A New and Robust Method for In‐situ EPR Electrochemistry

A new and simple design of electrochemical setup for in‐situ generation of free radicals to be measured using X‐band (9.5 GHz) electron paramagnetic resonance (EPR) spectrometer is described. The cell parts are all from commercially available components and requires no specially made glassware. The EPR performance of the setup is demonstrated by in‐situ electrochemical generation of organic radical cations and radical anions such as quinones, perylene‐diimide, pyrene, flavin, tryptophan and tyrosine through oxidation or reduction in solution. The well‐resolved EPR spectra of the radical products were simulated and analyzed and the hyperfine coupling constants have been assigned for the interactions of the unpaired electron with the various ring protons and the nitrogen nuclei.     

A radical ion-pair complex exhibiting strong C–H…O H-bonding interaction,moderately broaden near-IR absorbance and complicated EPR spectrum

An ion-pair complex of [Ni(mnt)₂]^2? with p-N-benzylpyridinium α-nitronyl nitroxide radical cations (p-BzPYNN) in acetonitrile shows a moderate and broad absorbance in near-IR region. In the crystal of this complex, two radical cations form a dimer via strong H-bonding interaction; such kinds of dimers are connected into H-bonding chain by [Ni(mnt)₂]^2? dianions through weak H-bonding interactions. The neighboring H-bonding chains are arranged into supramolecular sheet via π…π stacking interactions between the five-membered chelate-ring of anion and the superimposed pyridyl rings of cations. The polycrystalline EPR spectrum exhibits two isotropic EPR signals. Based on the analyses of crystal structure, variable temperature magnetic susceptibility and electronic spectrum, the stronger EPR signal is assigned to the radical cation, the weaker one probably arises from a trace amount of [Ni(mnt)₂]^1? species.     

The interaction of CO,N2 and NO with Cu cations in ZSM-5: quantum chemical description and IR study

In this paper we study the properties of Cu ions and their interactions with diatomic molecules in Cu-exchanged ZSM-5. We present DFT quantum chemical calculations for models composed of the Cu site and a diatomic molecule accompanied by IR investigations for various forms of CuZSM-5. Two series of calculations with density functional theory have been undertaken in order to investigate the influence of zeolitic framework on properties of exchanged cations: (i) for small models built of free mono- and divalent copper cation interacting with CO, N₂ and NO and (ii) 5- or 6-member ring models of ZSM-5 hosting the cation and a diatomic molecule. Comparison of calculated and experimental IR X–Y frequencies supports our model and brings some insight into the activation mechanism.

     

Zur Rolle von EDA-Komplexen bei kationischen Polymerisationen. VIII. Zur Charge-Transfer-Wechselwirkung von bπ-Donatoren mit organischen Kationen in Lösung

On the Role of EDA-complexes in Cationic Polymerization. VIII. The Charge-Transfer-Interaction of bπ-Donors with Organic Cations in Solution The interaction of 25 aromatic bπ-donors with organic cations of the type (RC₆H₄)₃ C^⊕X^⊖ (R =  CH₃,  C(CH₃)₃,  OCH₃,  H,  Cl,  I,  NO₂; X = BF₄^⊖, AsF₆^⊖, SbCl₆^⊖, FeCl₄^⊖) is investigated by means of u.v.-spectroscopy. The influence of the dissociation on the charge transfer energy will be discussed. With the decreasing concentration of the salt the charge transfer band is hardly shifted at longer wave length. The charge transfer energy can be described as a linear function of structural parameters (i.e. σ_p of R; I_p of the donor and reciprocal radius of the anions, respectively). The influence of the solvent is dependent on both the donicity and dielectric factors.     

