Structure,spectroscopy and magnetism of 1,4-dihydro-1,4-diazine radical cations: Exceptionally stable intermediates related to methylviologen and flavosemiquinone radicals

-1,4-Dialkyl-1,4-dihydro-1,4-diazine radical cations 1–3 have been established in recent years as unusually stable intermediates of corresponding two-step redox systems. The stability is evident from large comproportionation constants K_c > 10¹² and from the isolability of persistent radical cation salts with counter anions such as Br^-, I^-, I₃ ^-, PF₆ ^-, BPh4-, or (TCNQ₂)^-. The structures of several crystalline derivatives have been determined, showing planar π systems and, in one instance, an anion-dependent tendency to form π/π dimers. Effects of dimerization are also evident from comparative magnetic susceptibility measurements of 1,4-diethyl-1,4-dihydroquinoxalinium iodide and tetraphenylborate. UV/Vis absorption spectra of the radical cations have been determined and interpreted with the help of molecular orbital calculations. The most simple member of the series, 1,4-diethyl-1,4-dihydropyrazinum radical cation 1, exhibits a long wavelength forbidden band (²B_1u → ²A_u) with a conspicuous vibrational fine structure. The results obtained for the small but very stable new radical cations 1 and 2 provide clues to the stability of flavosemiquinone oxidation states in pertinent oxidoreductase enzymes and show ways to new components for the design of materials with anisotropic physical properties.     

Stepwise Two-Photon Excitation of 1,5-Dihydroflavin Mononucleotide: Study of Flavosemiquinone Properties

Abstract— Photoionization of 1,5-dihydroflavin mononucleotide in aqueous solution, leading to formation of hydrated electrons and flavosemiquinone radicals, was studied by nanosecond laser photolysis (excitation at LD = 354.7 nm). Analysis of the laser fluence dependence of the photoionization yield indicates that the reaction is a stepwise two-photon reaction with the fluorescent state of the flavin as intermediate step. Extinction coefficients of the neutral and anionic flavosemiquinone were obtained from transient absorbance measurements at 310-660 nm. The pro-tolytic dissociation constant of the neutral radical was determined from a study at pH 7.3-10.3 of the spectral evolution in the submicrosecond time range due to de-protonation of this radical. The value pK_a= 8.7 0.1 was obtained. The formation of tautomeric species of the neutral flavosemiquinone was suggested based on the existence of a spectral evolution of this species during the first microseconds after excitation.     

Study of solvent extraction of <Superscript>114m</Superscript>In(III) using quinaldic acid into isoamyl alcohol

A complementary study of hydroxyl radical formation in the depleted uranium (DU)-hydrogen peroxide (H₂O₂) system and the effect of biosubstances on the system were examined using the spin-trapping method. Hydroxyl radical was formed in the uranyl ion (UO₂ ²⁺), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), and hydrogen peroxide (H₂O₂) mixture solution. The pseudo first order rate constants of DMPO-OH formation were estimated to be 0.033 s⁻¹ for UO₂ ²⁺-H₂O₂-DMPO solution and 0.153 s⁻¹ for UO₂₊-H₂O₂-DMPO solution. The obtained results indicated that the hydroxyl radical formation in the UO₂ ²⁺-H₂O₂ solution could be described as a stepwise reaction process including the reduction of UO₂ ²⁺ to UO₂ ²⁺ by H₂O₂ and the Fenton-type reaction of UO₂ ⁺ with H₂O₂. Biosubstances, such as proteins, amino acids and saccharides, decreased the DMPO-OH formation, which was caused by the direct hydroxyl radical scavenging and the suppression of hydroxyl radical formation by coupling with uranyl ion.     

