The classic "brown-ring" reaction in a new medium: kinetics, mechanism, and spectroscopy of the reversible binding of nitric oxide to iron(II) in an ionic liquid

To elucidate the applicability and properties of ionic liquids (ILs) to serve as chemical reaction media for the activation of small molecules by transition-metal complexes, detailed kinetic and mechanistic studies were performed on the reversible binding of NO to FeCl(2) dissolved in the IL 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]) as a solvent. We report, for the first time, the application of laser flash photolysis at ambient and high pressure to study the kinetics of this reaction in an IL. The kinetic data and activation parameters for the "on" and "off" reactions suggest that both processes follow a limiting dissociative (D) ligand substitution mechanism, in contrast to that reported for the same reaction in aqueous solution, where this well-known "brown-ring" reaction follows an interchange dissociative (I(d)) ligand substitution mechanism. The observed difference apparently arises from the participation of the IL anion as a N-donor ligand, as evidenced by the formation of polymeric [Fe(dca)(3)Cl](x)[emim](2x) chains in the solid state and verified by X-ray crystallography. In addition, infrared (IR), Mo?ssbauer, and EPR spectra were recorded for the monomeric reaction product [Fe(dca)(5)NO](3-) formed in the IL, and the parameters closely resemble those of the {FeNO}(7) unit in other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of a high-spin Fe(III) (S = 5/2) center antiferromagnetically coupled to NO(-) (S = 1), yielding the observed spin quartet ground state (S(t) = 3/2).     

The Study of Kinetics of Poly[(R,S)-3-hydroxybutyrate) Degradation Induced by Carboxylate

Summary: The carboxylate induced degradation of the poly[(R,S)-3-hydroxybutyrate] (PHB) has been investigated with non-isothermal measurements. The apparent activation energies for PHB degradation have been determined. Application of the Kissinger's and Flynn-Wall-Ozawa's method for TG and DSC derived data gave good correlation of the results proving applicability of the non-isothermal DSC measurements for the study. Moreover, dependence of the apparent activation energies on the activity of the carboxylate has been found.     

Electronic structure, spectra, and magnetic circular dichroism of cyclohexa-, cyclohepta-, and cyclooctapyrrole

Three recently obtained expanded porphyrins represent nice examples of compounds for which the electronic and spectral properties can be predicted from symmetry considerations alone. Perimeter-model-based theoretical analysis of the electronic structure of doubly protonated cyclo[6], cyclo[7], and cyclo[8]pyrrole leads to the anticipation of qualitatively the same electronic absorption and magnetic circular dichroism patterns for all three compounds. These predictions are fully confirmed by experiments, as well as DFT and INDO/S calculations. Due to a characteristic pattern of frontier molecular orbitals, a degenerate HOMO and a strongly split LUMO pair, the three cyclopyrroles show comparable absorption intensity in the Q and Soret regions. Magnetic circular dichroism spectra reveal both A and B Faraday terms, of which the signs and magnitudes are in remarkably good agreement with theoretical expectations. The values of the magnetic moments of the two lowest degenerate excited states have also been obtained.     

Intra- and intermolecular electronic relaxation of the second excited singlet and the lowest excited triplet states of 1,3-dimethyl-4-thiouracil in solution

Intramolecular processes of deactivation of 1,3-dimethyl-4-thiouracil (DMTU) from the second excited singlet (S2) (pi, pi*) and the lowest excited triplet (T1) (pi, pi*) states have been studied using perfluoro-1,3-dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (tau(S2)) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of tau(S2) in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern-Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (tau0(T1)) and rate constants of self-quenching (k(sq)). The lifetimes, tau0(T1), of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.     

On the catalytic effect of thiolactams on the electrochemical reduction of Zn(II) ions

Summary The electroeduction of Zn(II) ions in 1M NaClO₄ in the presence of thiolactams has been studied by means of the faradaic impedance method in wide frequency ranges. The standard rate constants are found to be a linear function of the surface excesses of thiolactams. Catalytic activity of thiolactams increases in the following order: thiopyrrolidone, thiopiperidinone, thiocaprolactam; the enthalpies of activation decrease in this order. The differences in the catalytic activity of thiolactams result mainly from the double layer effect.

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Zum katalytischen Effekt von Thiolactamen auf die elektrochemische Reduktion von Zn(II)-Ionen

Zusammenfassung Die Elektroeduktion von Zn(II)-Ionen in 1M NaClO₄ in Gegenwart von Thiolactamen wurde mittels derFaradayschen Impedanzmethode über große Frequenzbereiche untersucht. Die Standardgeschwindigkeitskonstanten sind eine lineare Funktion der Überschußoberfläche der Thiolactame. Deren katalytische Aktivität steigt in der Reihenfolge Thiopyrrolidon — Thiopiperidinon — Thiocaprolactam; die Aktivierungsenthalpien sinken in der angegebenen Reihenfolge. Die Unterschiede in der katalytischen Aktivität der Thiolactame sind hauptsächlich auf den Doppelschichteffekt zurückzuführen.