Sorption of nitrogen,oxygen, and argon in Cd (II) exchanged zeolite X: volumetric equilibrium adsorption and grand canonical Monte Carlo study

The adsorption of nitrogen, oxygen and argon has been studied in cadmium (II) cations exchanged zeolite X at 288.2 and 303.2 K. Experimentally measured adsorption isotherms are compared with theoretically calculated data using grand canonical Monte Carlo (GCMC) simulation. Nitrogen showed higher adsorption capacity and selectivity than oxygen and argon in these zeolite samples. The cadmium exchanged zeolite X was showed that increased adsorption capacity for nitrogen, oxygen, and argon with increase in Cd (II)-exchange levels, indicating as charge density increases adsorption capacity also increase. Isosteric heat of adsorption data showed stronger interactions of nitrogen molecules with cadmium cations in zeolite samples. These observations have been explained in terms of higher electrostatic interaction of nitrogen with extra framework zeolite cations. The selectivity of oxygen over argon is explained in terms of its higher interaction with Cd (II)-exchanged zeolites than argon molecules. The selectivity of N₂/O₂ of cadmium-exchanged zeolite X is better than only sodium containing zeolite-X. Heats of adsorption and adsorption isotherms were also calculated using GCMC simulation algorithm. Simulation studies expectedly show the proximity of nitrogen molecules to the locations of extra framework sodium and cadmium cations.     

Diskussion der EPR-Spektren von Radikalanionen des TCNQ und TCNE auf der Basis von Solvensparametern

Discussion of the EPR-Spectra of TCNQ and TCNE Radical Anions on the Basis of Solvent Parameters The formation and stabilization of TCNQ and TCNE radical anions as well from neutral molecules as from EDA complexes in solution was investigated by EPR spectroscopy. The remarcable dependence of generated radical anion concentration, line width and resolution of EPR signals from the nature of used solvents was explaint in terms of selected solvent parameters. It was shown, that the donicity is a proper parameter for characterizing the generation of radical anions, whereas the parameters acceptor number and the quantity δ, which is proportional to the energy of solvent cage formation, can be used as a measure for stabilization of the generated radical anions in solution.     

Radikalreaktionen an N-Heterocyclen. IV. EPR-Untersuchung des H-Transfers zwischen aminischen Antioxidantien und Peroxy-Radikalen

Radical Reactions of N-Heterocyclic Compounds. IV. ESR Investigation of H-Transfer between Aminic Antioxydants and Peroxy Radicals The N-radicals of pyrazole-, imidazole- and indole-derivatives were prepared in comparatively high concentrations at room temperature in benzene or toluene and their e.s.r. signals were interpreted. In H-abstraction by free or to Co(III)-coordinated tert.-butylperoxy radicals from bifunctional heterocyclic substituted amines 2–9, 11 the following efficiency was ascertained: unhindered  OH > aniline-NH > heterocyclic-NH , hindered OH-groups. No radical attack on the exocyclic NH₂-group took place. The splitting constant concerning the heterocyclic nitrogen of nitroxyl radicals 5a, 10a was in the range a_N^endo = 0.7 to 0.85 mT in contrast to exocyclic nitroxyls 6a, 8a, 9a, 11a possessing a_N^exo = 1.0 to 1.1 mT. The interruption of the π-conjugation between the heterocyclic and the aniline groups by insertion of a CH₂-group led to the decrease of the stability of the generated N-radicals. In the case of anilinebenzimidazole 9 the Co(III)-complexed endocyclic aminyl radical 9b was observed simultaneously with the exocyclic nitroxyl radical 9a .     

Synthesis and Characterization of Energetic 1,2‐Bis(Oxyamino)Ethane Salts

The synthesis, characterization, theoretical calculations, and safety studies of energetic salts based on 1,2‐bis(oxyamino)ethane, (H₂N O CH₂ CH₂ O NH₂), were carried out. The salts were characterized by vibrational (infrared, Raman), multinuclear NMR studies (¹H, ¹³C), differential scanning calorimetry (DSC), elemental analysis, and initial safety testing (impact and friction sensitivity). Single crystal X‐ray diffraction studies were carried out on the mono‐perchlorate and the double nitrate salts, revealing the expected structures.     