Autocatalysis by the nitroxyl radical in the cyclic mechanism of chain termination in polymer oxidation

The activation energy and rate constant of the reaction between the nitroxyl radical and N-alkoxyamine as a concerted abstraction–fragmentation reaction have been calculated using the intersecting parabolas model. This reaction proceeds fairly rapidly and leads to nitroxyl radical autoregeneration as a result of the following consecutive reactions:AmO^? + AmOR → AmOH + >C=O + Am^?, RO ₂ ^? + AmOH → ROOH + AmO^?, Am^?+ O₂ → Am ₂ ^? , and 2AmO ₂ ^? → 2AmO^? + O₂. Thus, the nitroxyl radical is an effective radical catalyst for its own regeneration from N-alkoxyamine. The rates of regeneration of the nitroxyl radical from its N-alkoxyamine under the action of alkyl, alkoxyl, peroxyl, nitroxyl, and hydroperoxyl radicals under conditions of polypropylene oxidation inhibited by the nitroxyl radical are compared. It is demonstrated that only peroxyl, hydroperoxyl, and nitroxyl radicals are involved in AmO^? regeneration from AmOR.     

Removal of nickel and cobalt ions from aqueous solutions using electron-beam treatment

Radiation removal of Co²⁺ and Ni²⁺ ions from aqueous solutions containing Me²⁺ and various scavengers for OH radicals has been studied. In nondeaerated solutions containing HCOOK as OH radical scavenger, two mechanisms of removal were found: reduction leading to the insoluble colloidal metals and precipitation causing predominant formation of carbonate. The processes taking place are mainly affected by the concentration of formate scavenger and depend on the type of metal ions in solution.In the presence of aliphatic alcohols as OH radical scavengers at pH in the interval 6–8, the radiation treatment leads to the metallic product. The efficiency of reduction depends on the rate constant for the reaction of Me⁺ intermediate with corresponding alcohol radical.     

Synthesis and reactivity of metal-containing monomers 49. Synthesis and structure of low-valence transition metal acrylates and their polymers

The methods for synthesis of Cu¹⁺, Fe²⁺, Cr²⁺, and V³⁺ acrylates were elaborated. The compounds obtained were characterized by elemental analyses, data of IR, X-ray photoelectron, and Mössbauer spectroscopy, magnetochemistry, and mass spectrometry. Polymeric products were obtained by liquid-phase radical polymerization of metal acrylates. Changes in the electronic state of the ions and their nearest ligand environment during polymerization were observed.     

Oligotriarylamines with a Pyrene Core: A Multicenter Strategy for Enhancing Radical Cation and Dication Stability and Tuning Spin Distribution

Monoamine 1 , diamines 2 – 4 , triamine 5 , and tetraamine 6 have been synthesized by substituting dianisylamino groups at the 1‐, 3‐, 6‐, and/or 8‐positions of pyrene. Diamines 2 – 4 differ in the positions of the amine substituents. No pyrene–pyrene interactions are evident in the single‐crystal packing of 3 , 4 , and 6 . With increasing numbers of amine substituents, the first oxidation potential decreases progressively from the mono‐ to the tetraamine. These compounds show intense charge‐transfer (CT) emission in CH₂Cl₂ at around 530 nm with quantum yields of 48–68 %. Upon stepwise oxidation by electrolysis or chemical oxidation, these compounds were transformed into radical cations 1 ^?+– 6 ^?+ and dications 2 ²⁺– 6 ²⁺, which feature strong visible and near‐infrared absorptions. Time‐dependent density functional theory studies suggested the presence of localized transitions from the pyrene radical cation and aminium radical cation, intervalence CT, and CT between the pyrene and amine moieties. Spectroscopic studies indicated that these radical cations and dications have good stability. Triamine 5 and tetraamine 6 formed efficient CT complexes with tetracyanoquinodimethane in solution. The results of EPR spectroscopy and density functional theory calculations suggested that the dications 2 ²⁺– 4 ²⁺ have a triplet ground state, whereas 5 ²⁺ and 6 ²⁺ have a singlet ground state. The dication of 1,3‐disubstituted diamine 4 exhibits a strong EPR signal.     

