Surfactant control of gas uptake: effect of butanol films on HCl and HBr entry into supercooled sulfuric acid

The entry of HCl into 60-68 wt % D(2)SO(4) and HBr into 68 wt % acid containing 0-0.18 M 1-butanol was monitored by measuring the fractions of impinging HCl and HBr molecules that desorb as DCl and DBr after undergoing H --> D exchange within the deuterated acid. The addition of 0.18 M butanol to the acid creates butyl films that reach approximately 80% surface coverage at 213 K. Surprisingly, this butyl film does not impede exchange but instead enhances it: the HCl --> DCl exchange fractions increase from 0.52 to 0.74 for 60 wt % D(2)SO(4) and from 0.14 to 0.27 for 68 wt % D(2)SO(4). HBr --> DBr exchange increases even more sharply, rising from 0.22 to 0.65 for 68 wt % D(2)SO(4). We demonstrate that this enhanced exchange corresponds to enhanced uptake into the butyl-coated acid for HBr and infer this equivalence for HCl. In contrast, the entry probability of the basic molecule CF(3)CH(2)OH exceeds 0.85 at all acid concentrations and is only slightly diminished by the butyl film. The OD groups of surface butanol molecules may assist entry by providing extra interfacial protonation sites for HCl and HBr dissociation. The experiments suggest that short-chain surfactants in sulfuric acid aerosols do not hinder heterogeneous reactions of HCl or HBr with other solute species.     

Electrochemical detection of amino acids at carbon nanotube and nickel-carbon nanotube modified electrodes

The oxidation and enhanced detection of traditionally 'non-electroactive' amino acids at a single-wall carbon nanotube (SWNT) surface and at a nickel hydroxide film electrochemically deposited and generated upon the SWNT layer is reported. Different CNT are compared, with Nafion-dispersed SWNT offering the most favorable layer for constant-potential amperometric detection. Factors affecting the oxidation process, including the pH or applied potential, are assessed. The response of the SWNT-Nafion coated electrode compares favorably with that of copper and nickel disk electrodes under flow injection analysis (FIA) conditions. The electrodeposition of nickel onto the SWNT-Nafion layer (Ni-CNT) led to a dramatic enhancement of the analytical response (vs. that observed at the SWNT or nickel electrodes alone). The oxidative process at the Ni(OH)(2) layer has been studied and the increase in sensitivity rationalized. In the presence of amino acid the Ni-CNT layer undergoes an electrocatalytic process in which the amino acid reduces the newly formed NiO(OH) species. Furthermore, the attractive response of both the CNT and Ni-CNT layers has allowed these electrodes to be used for constant-potential FIA of various amino acids and indicates great promise for monitoring chromatographic effluents. Once again an improved signal was observed at the Ni-CNT electrode compared to nickel deposited upon a bare glassy carbon electrode (Ni-GC).     

Structural Isomers of Bis(diphenylarsino)methane‐dichloroplatinum(II)

From a range of preparative methods, three geometric isomers of bis(diphenylarsino)methane‐dichloroplatinum(II) have been isolated, viz. cis‐PtCl₂(dpam), trans, trans‐Pt₂Cl₄(dpam)₂ and cis, trans‐Pt₂Cl₄(dpam)₂. Their structures were determined by single‐crystal X‐ray diffraction.     

Ab initio characterization of several states of nitroxylium (NO). Comparison of fragmentation energies of nitroxylium,nitroxyl (NO3), and nitrate

Ab initio calculations have been performed at the self-consistent field (HF) level, and its perturbative extensions up to fourth-order (MPn), for several electronic states of nitroxylium (NO) as well as for a large number of reference species. Geometries are optimized at the HF/DZ and HF/DZP levels (double zeta and double zeta plus polarization bases). The ground state is found to be the D_3h ¹A₁′ state, with the C_2v ¹A₁ (closed Y) state higher by 0.94 eV. The relationship between adding electrons or oxygen atoms to NO⁺ and NO is explored, especially in relation to fragmentation energies of NO (q = 0 or 1). A comparison is drawn between NO and two isoelectronic species, CO₃ and C(CH₂)₃, where no surprises are found.     

Interconversions of aryl radicals by 1,4-shifts of hydrogen atoms. A synthesis of benzo[a]corannulene

Aryl radicals generated ortho to aryl substituents by flash vacuum pyrolysis (FVP) of the corresponding aryl chlorides are shown to be capable of transferring hydrogen atoms between the ortho and ortho' positions (1,4-shifts of hydrogen atoms). In the examples described here, the rearranged aryl radicals are trapped by subsequent radical cyclization reactions. For example, FVP of 2-(o-chlorophenyl)benzo[c]phenanthrene gives 1-phenylbenzo[ghi]fluoranthene as the major product by homolysis of the C-Cl bond, 1,4-shift of a hydrogen atom out of the sterically congested cove region to the radical center, cyclization of the rearranged radical, and rearomatization of the molecule by loss of the other cove region hydrogen. In a control experiment run under the same conditions, FVP of 2-phenylbenzo[c]phenanthrene, which lacks a radical precursor, gave primarily recovered starting material. When the FVP was repeated using 2-(2,6-dichlorophenyl)benzo[c]phenanthrene as the starting material, benzo[a]corannulene was obtained as the major product, presumably by the same cascade of events to produce 1-(o-chlorophenyl)benzo[ghi]fluoranthene, which then suffers a second radical cyclization spontaneously under the high-temperature conditions to give the geodesic polyarene.     