Elektrochemische Oxydation organischer Verbindungen in Fluorsulfonsäure. X. Effekt der Aciditätserhöhung auf das anodische Verhalten einiger aromatischer Verbindungen im System HSO3F?SbF5

Electrochemical Oxidation of Organic Compounds in Fluorsulfuric Acid. X. Effect of the Increase in Acidity on the Anodic Behavior of Some Aromatic Compounds in the HSO₃F SbF₅-System The effect of the acidity on the anodic behavior of some aromatic nitrils, nitro compounds, amines, sulfonyl fluorides, carbonyl compounds, hydrocarbons, benzylic acetates and methoxy compounds is investigated by cyclic voltammetry in HSO₃F and HSO₃F SbF₅ (molecular ratio 15:1 and 3:1) at −76°C and in some cases in HSO₃F SbF₅ (1:1) at −40°C. With increasing content of SbF₅ in most cases a positive shift of the anodic half-peak potentials Δ E_p/2 is found, which depends on the type and on the strength of the interaction with the superacidic solvent. Δ E_p/2 increases from the protonated functional groups in the series  NH₃^⊕ (0.00 V),  CN (0.05–0.10 V),  SO₂F (0.10–0.20 V),  COR (0.20–0.40 V),  NO₂ (0.30–0.60 V),  OCH₃ (>0.8 V) to benzoylium ions (0.45–0.60 V), benzyl cations (0.60–0.80 V) and arenium ions (1.10–1.30 V). Besides, the anodic behavior depends to a large extent on the deprotonation rate of the substrate, which occurs precedingly to, simultaneously with or subsequent to the electron transfer.     

Wechselwirkungen in Kristallen. 136. Protonierte Di(pyrid-2-yl)amin-Salze mit verschiedenen Anionen: Monomeres Tetraphenylborat sowie Bis(trifluormethylsulfonat), dimeres Squarat und polymeres Chlorid-Dihydrat

Interactions in Crystals. 136. Protonated Dipyridylamine Salts with Different Anions: Monomeric Tetraphenylborate as well as Bis(trifluormethylsulfonate), Dimeric Squarate and Polymeric Chloride Dihydrate Di(pyrid-2-yl)amine is monoprotonated by acetic, squaric as well as hydrochloric acid and diprotonated by trifluoromethylsulfonic acid. To explore the anion and hydrogen-bridge dependency of its salts, crystals of the monomeric tetraphenylborate, the dimeric squarate as well as the bis(trifluoromethylsulfonate) and the polymeric chloride hydrate have been grown and their structures determined: Separated by bulky tetraphenylborate anions, pyrid-2-yl(2′-pyridinium) amine cation contains an intramolecular hydrogen bridge N^⊕H…︁N and is slightly folded. The squarate salt dimer exhibits additional bridges, N H…︁O^⊖ and O H…︁O. The severely twisted di(2-pyridinium)amine dication is connected to its trifluormethylsulfonate counter anions by altogether three H-bridges, two N^⊕H…︁O and one N H…︁O. The chloride dihydrate crystallizes in layers of 16-membered rings, formed from three Cl anions and five water molecules. The diversity of the hydrogen-bridge dominated salt structures is discussed based on comparison with selected literature examples and on PM3 calculations.     

In situ EPR spectroelectrochemistry of single-walled carbon nanotubes and C60 fullerene peapods