Measurements of the kinetics of the OH‐initiated oxidation of methyl vinyl ketone and methacrolein

The mechanisms of the OH‐initiated oxidation of methyl vinyl ketone and methacrolein have been studied at 300 K and 100 Torr total pressure, using a turbulent flow technique coupled with laser‐induced fluorescence detection of the OH radical. The rate constants for the OH + methyl vinyl ketone and OH + methacrolein reactions were measured to be (1.78 ± 0.08) × 10^?11 and (3.22 ± 0.10) × 10^?11 cm³ molecule^?1 s^?1, respectively, and were found to be in excellent agreement with previous studies. In the presence of O₂ and NO, the OH radical propagation and the loss of OH through radical termination resulting from the production of methyl vinyl ketone‐ and methacrolein‐based alkyl nitrates were measured at 100 Torr total pressure and compared to the simulations of the kinetics of these reaction systems. The results of these experiments are consistent with an overall rate constant of (2.0 ± 1.3) × 10^?11 cm³ molecule^?1 s^?1 for both the methyl vinyl ketone‐based peroxy radical + NO and methacrolein‐based peroxy radical + NO reactions, each with branching ratios of 0.90 ± 0.10 for the bimolecular channel (oxidation of NO to NO₂) and 0.10 ± 0.10 for the termolecular channel (production of methyl vinyl ketone‐ and methacrolein‐based alkyl nitrates). © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 12–25, 2003     

Kinetics and products of the gas-phase reactions of the OH radical and O3 with allyl chloride and benzyl chloride at room temperature

The kinetics of the gas-phase reactions of allyl chloride and benzyl chloride with the OH radical and O₃ were investigated at 298 ± 2 K and atmospheric pressure. Direct measurements of the rate constants for reactions with ozone yielded values of ??(O₃ + allyl chloride) = (1.60 ± 0.18) × 10^?18 cm³ molecule^?1 s^?1 and ??(O₃ + benzyl chloride) < 6 × 10^?20 cm³ molecule^?1 s^?1. With the use of a relative rate technique and ethane as a scavenger of chlorine atoms produced in the OH radical reactions, rate constants of ??(OH + allyl chloride) = (1.69 ± 0.07) × 10^?11 cm³ molecule^?1 s^?1 and ??(OH + benzyl chloride) = (2.80 ± 0.19) × 10^?12 cm³ molecule^?1 s^?1 were measured. A study of the OH radical reaction with allyl chloride by long pathlength FT-IR absorption spectroscopy indicated that the co-products ClCH₂CHO and HCHO account for ca. 44% of the reaction, and along with the other products HOCH₂CHO, (ClCH₂)₂CO, and CH₂ ? CHCHO account for 84 ± 16% of the allyl chloride reacting. The data indicate that in one atmosphere of air in the presence of NO the chloroalkoxy radical formed following OH radical addition to the terminal carbon atom of the double bond decomposes to yield HOCH₂CHO and the CH₂Cl radical, which becomes a significant source of the Cl atoms involved in secondary reactions. A product study of the OH radical reaction with benzyl chloride identified only benzaldehyde and peroxybenzoyl nitrate in low yields (ca. 8% and ?4%, respectively), with the remainder of the products being unidentified.     

Photoelectrochemical and characterization of intercalation compound of KTiNbO5 with methylviologen

Methylviologen (1,1′-dimethyl-4,4′dipyridinium ion, MV²⁺) has been intercalated into the interlayer space of layered potassium titanoniobate (KTiNbO₅) by a method involving the displacement of guest molecules using a n-hexylammonium titanoniobate (HeNH₃⁺-TiNbO₅) intercalation compound. The methylviologen titanoniobate (MV²⁺-TiNbO₅) intercalation compound was characterized using SEM, TEM, XRD, IR and elemental analysis. The photochemical and electrochemical behaviors of the MV²⁺-TiNbO₅ hybrid thin film were investigated. The photo excitation of oxygen-present MV²⁺-TiNbO₅ thin film with UV light indicated the electron transfer from the titanoniobate layer to MV²⁺ to form MV⁺ radical cation. The cyclic voltammogram of the MV²⁺-TiNbO₅ hybrid thin film exhibited two consecutive electron-transfer steps.     

