Non-resonant microwave absorption studies of superconducting MgB2 and MgB2 + MgO

Non-resonant microwave absorption (NRMA) studies of superconducting MgB₂ and a sample containing ∼10% by weight of MgO in MgB₂ are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB₂ sample. A linear temperature dependence of the lower critical field H _c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB₂ + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links. The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB₂ sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong pinning of flux lines. This hysteresis saturates a few degrees below T _c while in the MgB₂ + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning.     

Very fast electron migrations within p-doped aromatic cofacial arrays leading to three-dimensional (toroidal) pi-delocalization

The charge-resonance phenomenon originally identified by Badger and Brocklehurst lies at the core of the basic understanding of electron movement and delocalization that is possible within p-doped aromatic (face-to-face) arrays. To this end, we now utilize a series of different aryl-donor groups (Ar) around a central platform to precisely evaluate the intramolecular electron movement among these tethered redox centers. As such, the unique charge-resonance (intervalence) absorption bands observed upon the one-electron oxidation or p-doping of various hexaarylbenzenoid arrays (Ar6C6) provide quantitative measures of the reorganization energy (lambda) and the electronic coupling element (H(ab)) that are required for the evaluation of the activation barrier (deltaG(ET)) for electron-transfer self-exchange according to Marcus-Hush theory. The extensive search for viable redox centers is considerably aided by the application of a voltammetric criterion that has led in this study to Ar = N,N-dialkyl-p-anilinyl, in which exceptionally low barriers are shown to lie in the range deltaG(ET) = 0.3-0.7 kcal mol(-1) for very fast electron hopping or peregrination around the hexagonal circuit among six equivalent Ar sites. Therefore, at transition temperatures T(t) > 0.5/R or roughly -20 degrees C, the electron-transfer dynamics become essentially barrierless since the whizzing occurs beyond the continuum of states and effectively achieves complete pi-delocalization.     

Quinones as electron acceptors. X-ray structures, spectral (EPR, UV-vis) characteristics and electron-transfer reactivities of their reduced anion radicals as separated vs contact ion pairs

Successful isolation of a series of pure (crystalline) salts of labile quinone anion radicals suitable for X-ray crystallographic analysis allows for the first time their rigorous structural distinction as "separated" ion pairs (SIPs) vs "contact" ion pairs (CIPs). The quantitative evaluation of the precise changes in the geometries of these quinones (Q) upon one-electron reduction to afford the anion radical (Q-*) is viewed relative to the corresponding (two-electron) reduction to the hydroquinone (H2Q) via the Pauling bond-length/bond-order paradigm. Structural consequences between such separated and contact ion pairs as defined in the solid state with those extant in solution are explored in the context of their spectral (EPR, UV-vis) properties and isomerization of tightly bound CIPs. Moreover, the SIP/CIP dichotomy is also examined in intermolecular interactions for rapid (self-exchange) electron transfer between Q-* and Q with second-order rate constants of kET approximately equal to 10(8) M-1 s-1, together with the spectral observation of the paramagnetic intermediates [Q,Q-*]leading to 1:1 adducts, as established by X-ray crystallography.     

Molecular Bases for Anesthetic Agents: Halothane as a Halogen‐ and Hydrogen‐Bond Donor

Although instrumental for optimizing their pharmacological activity, a molecular understanding of the preferential interactions given by volatile anesthetics is quite poor. This paper confirms the ability of halothane to work as a hydrogen‐bond (HB) donor and gives the first experimental proof that halothane also works as a halogen‐bond (HaB) donor in the solid state and in solution. A halothane/hexamethylphosphortriamide co‐crystal is described and its single‐crystal X‐ray structure shows short HaBs between bromine, or chlorine, and the phosphoryl oxygen. New UV/Vis absorption bands appear upon addition of diazabicyclooctane and tetra(n‐butyl)ammonium iodide to halothane solutions, indicating that nitrogen atoms and anions may mediate the HaB‐driven binding processes involving halothane as well. The ability of halothane to work as a bidentate/tridentate tecton by acting as a HaB and HB donor gives an atomic rationale for the eudismic ratio shown by this agent.     

Complexes of Ni2+ with dioxotetramines and macrocyclic tetraaza-ligands as inhibitors of oxidation of dialkyl sulfoxides

Nickel complexes with acyclic dioxotetramines and macrocyclic tetraaza-ligands, when present in dimethyl sulfoxide, inhibit its radical chain oxidation with oxygen, as shown by the volumetric method. A scheme of the processes involving the decay of -sulfinylperoxyl radicals upon interaction with both Ni²⁺ and Ni³⁺ ions is proposed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 879–883, May, 1993.We are grateful to S. P. Gavrish who has kindly provided us with the samples of compounds 6 and 7.     

New NiIII/II redox systems with pH controlled potentials based on macrocyclic ligands with azacyclam ligands

Replacement of carbon atoms by non-coordinating nitrogen atoms in 14-membered macrocyclic ligands L increased the redox potentials of Ni_III/IIL couples. Compounds of this type are new systems whose redox potentials can be varied by changing the pH of the medium.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 6, pp. 375–378, November–December, 1995.This study was supported by the State Fund for Fundamental Research, Ukraine State Committee on Science and Technology (project 33/52) and in part by the International Science Fund (grant UBG 000).     

The spectral elucidation versus the X-ray structure of the critical precursor complex in bimolecular electron transfers: application of experimental/theoretical solvent probes to ion-radical (redox) dyads

The mechanistic conundrum is commonly posed by the intrinsic structural disconnect between a bimolecular (reactive) intermediate that is fleetingly detected spectroscopically in solution versus that rigorously defined by isolation and X-ray crystallography. We resolve this ambiguity by the combined experimental and theoretical application of the solvent media probe to the transient (1:1) precursor complex in the simplest chemical reaction involving direct adiabatic electron transfer (ET) among various donor/acceptor pairs. Of particular help in our resolution of such an important ET problem is the characterization of the bimolecular precursor complex as Robin-Day class II (localized) or class III (delocalized) from either the solvent-dependent or the solvent-independent response of the diagnostic intervalence absorption bands for the quantitative evaluation of the electronic coupling elements. The magnitudes of these intracomplex bindings are confirmed by theoretical (ab initio and DFT) computations that derive from X-ray structures and Marcus-Hush theories. Most importantly, the experimental solvent-induced ET barriers evaluated from the intervalence absorption bands are also quantitatively verified by the calculated outer-shell reorganization energies to establish unambiguously the intimate interconnection between the loosely bound bimolecular intermediate identified concurrently in solution and in the solid state.     

Kinetic determination of ruthenium using the oxidation of a macrocyclic complex of Nickel(II) by iodate

It was found that complex ruthenium and iridium sulfates catalyze the oxidation of macrocyclic nickel(II) complexes by iodate. Dependences of the reaction rate on the procedure of the preparation of the catalyst, concentrations of components, the acidity of the reaction mixture, and temperature were determined. Based on these studies, a procedure was developed for the determination of ruthenium compounds in solutions. It was demonstrated that this reaction can be used for the determination of iridium.