The first in situ electron paramagnetic resonance (EPR) spectroelectrochemical study of C₆₀ fullerene peapods (C₆₀@SWCNT) as well as that of single-walled carbon nanotubes (SWCNTs) in different electrolyte solutions describes the formation of spin states by charge transfer reactions. Electrochemical reduction of peapods at high negative potentials causes the production of spins at the SWCNT site, while the intratubular fullerene is unchanged.Slightly anisotropic EPR signals were detected during electrochemical reduction of single-walled carbon nanotubes and fullerene peapods in the potential region from −1.75 to −2.15 V vs. decamethylferrocene/decamethylferrocinium couple. They are centered at g = 2.0038 and exhibit a hyperfine structure indicating the presence of functional groups containing N, O, H atoms in neighborhood. They differ from the EPR signals of chemically (potassium) doped SWCNT and C₆₀@SWCNT. As the EPR signal is influenced by the electrolyte counter ions a reaction with electrolysis products of tetraalkylammonium cations is taken into consideration. No EPR lines of fullerene anions were found in electrochemically treated peapods, but these anions are detectable, if a free C₆₀ in solution is cathodically reduced on a SWCNT electrode.     

Synthese und UV-spektroskopische Charakterisierung von Carbeniumionen an Aerosil

Synthesis and UV-spectroscopic Characterization of Carbenium Ions on Aerosil The interaction of aerosil with various triphenylmethyl derivatives in the cationic vinyl polymerization in 1,2-dichloroethane was studied. The adsorption of the triphenylmethylium ions onto aerosil surfaces strongly depends on both the type of substituents (R   CH₃,  OCH₃,  Cl,  H and  NO₂) and the nature of the counter anion (SbCl₆^⊖, FeCl₄^⊖, SnCl₅^⊖, HSO₄^⊖, SO₄^2⊖, F^⊖, Cl^⊖, Br^⊖, CF₃COO^⊖, N₃^⊖ and SCN^⊖). The binding of the triphenylmethylium ions with aerosil occurs via nucleophilic anions such as halides. Counter anions derived from metal halides as MtX_n+1^⊖ are not able bind on aerosil. Covalent compounds as triphenylmethyl halides are ionized by the acidic aerosil surface. The influence of substituents at the triphenylmethylium ion, the type of counter anions and the influence of water traces on surface — ionization are discussed.     

Novel heterometallic Cu(II)/Cr(III) complex with unique open-chain N-ligand produced in conditions of direct template synthesis

The one-pot self-assembly reaction of copper powder, Reineckes salt, acetone and tris(2-aminoethyl)amine (tren) in dmso affords to obtain a heterometallic compound [Cu(trenac)][Cr(NCS)₄(NH₃)₂](NCS) · 6dmso with novel Schiff-base ligand N-(2-{[(1E)-3-amino-1,3-dimethylbutylidene]amino}ethyl)-N-(2-aminoethyl)ethane-1,2-diamine (trenac). The ligand trenac is generated by metal-directed condensation of tren, acetone and ammonia, formed during the synthetic procedure. X-ray structural investigations showed that the complex consists of [Cu(trenac)]²⁺ cations, anions of the Reineckes salt [Cr(NCS)₄(NH₃)₂]^?, uncoordinated thiocyanate groups and dmso molecules, which are held together by electrostatic forces and hydrogen-bonded interactions. The compound was characterized by EPR spectroscopic and magnetic susceptibility measurements.     

Capture of H2S from binary gas mixture by imidazolium‐based ionic liquids with nonfluorous anions: A theoretical study

Selective capture of H₂S from gas mixture is essential to reduce its undesirable high corrosiveness and toxicity. Ionic liquids have been proposed as a promising material, and there is a need to clarify the capture mechanisms and search for optimal combination of cation and anion for application. This work aims to elucidate the interactions between H₂S and nonfluorous imidazolium ionic liquids (NIILs) at a molecular level. The effects of hydroxyl group on the tail of alkyl chain, and combinations of imidazolium cations and nonfluorous anions on H₂S capture are explored using quantum chemistry calculations. Furthermore, molecular dynamics simulations are used to explore the microstructural features of NIIL–H₂S–CH₄ mixture systems. It is found that the hydroxyl groups in the cations is essential in governing the absorption properties of NIILs, including the interaction sites for hydrogen bonding, interaction geometries and energies, diffusion coefficients, and organization of H₂S and CH₄ around cations and anions. A molecular viewpoint to design appropriate ionic liquids for promoting their applications for H₂S capture is provided. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3824–3833, 2013     