ESR study on photoreaction of formato complex of europium(III) in HCOOH-HCOONa buffer solution

Matrix isolation ESR study showed that the ligated HCCO^? ion was decomposed into H⁺ and ·COO^? radical anion through CTTM process at λ = 254 nm, by contrast, ·CH₃ radical and CO₂ were produced from CH₃COO^? ligand. In order to explain the photo- and related reactions in the liquid solution, a proposal is made for a cyclic scheme conjugated with the photo-decomposition of the complex. The cycle consists of three steps; photo-reduction of H⁺ by Eu²⁺, radical alternation from ·H to ·COO^?, and oxidation of ·COO^? by Eu³⁺.     

Polymeric Electron Carriers

Polymers containing 1,4-dihydronicotinamide (P-NAH) alloxan (P-A), and viologen (P-V²⁺) moieties were synthesized and characterized. P-NAH reduced various organic substances such as lipoic acid, alloxan, and viologens and also immobilized quinone mediated by alloxan. P-A was reduced to the polymer-bearing alloxan radical and the dialuric acid structure without crosslinks by one- and two-electron reduction, respectively, and P-A also mediated the redox reaction occurring between aqueous and organic (water-immiscible) layers. P-V²⁺ was converted to the stable viologen radical reversibly by one-electron reduction. Electric potentials and currents on photo-reduction of P-V²⁺ and catalytic behavior of P-V²⁺ in the reduction of carbonyl compounds were examined.     

Fast atom bombardment (FAB) mass spectrometry of piperidine N-oxide derivatives and free radical quenching under FAB conditions

Mechanisms are proposed for the formation of M⁺, [M + 2H]⁺ and [M + 3H]⁺ ions in the fast atom bombardment (FAB) mass spectra of 4-(2,2,6,6-tetramethyl-1-oxyl)-piperidol and its carboxylates. Free radical quenching induced by the fast atom beam has been observed. The effects of temperature on the radical quenching and of acid on the FAB mass spectra are discussed. The experiment showed that the volatile liquid samples with vapour pressures higher than that for glycerol produced M⁺ even-electron molecular ions, and the FAB mass spectra were similar to the corresponding electron ionization mass spectra. For the solid samples, it was found that the free radicals were quenched during the FAB process so that the mononitroxide and dinitroxide compounds produced [M + 2H]⁺ and [M + 3H]⁺ ions, respectively. Further experiments showed that the intensities and stabilities of [M + 2H]⁺ and [M + 3H]⁺ ions could be improved by addition of acids.     

Synthesis,characterization, molecular modelling and biological activity of 2‐(pyridin‐1‐ium‐1‐yl) acetate and its Cu2+, Pt4+, Pd2+, Au3+ and Nd3+ complexes

A new series of Cu²⁺, Pt⁴⁺, Pd²⁺, Au³⁺ and Nd³⁺ complexes derived from 2‐(pyridin‐1‐ium‐1‐yl) acetate have been synthesized and characterized using elemental analyses, spectral (infrared (IR), UV–visible, mass, ¹³C NMR and ¹H NMR), magnetic and thermal measurements. IR results suggest that the ligand acts in a neutral monodentate fashion in all complexes. Octahedral geometry is proposed for Cu²⁺ and Pt⁴⁺ complexes and square‐planar for Pd²⁺ and Au³⁺ complexes. The bond lengths, bond angles, and HOMO and LUMO were calculated. Superoxide dismutase‐like radical scavenger activity and cytotoxic activity of the isolated complexes on HepG2 liver cancer cells have been screened. Ligand and complexes (Pt⁴⁺ and Nd³⁺) exhibit potent antioxidant activity upon coordination while Cu²⁺ and Au³⁺ complexes do not show superoxide dismutase‐like radical scavenger activity. The cytotoxic activity assay against HepG2 cell line proves that the ligand and its Pt⁴⁺ complex have a high cytotoxic activity, while the other complexes showed no cytotoxic activity. Copyright © 2014 John Wiley & Sons, Ltd.     

