Model calculations of harpooning elementary processes in the system K + Br2

The dynamics of non-adiabatic elementary processes in the system K + Br₂ (exchange reaction, electron transfer, dissociation) are investigated using the quasiclassical trajectory method. Based on a estimation of the transition probability from the covalent to the ionic potential surface, the treatment is restricted to a calculation of the motion (1D, 2D, and 3D) on the ionic potential which is represented by a model function. Differential and total cross sections are determined, briefly discussed and compared with molecular-beam data.     

Polymerization Processes in Ionic Liquids. Cationic Polymerization of Styrene

There is an increasing interest in using ionic liquids as solvents for polymerization processes. Most published data deals with controlled radical polymerization. It has been shown that ionic liquids offer several advantages for conducting Atom Transfer Radical Polymerization (ATRP), such as good solubility of catalyst and improved k_p/k_t ratio. Ionic liquids are highly polar therefore they seem to be suitable solvents for conducting also ionic polymerization processes. In our preliminary communication we reported on cationic polymerization of styrene initiated by R-Cl/TiCl₄ system in ionic liquid. To clarify the mechanism of this process, racemization of optically active 1-phenylethyl choride (initiator and the model of dormant species) was studied and it was shown that in ionic liquid racemization proceeds even in the absence of coinitiator (TiCl₄). Because racemization proceeds through ionization of C Cl bond, this explains the cationic polymerization of styrene initiated by R-Cl alone (in the absence of coinitiator). Chain transfer, however, cannot be eliminated, therefore polymerization is not controlled.     

Redox Potential and Crystal Chemistry of Hexanuclear Cluster Compounds

Most of TM₆-cluster compounds (TM = transition metal) are soluble in polar solvents, in which the cluster units commonly remain intact, preserving the same atomic arrangement as in solids. Consequently, the redox potential is often used to characterize structural and electronic features of respective solids. Although a high lability and variety of ligands allow for tuning of redox potential and of the related spectroscopic properties in wide ranges, the mechanism of this tuning is still unclear. Crystal chemistry approach was applied for the first time to clarify this mechanism. It was shown that there are two factors affecting redox potential of a given metal couple: Lever’s electrochemical parameters of the ligands and the effective ionic charge of TM, which in cluster compounds differs effectively from the formal value due to the bond strains around TM atoms. Calculations of the effective ionic charge of TMs were performed in the framework of bond valence model, which relates the valence of a bond to its length by simple Pauling relationship. It was also shown that due to the bond strains the charge depends mainly on the atomic size of the inner ligands.     

Polyurethane anionomers. I. Structure properties of polyurethane anionomers

The influence of mono and divalent nontransition and transition metals on the glass transition and mechanical properties of polyurethane anionomers have been investigated. NCO-terminated prepolymer prepared from 4,4′-methylene-bis(phenyl isocyanate) (MDI) and poly(caprolactone) glycol (PCL) was chain extended with dimethylolpropionic acid (DMPA), and the anionomers obtained by neutralization of the prepolymer. The glass transition temperatures of polyurethane anionomers have been studied as a function of the counterion. From simple electrostatic considerations, it is shown that a linear relationship exists between the glass transition temperature and ionic potential (?) for these particular materials. The relation is; T_g = A? + B. The mechanical properties are greatly affected by the type of the counterion, and in some cases, such as monovalent and nontransition metals, the mechanical properties of the anionomers improved by increasing the ionic potential. On the other hand, transition metals containing anionomers exhibited good mechanical properties but no relationship was observed between the mechanical properties and the ionic potential. The extent of water absorption of PU anionomers follows the same relative trends as the tensile strengths of the transition metals with filled and partially filled d-orbitals.     

Computer simulation of active layers in the electric double layer supercapacitor: Optimization of active layer charging modes and structure,calculation of overall characteristics

The structure and functioning modes of active layers in an electric double layer capacitor (EDLC) with an aqueous electrolyte are simulated by means of a computer. A model of active layers prepared from activated carbon materials is proposed, percolation estimates are performed and effective ionic conductivities are calculated. The polarization of active layers includes a sequence of two charging processes: first, galvanostatic and then potentiostatic. The proposed program of calculations involves mutual matching and optimization of seven parameters characterizing the active layer and conditions of charging processes. According to calculations, galvanostatic polarization of wide pores in the EDLC biporous active layer up to the limiting potential followed by potentiostatic polarization of fine pores allows the capacity C _sp = 246 F/g and the energy W _sp = 107 kJ/kg to be obtained in fractions of second.     