Structural Characterization of Frustrated Lewis Pairs and Their Reaction Products Using Modern Solid-State NMR Spectroscopy Techniques

Frustrated Lewis pair (FLP) chemistry has provided a new strategy for small-molecule binding and/or catalytic activation. The most prominent FLPs are based on intramolecular phosphane borane adducts, the catalytic properties of which can be tailored over wide ranges of reactivity and selectivity. Advanced solid-state NMR spectroscopic techniques, together with DFT calculations, can provide new structural insights in these systems. This review illustrates the utility of ³¹P and ¹¹B NMR chemical shifts, ¹¹B electric field gradient tensors, and ³¹P ¹¹B indirect and direct dipole dipole interactions for characterizing intramolecular borane phosphane FLPs. We demonstrate the potential of this method to 1) quantify the extent of boron phosphorus bonding interactions (and hence, the “degree of frustration”); 2) reveal specific structural details (i.e., boron phosphorus distances and other local geometric aspects) related to their catalytic activities; and (3) characterize products of FLP reactions with regard to molecular structure, stereochemistry, and aggregation properties in terms of internuclear distances, bonding connectivities, and orientational parameters.     

Komplikationen bei der Initiierung kationischer Polymerisationen mit Lewis-Säuren - Modellrechnungen an Antimon-(V)-halogeniden

Complications in the Initiation of Cationic Polymerizations by Lewis Acids - Model Calculations for Antimony(V)halogenids Quantum-chemical CNDO/2-calculations including geometry-optimization show that complex anions (X₅Sb-Z-SbY₅)⊖ are formed as a result of interaction between Lewis acids SbX₅ and anions SbY₅Z⊖ (X, Y, Z = F, Cl, Br). This interaction is very strong in the case of X, Z = F and Y = Br. When the complex anions decompose the simple anions with the greatest number of fluorine ligands are formed. The interaction between the two different Lewis acids SbX₅ and SbY₅ yields a random distribution of the ligands of both acids. During formation of the ions SbX₄⊕ and SbXY₅⊖ the anion with the greatest number of fluorine ligands is formed. The probability of the conversion Sb(V)-Sb(III) is higher for SbCl₅ than for SbCl₆⊖.     

Controlled free radical photopolymerization of styrene initiated by trithiocarbonate

Density functional theory calculations are reported for prediction of the trends in C S bond dissociation energies and atomic spin densities for radicals using S,S′‐bis(α,α′‐dimethyl‐α‐acetic acid) trithiocarbonate (TTCA) and bis(2‐oxo‐2‐phenylethyl) trithiocarbonate (TTCB) as reversible addition fragmentation chain transfer (RAFT) reagents. The calculations predict that the value of the C S bond length (1.865 Å) of TTCA is longer than that (1.826 Å) of TTCB, and TTCA is more effective for the polymerization of styrene (St) compared to TTCB as predicted by density functional theory. In photopolymerizations, pseudo‐first‐order kinetics were confirmed for TTCB‐mediated photopolymerization of St due to the linear increase of ln([M]₀/[M]) up to about 28% conversion, suggesting the living characteristics behavior of the photopolymerization of St in the presence of TTCB. For both TTCA and TTCB the polydispersities change with increasing conversion in the range 1.10–1.45, typical for RAFT‐prepared (co)polymers and well below the theoretical lower limit of 1.50 for a normal free radical polymerization. In addition, the triblock copolymer polystyrene‐block‐poly(butyl acrylate)‐block‐polystyrene (PS PBA PS) was successfully prepared, with very good control over molecular weight and narrow polydispersity (M_w/M_n = 1.45), using PS S C(S) S PS as macro‐photoinitiator under UV irradiation at room temperature. This indicated that this reversible and valid strategy led to a better controlled block copolymer with defined structures. Copyright © 2007 Society of Chemical Industry     