The ESR Study of O2 – Radical Anion Formation during Adsorption of an NO + O2 Mixture and O2 on ZrO2 and (0.1–2.0)% MoO3/ZrO2

The influence of conditions of the preliminary thermal treatment of ZrO₂, ammonia and methanol adsorption, and MoO₃ supporting on O₂ ^– formation during the adsorption of an NO + O₂ mixture was studied. The interaction of O₂ ^– with different molecules was studied. Adsorbed ammonia and methanol, as well as supported Mo⁶⁺ ions, were shown to inhibit this reaction. The involvement of the Zr⁴⁺ and O^2– Lewis sites in the reaction was concluded. The interaction of ammonia and methanol with the O₂ ^– radical anions changed the g tensor parameters and decreased the thermal stability of O₂ ^– in the case of methanol. O₂ ^– radical anions were formed on the reduced (0.1–2.0)% MoO₃/ZrO₂ samples during the interaction of O₂ with the Mo⁵⁺ ions in the octahedral configuration. As in the case of O₂ ^– formation during NO + O₂ adsorption on ZrO₂, the radical anions were localized in the coordination spheres of the coordinately unsaturated Zr⁴⁺ ions. A change in the MoO₃ content of the samples from 0.1 to 0.5% led to an increase in the amount of O₂ ^–, whereas a change from 0.5 to 2.0% led to a decrease in the O₂ ^– amount due to the screening of the Zr⁴⁺ ions by oxo complexes and polymolybdates.     

Electron Capture Dissociation of Hydrogen-Deficient Peptide Radical Cations

Hydrogen-deficient peptide radical cations exhibit fascinating gas phase chemistry, which is governed by radical driven dissociation and, in many cases, by a combination of radical and charge driven fragmentation. Here we examine electron capture dissociation (ECD) of doubly, [M + H]^2+?, and triply, [M + 2H]^3+?, charged hydrogen-deficient species, aiming to investigate the effect of a hydrogen-deficient radical site on the ECD outcome and characterize the dissociation pathways of hydrogen-deficient species in ECD. ECD of [M + H]^2+? and [M + 2H]^3+? precursor ions resulted in efficient electron capture by the hydrogen-deficient species. However, the intensities of c- and z-type product ions were reduced, compared with those observed for the even electron species, indicating suppression of N?CC_?? backbone bond cleavages. We postulate that radical recombination occurs after the initial electron capture event leading to a stable even electron intermediate, which does not trigger N?CC_?? bond dissociations. Although the intensities of c- and z-type product ions were reduced, the number of backbone bond cleavages remained largely unaffected between the ECD spectra of the even electron and hydrogen-deficient species. We hypothesize that a small ion population exist as a biradical, which can trigger N?CC_?? bond cleavages. Alternatively, radical recombination and N?CC_?? bond cleavages can be in competition, with radical recombination being the dominant pathway and N?CC_?? cleavages occurring to a lesser degree. Formation of b- and y-type ions observed for two of the hydrogen-deficient peptides examined is also discussed.     

