indicating that back electron transfer, associated with Δ G back values of −2.2 to −2.8 eV, increases with decreasing energy content of the SSIP. This result represents another example for back electron transfer occurring in the Marcus ‘inverted region’. However, by assuming k L ⪕ k diff, the absolute values of k back are about one to two orders of magnitude smaller than those obtained by using the generally accepted value of 5 × 10 8 s −1 for k sep. To explain the lower values of k sep and k back, it is proposed that the encounter distance for electron transfer with the electron donors used in this study may be somewhat larger (about 7–10 Å) than the distance (of about 6.5–7.5 Å) usually assumed for these processes with less bulky electron donors.

Efficiencies of photoinduced electron-transfer reactions: role of the Marcus inverted region in return electron transfer within geminate radical-ion pairs

In photoinduced electron-transfer processes the primary step is conversion of the electronic energy of an excited state into chemical energy retained in the form of a redox (geminate radical-ion) pair (A + D A'-/D'+). In polar solvents, separation of the geminate pair occurs with formation of free radical ions in solution. The quantum yields of product formation, from reactions of either the free ions, or of the geminate pair, are often low, however, due to the return electron transfer reaction (A'-/D'+ - A + D), an energy-wasting step that competes with the useful reactions of the ion pair. The present study was undertaken to investigate the parameters controlling the rates of these return electron transfer reactions. Quantum yields of free radical ion formation were measured for ion pairs formed upon electron-transfer quenching of the first excited singlet states of cyanoanthracenes by simple aromatic hydrocarbon donors in aceonitrile at room temperature. The free-ion yields are determined by the competition between the rates of separation and return electron transfer. By assuming a constant rate of separation, the rates of the return electron transfer process are obtained. These highly exothermic return electron transfer reactions (-AG,, = 2-3 eV) were found to be strongly dependent on the reaction exothermicity. The electron-transfer rates showed a marked decrease (ea. 2 orders of magnitude in this AG, range) with increasing exothermicity. This effect represents a clear example of the Marcus inverted region. Semiquantum mechanical electron-transfer theories were used to analyze the data quantitatively. The electron-transfer rates were found also to depend upon the degree of charge delocalization within the ions of the pair, which is attributed to variations in the solvent reorganization energy and electronic coupling matrix element. Accordingly, mostly on the basis of redox potentials, one can vary the quantum yield of free-ion formation from a few percent to values approaching unity. Use of a strong donor with a strong acceptor to induce reactions based on electron transfer is likely to be inefficient because of the fast return electron transfer in the resulting low-energy ion pair. A system with the smallest possible driving force for the initial charge-separation reaction results in a high-energy, and therefore long-lived ion pair, which allows the desired processes to occur more efficiently. The use of an indirect path based on secondary electron transfer, a concept called cosensitization, results in efficient radical-ion formation even when the direct path results in a very low quantum yield.

Photoinduced Free—Radical Ion Formation from Some Charge Transfer Complexes

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