Zur Anwendung von Lanthanoid-Verschiebungsreagenzien bei der NMR-spektroskopischen Untersuchung von Gemischen monofunktioneller Kohlenwasserstoffe

Application of Lanthanide Shift Reagents in ¹H-N.M.R.-Spectroscopic Investigations of Mixtures of Monofunctional Hydrocarbons The influence of n.m.r. shift reagents on the ¹H-n.m.r.-spectra of mixtures has been investigated. Hydrocarbons of the type CH₃(CH₂)_nX (X =  OH,  CHO,  COR,  COOCH₃,  COOH,  OR with R = Methyl-, Ethyl-, n-Propyl-, n-Butyl- and n = 1,2) served as model substances. The components of the mixtures compete with the shift reagent. Relative equilibrium constants have been calculated to describe this behavior in mixtures. Several equations have been deduced and tested to describe the induced shifts of the protons in mixtures. These equations are applicable to qualitative and quantitative analysis of unknown mixtures.     

The Role of pH in the Mechanism of .OH Radical Induced Oxidation of Nicotine

The ^.OH radical induced oxidation of nicotine was studied using pulse radiolysis techniques from pH 1 to 13.6. Theoretical calculations were used to help interpret the experimental results. The bond dissociation enthalpies of all of the C H bonds of nicotine were determined using DFT calculations, coupled with the isodesmic reaction. From time-dependent density functional response theory, estimates were obtained of the location of the dominant transient absorption bands (λ_max), their intensities (electronic oscillator strength, f), and the electronic composition of these transitions. OH radicals as well as other potent oxidants reacted with free nicotine through separated, concerted electron proton transfer, leading mostly to the formation of an alpha-aminoalkyl radical located on the C_2′ carbon of the aliphatic ring ( A_2′ ). Protonated nicotine underwent hydrogen atom abstraction at the C_2′ and N_1′ positions, resulting in the formation of the conjugate acid of A_2′ ( A_2′H⁺ ) and the alkylamine radical cation ( N⁺ ), respectively. Doubly protonated nicotine underwent the same reaction pathways, leading to two corresponding conjugate acid species, protonated at the aromatic nitrogen position: PyrH⁺A_2′H⁺ and PyrH⁺N⁺ _. All these radicals interconverted between each other through hydrolytic reactions. The radical A_2′ and its conjugate acid PyrH⁺A_2′ absorbed 10 times stronger than the N⁺ species, based on calculations of f. From the growth of the transient absorption of A_2′ (λ_max=330 nm, ε=8080 M ⁻¹ cm⁻¹), second-order rate constants were determined: k_(OH+Nic)=6.7×10⁹ M ⁻¹ s ⁻¹, k_(OH+NicH)=1.0×10⁹ M ⁻¹ s⁻¹. The alpha-aminoalkyl radicals decayed by disproportionation to form iminium cations 1 – 5 , which contributed to an increase in the specific conductivity of the basic solutions of nicotine following electron pulse irradiation.     

Theoretical Study on Pyrolysis and Sensitivity of Energetic Compounds. Part I: Simple model molecules containing NO2 Group

UHF (Unrestricted Hartree-Fock) Molecular Orbital calculations have been first performed for studying the pyrolysis mechanism of five compounds (nitromethane, methyl nitrate, nitroamine, methyl nitroamine and dimethyl nitroamine) containing NO₂ group as the simple models of organic explosives by using PM3 and AM1 methods. The potential energy curves and activation energies of the five pyrolysis reactions (into radicals) have been obtained. The activation energies are consistent with the experimental impact sensitivity of these three kinds of explosives: C-nitro < N-nitro < O-nitro compounds. It is found that there is a parallel linear relationship between the bond orders of N NO₂ bond in the molecules of three nitroamine compounds and the activation energies of their initiation reactions breaking N NO₂ bond. The obtained correlation coefficients between bond orders and activation energies from PM3 and AM1 calculations are 0.9962 and 0.9999, respectively.