Activation of Hydrogen Peroxide by Ionic Liquids: Mechanistic Studies and Application in the Epoxidation of Olefins

Imidazolium‐based ionic liquids that contain perrhenate anions are very efficient reaction media for the epoxidation of olefins with H₂O₂ as an oxidant, thus affording cyclooctene in almost quantitative yields. The mechanism of this reaction does not follow the usual pathway through peroxo complexes, as is the case with long‐known molecular transition‐metal catalysts. By using in situ Raman, FTIR, and NMR spectroscopy and DFT calculations, we have shown that the formation of hydrogen bonds between the oxidant and perrhenate activates the oxidant, thereby leading to the transfer of an oxygen atom onto the olefin demonstrating the special features of an ionic liquid as a reaction environment. The influence of the imidazolium cation and the oxidant (aqueous H₂O₂, urea hydrogen peroxide, and tert‐butyl hydrogen peroxide) on the efficiency of the epoxidation of cis‐cyclooctene were examined. Other olefinic substrates were also used in this study and they exhibited good yields of the corresponding epoxides. This report shows the potential of using simple complexes or salts for the activation of hydrogen peroxide, owing to the interactions between the solvent medium and the active complex.     

Ab initio study of the reaction mechanism of water dissociation into the ionic species OH− and H3O+

A reaction mechanism of water dissociation is proposed where solvent effects are accounted for via a minimum stable model that considers the interaction of five water molecules. It is based on the fully self-consistent field (SCF) optimized structures of the reactant, product, and transition state, the calculations being at the Hartree–Fock and configuration interaction level [Møller–Plesset second-order perturbation (MP2) and coupled-cluster single and double excitations (CCSD)]. They were performed with four different basis sets that included polarized and diffuse orbitals. The dissociative mechanism leads to the ionic species OH⁻+H₃O⁺ as stable products and upon analysis of the energy hypersurface, a transition state is found which yields an activation barrier of 21.2 kcal/mol. This value is in good agreement with the experimentally determined enthalpy for the reaction. The contribution of the aggregation energy is emphasized. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 253–259, 1998     

Mechanism and kinetics of electrode processes in systems comprising solid polyaluminate electrolyte

Cathodic reduction of organic semiconductors (charge-transfer complexes and radical-ion salts) at interfaces in Na(Hg)/β-Al₂O₃/organic semiconductor systems is studied by inversion voltammetry and chronopotentiometry. Formation of transition layer at the organic semiconductor/solid electrolyte interface is revealed. The mechanism of the charge transfer complex and radical-ion salt cathodic reduction depends on the potential scan rate; the cathodic process at nonmetal electrodes occurs under the conditions of double injection of electronic and ionic charge carriers to electrode bulk.     

A non-radical mechanism for methane hydroxylation at the diiron active site of soluble methane monooxygenase

We propose a non‐radical mechanism for the conversion of methane into methanol by soluble methane monooxygenase (sMMO), the active site of which involves a diiron active center. We assume the active site of the MMOH_Q intermediate, exhibiting direct reactivity with the methane substrate, to be a bis(μ‐oxo)diiron(IV ) complex in which one of the iron atoms is coordinatively unsaturated (five‐coordinate). Is it reasonable for such a diiron complex to be formed in the catalytic reaction of sMMO? The answer to this important question is positive from the viewpoint of energetics in density functional theory (DFT) calculations. Our model thus has a vacant coordination site for substrate methane. If MMOH_Q involves a coordinatively unsaturated iron atom at the active center, methane is effectively converted into methanol in the broken‐symmetry singlet state by a non‐radical mechanism; in the first step a methane C? H bond is dissociated via a four‐centered transition state (TS1) resulting in an important intermediate involving a hydroxo ligand and a methyl ligand, and in the second step the binding of the methyl ligand and the hydroxo ligand through a three‐centered transition state (TS2) results in the formation of a methanol complex. This mechanism is essentially identical to that of the methane–methanol conversion by the bare FeO⁺ complex and relevant transition metal–oxo complexes in the gas phase. Neither radical species nor ionic species are involved in this mechanism. We look in detail at kinetic isotope effects (KIEs) for H atom abstraction from methane on the basis of transition state theory with Wigner tunneling corrections.     