Greatly enhanced intermolecular π-dimer formation of a porphyrin trimer radical trications through multiple π bonds

A trefoil‐like porphyrin trimer linked by triphenylamine (TPA‐TPZn₃) was synthesized. A three‐electron oxidation of TPA‐TPZn₃ forms a radical trication (TPA‐TPZn₃³⁺), in which each porphyrin ring undergoes a one‐electron oxidation. The radical trication TPA‐TPZn₃³⁺ spontaneously dimerizes to afford (TPA‐TPZn₃)₂⁶⁺ in CH₂Cl₂. The characteristic charge‐resonance band due to the charge delocalization over the π system of (TPA‐TPZn₃)₂⁶⁺ was observed in the NIR region. The initial oxidation potential of TPA‐TPZn₃ is negatively shifted relative to that of the corresponding monomer porphyrin, which results from the stabilization of the oxidized state of TPA‐TPZn₃ associated with the dimerization. The thermodynamic parameters (i.e., ΔH, ΔS, and ΔG) for the formation of (TPA‐TPZn₃)₂⁶⁺ were determined by measuring Vis/NIR spectra at various temperatures. The formation constant of (TPA‐TPZn₃)₂⁶⁺ is significantly larger than that of the radical cation dimer of the corresponding monomer porphyrin (e.g., over 2000‐fold at 233 K). The electronic states were investigated using EPR spectroscopic analysis. The greatly enhanced dimerization of TPA‐TPZn₃³⁺ results from multiple π‐bond formation between the porphyrin radical cations.     

Spectroelectrochemical and spectrophotochemical properties of N-tetradecyl-N '-ethyl-viologen: Micellar effects

The results of Spectroelectrochemical studies in homogenous solutions have shown that below the cmc value the cation radical of N-tetradecyl-N '-ethyl viologen (TDEV) dimerizes. The TDEV and tetradecyltriethyl-ammonium bromide (TDEA) micelles were found to stabilize the cation radical TDEV^.+ and increase the rate constant for the reaction TDEV+TDEV²⁺ = TDEV^.+ as compared with the results obtained at concentrations below cmc.Based on the spectrophotochemical measurements for TDEV it was found that the quantum yield (Φ) of photoreduction in micellar evironment of TDEA was twice as large as Φ for reactions performed in homogenous solution. Moreover, in micellar solutions photoreduction of TDEV leads to a cation radical of reduced TDEV (TDEV⁺), but in homogenous solution to the dimer of TDEV [TDEV]₂. Therefore, the process of dimerization of TDEV^.+ cation radical is inhibited by micellar catalysis.     

Effects of ligands on the “living” radical polymerization initiated by the aged Cr2+ plus benzoyl peroxide system

The “living” radical polymerization of methyl methacrylate with the aged Cr²⁺ plus benzoyl peroxide (BPO) system in the presence of various amines as ligand has been studied in N,N′-dimethylformamide. Aliphatic amines such as ethylenediamine diminished the rate of polymerization, while dipyridyl (dipy) and o-phenanthroline (phen) accelerated the polymerization rate as follows: phen > dipy > pyridine ? none. Specifically, the rate of polymerization in the presence of phen had a maximum value at [phen]/[Cr²⁺] = 0.5. The retardation of polymerization by aliphatic amines was explained by the interaction of BPO with free and coordinated amines. The latter result may support the mechanism involving the complexed radical proposed for the living radical polymerization with the aged Cr²⁺ + BPO system. In the presence of phen the effects of aging period and aging temperature as well as polymerization temperature were studied and the nature of complexed radicals is discussed.     

The mechanism of electroreduction of nitrobenzene and 3-nitropyridine in DMF in the presence of divalent cations

The differences in electrochemical behaviour of nitrobenzene and 3-nitropyridine connected with a change of supporting electrolyte are discussed. It is shown that in the presence of alkaline earch metal cations a deposit of composition (NB^?. Me²⁺) is formed at the electrode surface. The cations: Ba²⁺, Ca²⁺ and Mg²⁺ cause the instability of the nitrobenzene radical anion formed in the first electroreduction step. It was found that in the presence of Ba²⁺ and Ca²⁺ the NOB/NO^?. couple is stabilized.