A polymorphic pocket at the P10 position contributes to peptide binding specificity in class II MHC proteins

Peptides bind to class II major histocompatibility complex (MHC) proteins in an extended conformation. Pockets in the peptide binding site spaced to accommodate peptide side chains at the P1, P4, P6, and P9 positions have been previously characterized and help to explain the obtained peptide binding specificity. However, two peptides differing only at P10 have significantly different binding affinities for HLA-DR1. The structure of HLA-DR1 in complex with the tighter binding peptide shows that the peptide binds in the usual polyproline type II conformation, but with the P10 residue accommodated in a shallow pocket at the end of the binding groove. HLA-DR1 variants with polymorphic residues at these positions were produced and found to exhibit different side chain specificity at the P10 position. These results define a new specificity position in HLA-DR proteins.     

Citrate selectivity of poly(neutral red) electropolymerized films

The synthesis and the spectroscopic and potentiometric characterization of electropolymerized films obtained from neutral red (NR) on Pt surface are described. FT Raman and NMR spectroscopy were used for evaluation of the resulting poly(NR) layers. Then potentiometry was applied to study the characteristics of the complexation of carboxylates with the poly(NR) films. A potentiometric poly(NR) sensor showing high selectivity for citrate anion was employed for its determination in soft drinks.     

Organothallium compounds : XVII. Halogenobis(polyfluorophenyl)thallium(III) complexes

The preparations, stabilities and structures of the complexes R₂TlX and R₂ LTlX (R = C₆F₅, p-HC₆F₄, or o-HC₆F₄; X = Br or Cl; L = Ph₃PO, 2,2′-bipyridyl (bpy) or Ph₃P) have been examined or (R = C₆ F₅) reinvestigated. The derivatives R₂TlX are monomeric in acetone, from which the complex (p-HC₆F₄)₂ Me₂COTIBr has been isolated. In this solvent, the complexes R₂LTlX (L = Ph₃PO, bpy, or Ph₃P) undergo partial dissociation by loss of L. When L = bpy, there is also slight ionization into R₂LTl⁺ and R₂TlX^?₂. The acceptor properties of R₂TlX compounds towards uncharged ligands decrease R = C₆F₅ ? p-HC₆F₄ > o-HC₆F₄ > Ph. Dimeric behaviour is observed for R₂TIX compounds in benzene, whilst R₂LTlX (L = Ph₃PO or bpy) derivatives show slight but significant association. In the solid state, R₂TlX compounds are considered to be polymeric with five coordinate thallium, and R₂LTlX derivatives to be dimeric with five (L = Ph₃PO) or six (L = bpy) coordinate thallium by contrast with four coordinate dimeric and four or five coordinate monomeric structures previously proposed for the respective pentafluorophenyl derivatives. Halogen bridging is unsymmetrical for R = C₆F₅ or p-HC₆F₄, but may be more symmetrical for R = o-HC₆F₄ when L = Ph₃PO or bpy. Reported structural data for the complexes (C₆F₅)LTlX (L = Ph₃AsO, Ph₃P, Ph₃As, or 1,10-phenanthroline; X = Br or Cl) and (C₆F₅)₂TlCl^?₂ are reinterpreted and the proposed structures revised.     

The structure of di-μ-pentafluorobenzoatobis[bis(pentafluorophenyl)(triphenylphosphine oxide)thallium(iii)]

An X-ray crystallographic study has shown that the complex (C₆H₅)₂TlO₂CC₆F₅(OPPh₃) has a dimeric structure with unsymmetrical pentafluorobenzoate bridging (TlO 2.531 and 2.789 Å) but an exact crystallographic centre of symmetry. The pentafluorobenzoate groups are also unsymmetrically chelated to thallium (TlO 2.389 and 2.531 Å.), which overall has irregular six coordination.     

Mechanistic studies on the formation and reactivity of dioxygen adducts of diiron complexes supported by sterically hindered carboxylates

Dioxygen activation by enzymes such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases occurs at a nonheme diiron active site supported by two histidines and four carboxylates, typically involving a (peroxo)diiron(III,III) intermediate in an early step of the catalytic cycle. Biomimetic tetracarboxylatodiiron(II,II) complexes with the familiar "paddlewheel" topology comprising sterically bulky o-dixylylbenzoate ligands with pyridine, 1-methylimidazole, or THF at apical sites readily react with O(2) to afford thermally labile peroxo intermediates that can be trapped and characterized spectroscopically at low temperatures (193 K). Cryogenic stopped-flow kinetic analysis of O(2) adduct formation carried out for the three complexes reveals that dioxygen binds to the diiron(II,II) center with concentration dependences and activation parameters indicative of a direct associative pathway. The pyridine and 1-methylimidazole intermediates decay by self-decomposition. However, the THF intermediate decays much faster by oxygen transfer to added PPh(3), the kinetics of which has been studied with double mixing experiments in a cryogenic stopped-flow apparatus. The results show that the decay of the THF intermediate is kinetically controlled by the dissociation of a THF ligand, a conclusion supported by the observation of saturation kinetic behavior with respect to PPh(3), inhibition by added THF, and invariant saturation rate constants for the oxidation of various phosphines. It is proposed that the proximity of the reducing substrate to the peroxide ligand on the diiron coordination sphere facilitates the oxygen-atom transfer. This unique investigation of the reaction of an O(2) adduct of a biomimetic tetracarboxylatodiiron(II,II) complex provides a synthetic precedent for understanding the electrophilic reactivity of like adducts in the active sties of nonheme diiron enzymes.