The influence of the rate of bond stretching on the differential cross sections for ion-pair formation in K + XY collisions

Experimental differential cross sections are presented for ion-pair formation in collisions between K and I₂, IBr, Br₂, ICl, Cl₂. In this sequence the vibrational frequency of the halogen molecule is increased gradually from one system to the next. Therefore this is a suitable set of cros sections for analysing the influence of the rate at which the halogen bond stretches after transition to the ionic state. The experimental data are compared with trajectory calculations on diabatic and on adiabatic potential energy surfaces. The results of the two types of calculations differ significantly; the adiabatic model gives the better agreement with experiment. The essential difference between the two models is that in the diabatic model stretching of the halogen bond starts only when the crossing seam R_c between the ionic and covalent configurations is reached, while in adiabatic scattering, pre-stretching of the halogen bond occurs as the alkali atom approaches the crossing seam. As a result of pre-stretching the electron affinity is increased, which in turn affects the endoergicity and the total and differential cross sections for ion-pair formation. At energie below the threshold for ion-pair formation, the cross section for reactive scattering is also affected.     

酸性AlCl3-BMIC离子液体中的铝阳极溶解

采用线性扫描伏安法研究了Lewis 酸性AlCl₃-BMIC (BMIC: 1-butyl-3-methylimidazolium chloride)离子液体中铝电极的溶解. 铝电极在阳极极化时出现了钝化现象, 钝化是由于在铝电极表面形成了固体AlCl₃钝化膜造成的. 铝的电化学溶解过程可以依次分为三个区: 电化学控制区、过渡区和钝化区. 在电化学控制区, 铝的电化学溶解速率随着电位的正移而逐渐增加; 在过渡区, 由于电极表面AlCl₄^-和Al₂Cl₇^-浓度发生改变而析出固体AlCl₃使得铝电化学溶解速率随着电位的正移而逐渐减小; 当钝化膜形成之后, 铝的电化学溶解速率不再随着电位的正移而发生改变, 铝溶解进入钝化区. 增加搅拌、升高温度、降低离子液体AlCl₃摩尔分数都可以增加铝溶解阳极极限电流密度.     

The hydrogen-ion selective glass electrode

The properties and mechanism of operation of hydrogen-ion selective glass electrodes are reviewed and a model using information from recent research is presented. In the gel layer of a pH-glass protons or hydronium ions are bound to negative charges in a silicon network cross-linked with bi- or tervalent metal ions. The composition of the gel layer is discussed; it is known that the outer part contains water. The formation of a gel layer and its destruction in alkaline solutions are discussed. Between the gel layer and the dry glass there is a transition layer characterized by a very high resistivity. This forms a barrier to ion transport and prevents rapid corrosion of the glass. The ionic mobilities of various ions change drastically with distance inwards towards the dry glass. The alkaline and acid errors are discussed as well as the asymmetry potential. By etching away the gel layers and the transition layers on both sides of the glass and then starting hydration simultaneously on both sides, two symmetric layers can be produced. This will result in an almost complete cancellation of the asymmetry potential. Some applications to precision measurements and measurement in non-aqueous solvents are discussed.     

Theoretical study of DABCO-based ionic liquid: synthesis and reaction mechanism

Quaternary ammonium salt obtained from the Menshutkin reaction between DABCO and benzyl chloride has been used in the synthesis of a novel Bronsted acidic ionic liquid (IL), namely 1-benzyl-4-(sulfobutyl)-diaza-bicyclo-octane hydrogen sulfate. The reaction of DABCO with benzyl chloride is a crucial step in the synthesis of this IL. Density functional theory calculations at B3LYP/6-31G(d,p) level have been employed to investigate the mechanism of Menshutkin reaction by calculating the energy barriers through possible transition states i.e., five-membered ring transition state and S_N2 transition state in gas phase and in diethyl ether as a solvent. It was found that while DABCO reacts with benzyl chloride through the well-known S_N2 transition state mechanism, the corresponding reaction with chlorodiphenylmethane can proceed through both S_N2 and five-membered ring transition state mechanism. However, S_N2 transition state mechanism is still the strongly preferred one out of the two possible mechanisms. The electronic structure analysis shows that solvent effects and enhanced resonance stabilization may play a decisive role in guiding the reaction pathway.     

Electrochemical behaviour of Ni-Co amorphous alloys in alkaline solutions

The dissolution processes of amorphous G15 (Ni₅₈Co₂₀B₁₂Si₁₀) and G16 (Ni₂₅Co₅₀B₁₀Si₁₅) alloys in carbonate-bicarbonate buffers (8.9≤pH≤10.5) have been studied using voltammetry and stationary polarization techniques combined with electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements. Results indicate that the electrochemical processes are dependent on the applied potential, the alloy composition, and the pH and ionic strength of the electrolyte. An enhancement of the corrosion processes is observed when the pH and the ionic strength are increased and when the Co content of the alloy diminishes. Furthermore, XPS provided information about the composition of the surface layer. Electronic Publication     

An analytic hindered rotor model for calculating microcanonical variational unimolecular rate constants from reaction path properties