     

Mechanistic studies on the interaction of reduced cobalamin (vitamin B12r) with nitroprusside

The electron-transfer reaction between reduced cobalamin (Cbl(II)) and sodium pentacyanonitrosylferrate(II) (sodium nitroprusside, NP), as well as the subsequent processes following the electron-transfer step, were investigated by spectroscopic (UV-vis, (1)H NMR, EPR), electrochemical (CV, DPV) and kinetic (stopped-flow) techniques. In an effort to clarify the complex reaction pattern observed at physiological pH, systematic spectroscopic and kinetic studies were undertaken as a function of pH (1.8-9) and NP concentration (0.0001 - 0.09 M). The kinetics of the electron-transfer reaction was studied under pseudo-first-order conditions with respect to NP. The reaction occurs in two parallel paths of different order, viz. pseudo-first and pseudo-second order with respect to the NP concentration, respectively. The contribution of each path depends on pH and the [NP]/[Cbl(II)] ratio. At low pH and total NP concentration (pH < 3, [NP]/[Cbl(II)] approximately 1), the cyano-bridged successor complex [Cbl(III)-(mu-NC)-Fe(I)(CN)(3)(NO(+))](-) (1(s)()) is the final reaction product formed in an inner-sphere electron transfer reaction that is coupled to the release of cyanide from coordinated nitroprusside. At higher pH, subsequent reactions were observed which involve the attack of cyanide released in the electron transfer step on the initially formed cyano-bridged species, and lead to the formation of Cbl(III)CN and [Fe(I)(CN)(4)(NO(+))](2)(-). The strong dependence of the rate and mechanism of the subsequent reactions on pH is attributed to the large variation in the effective nucleophilicity of the cyanide ligand in the studied pH range. An alternative electron-transfer pathway observed in the presence of excess NP involves the reaction of the precursor complex [Cbl(II)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](2)(-) (1(p)()) with NP to give [Cbl(III)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](-) (2) and reduced nitroprusside, [Fe(CN)(5)NO](3)(-), as the initial reaction products. Analysis of the kinetic data allowed elucidation of the rate constants for the inner- and outer-sphere electron-transfer pathways. The main factors which influence the kinetics and thermodynamics of the observed electron-transfer steps are discussed on the basis of the spectroscopic, kinetic and electrochemical results. A general picture of the reaction pathways that occur on a short (s) and long (min to h) time scale as a function of pH and relative reactant concentrations is derived from the experimental data. In addition, the release of NO resulting from the one-electron reduction of NP by Cbl(II) was monitored with the use of a sensitive NO electrode. The results obtained in the present study are discussed in reference to the possible influence of cobalamin on the pharmacological action of nitroprusside.     

Atom movement on a dislocated surface

The standard picture of growth at a screw dislocation assumes that the movement of adatoms on a dislocation loop is the same as on an ideal plane. We have examined this proposition by investigating the movement of a single tungsten adatom on a W(110) plane intersected by a screw dislocation. Surprisingly enough, adatom movement was entirely different than on a normal (110) plane: the overall diffusivity was higher, and the mobility varied with the location of the adatom relative to the dislocation core. This study demonstrates that surface transport is strongly affected in the vicinity of dislocations.     

Site-specific incorporation of 5-methylaminomethyl-2-thiouridine and 2-thiouridine(s) into RNA sequences

We present a reproducible protocol for the site-specific incorporation of 5-methylaminomethyl-2-thiouridine (mnm⁵s²U) into a model RNA fragment and, together with 2-methyladenosine (m²A), into the native sequence of the Escherichia coli tRNA^Glu2 anticodon arm (E. coli ASL^Glu2). This approach is also utilized for the synthesis of oligomers modified with multiple 2-thiouridines.     

Effect of the reference solution in the measurement of ion activity coefficients using cells with transference at T = 298.15 K

This work reports individual activity coefficients of ions at T = 298.15 K in aqueous solutions obtained from voltage values of the respective half-cell ion-selective-electrode and a single-junction Ag–AgCl reference electrode, filled with different reference solutions at different concentrations. For potassium and chloride ions in KCl aqueous solutions, reference solutions of KCl, NaCl, or CsCl were used. For sodium and chloride ions in aqueous NaCl solutions, reference solutions of CsCl were used. Experimental runs were performed at molalities (1, 2, and 3) m of the reference solution. The concentration of the sample solution was increased, starting from around 1 · 10^?3 m, up to the molality of the reference solution. The values of activity coefficients are calculated using the Henderson equation to estimate the liquid-junction potential. Results show that the ionic activity coefficients are independent of the nature and concentration of reference solution.