A model is proposed for performing microcanonical variational transition state theory calculations which incorporates ideas from vibrator and flexible variational transition state models. Vibrational frequencies, moments of inertia, and potential energy for the variational transition state are found by reaction path following as for the vibrator model. However, the transitional modes are treated as hindered rotors using an analytic potential and an analytic density of states, which are fit to barriers for hindered rotation determined from reaction path following. The model proposed here differs from the flexible transition state model in that the density of states for the transitional modes is analytic and transitional modes and external rotational angular momenta are uncoupled. For the H + CH₃ ? CH₄ system, rate constants calculated with this new model are only 6–23% smaller on average from those of the flexible transition state model for values of total angular momentum which correspond to average rotational temperatures of 0–2000 K. Harmonic frequencies calculated for the transitional modes from the hindered rotor Hamiltonian are in good agreement with the exact values found by a reaction path analysis. © 1994 John Wiley & Sons, Inc.     

Effects of pH,salt, surfactant and composition on phase transition of poly(NIPAm/MAA) nanoparticles

The volume phase transition of poly(NIPAm/MAA) copolymer nanoparticles in buffer solutions at various pH and in aqueous solutions of KCl or ionic surfactants (SDS and DTMAB) was systematically studied using dynamic laser scattering technique. It was found that ionizable MAA groups imparted a responsiveness of the particles to pH and electrolytes. At pH > pKa of the copolymer, electrostatic repulsion of negative charges, mostly from COO⁻ groups, was a governing mechanism for preventing the particles from collapse at T > T_tr. The particles exhibited a sharp volume phase transition upon elimination of the negative charges by decreasing the pH of the medium or by the addition of cationic surfactant. At pH < pKa, the presence of MAA groups enhanced the hydrophobicity of the particles as indicated by a lower T_tr and a sharper volume phase transition. A pH 4 buffer at the same ionic strength exhibited the most significant effect on the particle size and phase transition, followed by the ionic surfactant with an opposite charge (e.g., DTMAB), KCl, and finally the ionic surfactant with the same charge (e.g. SDS). © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2667–2676, 1999     

On the ion‐pair dissociation mechanisms in the small NaCl·(H2O)6 cluster: A perspective from reaction path search calculations

We performed reaction path search calculations for the NaCl·(H₂O)₆ cluster using the global reaction route mapping (GRRM) code to understand the atomic‐level mechanisms of the NaCl → Na⁺ + Cl⁻ ionic dissociation induced by water solvents. Low‐lying minima, transition states connecting two local minima and corresponding intrinsic reaction coordinates on the potential energy surface are explored. We found that the Na Cl distances at the transitions states for the dissociation pathways were distributed in a relatively wide range of 2.7–3.7 Å and that the Na Cl distance at the transition state did not correlate with the commonly used solvation coordinates. This suggests that the definition of the transition states with specific structures as well as good reaction coordinate is very difficult for the ionic dissociation process even in a small water cluster. © 2018 Wiley Periodicals, Inc.     

Appearance of nonuniform electric fields in solid ionic conductors

A computer model is used to study charge transport in ionic materials upon the application of an electrochemical potential gradient. The model incorporates both the diffusion and drift mechanisms, permitting the evaluation of the contribution of each mechanism. Each mechanism is shown to be dominant depending on the material parameters and the experimental conditions. The results obtained are compatible with experimental and theoretical ones as reported in the literature, suggesting that in several cases studying the transport of mobile species in ionic materials upon the application of an electrochemical potential gradient, the existence of an internal field cannot be ignored. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 4, pp. 398–401. The text was submitted by the authors in English.     

Recent advances in spectroelectrochemistry related to molecular catalytic processes

Continuing interest in renewable energy utilization, the depletion of nonrenewable petrochemical feedstocks and rising atmospheric concentrations of CO₂ from anthropogenic emissions have made molecular electrocatalytic processes involving CO₂, H₂, and O₂ important research foci. One of the touted advantages of molecular electrocatalytic processes, in comparison to heterogeneous systems, is the relative ease with which the active species can be characterized and the catalyst optimized using synthetic methodology. This requires, however, that species generated by the application of potential be spectroscopically studied, which can be difficult given that changes in reactivity can occur. Spectroelectrochemical methods offer a way to study speciation as a function of potential and time, such that catalytic and noncatalytic reactivity can be understood in the context of an overall mechanism. Paired with steadily advancing electrochemical techniques for quantifying the thermodynamic and kinetic parameters of molecular electrocatalysts, spectroelectrochemical data sets can be used to generate a rich understanding of molecular behavior. Recent reports on the use of spectroelectrochemistry to understand molecular electrocatalytic reactions using transition metal complexes are summarized herein.