School of Chemistry - Research Publications
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ItemSelectivity Effects in Bimetallic Catalysis: Role of the Metal Sites in the Decomposition of Formic Acid into H-2 and CO2 by the Coinage Metal Binuclear Complexes [dppmMM(H)](+)(Wiley, 2017-04-07)Design of new bimetallic catalysts requires an understanding of how cooperative effects of the metal sites influences reactivity. Here we show how switching one or both of the silver atoms in binuclear silver hydride cations, [dppmAg2(H)]+ (dppm=1,1‐Bis(diphenylphosphino)‐methane), with all combinations of copper and/or gold maintains selective dehydrogenation of formic acid, enhancing reactivity by up to 2 orders of magnitude. This is a key step in the selective, catalyzed extrusion of carbon dioxide from formic acid, HO2CH, with important applications in hydrogen storage and in situ generation of H2. Decarboxylation of [dppmMM′(O2CH)]+ through collision induced dissociation regenerates [dppmMM′(H)]+. DFT calculations provide insights into these cooperative effects. The copper homobinuclear catalyst performs best overall.
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ItemChemical Ionization Mass Spectrometry: 50 Years on(SPRINGER, 2016-11)
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ItemGas-Phase Intercluster Thiyl-Radical Induced C-H Bond Homolysis Selectively Forms Sugar C2-Radical Cations of Methyl D-Glucopyranoside: Isotopic Labeling Studies and Cleavage Reactions(SPRINGER, 2017-07)A suite of isotopologues of methyl D-glucopyranosides is used in conjunction with multistage mass spectrometry experiments to determine the radical site and cleavage reactions of sugar radical cations formed via a recently developed 'bio-inspired' method. In the first stage of CID (MS2), collision-induced dissociation (CID) of a protonated noncovalent complex between the sugar and S-nitrosocysteamine, [H3NCH2CH2SNO + M]+, unleashes a thiyl radical via bond homolysis to give the noncovalent radical cation, [H3NCH2CH2S• + M]+. CID (MS3) of this radical cation complex results in dissociation of the noncovalent complex to generate the sugar radical cation. Replacement of all exchangeable OH and NH protons with deuterons reveals that the sugar radical cation is formed in a process involving abstraction of a hydrogen atom from a C-H bond of the sugar coupled with proton transfer to the sugar, to form [M - H• + D+]. Investigation of this process using individual C-D labeled sugars reveals that the main site of H/D abstraction is the C2 position, since only the C2-deuterium labeled sugar yields a dominant [M - D• + H+] product ion. The fragmentation reactions of the distonic sugar radical cation, [M - H•+ H+], were studied by another stage of CID (MS4). 13C-labeling studies revealed that a series of three related fragment ions each contain the C1-C3 atoms; these arise from cross-ring cleavage reactions of the sugar. Graphical Abstract ᅟ.
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ItemSeymour "Sy" Meyerson, December 4, 1916-May 28, 2016 OBITUARY(SPRINGER, 2016-12)
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ItemLigand-induced substrate steering and reshaping of [Ag2(H)]+ scaffold for selective CO2 extrusion from formic acid(NATURE PUBLISHING GROUP, 2016-06)Metalloenzymes preorganize the reaction environment to steer substrate(s) along the required reaction coordinate. Here, we show that phosphine ligands selectively facilitate protonation of binuclear silver hydride cations, [LAg2(H)](+) by optimizing the geometry of the active site. This is a key step in the selective, catalysed extrusion of carbon dioxide from formic acid, HO2CH, with important applications (for example, hydrogen storage). Gas-phase ion-molecule reactions, collision-induced dissociation (CID), infrared and ultraviolet action spectroscopy and computational chemistry link structure to reactivity and mechanism. [Ag2(H)](+) and [Ph3PAg2(H)](+) react with formic acid yielding Lewis adducts, while [(Ph3P)2Ag2(H)](+) is unreactive. Using bis(diphenylphosphino)methane (dppm) reshapes the geometry of the binuclear Ag2(H)(+) scaffold, triggering reactivity towards formic acid, to produce [dppmAg2(O2CH)](+) and H2. Decarboxylation of [dppmAg2(O2CH)](+) via CID regenerates [dppmAg2(H)](+). These gas-phase insights inspired variable temperature NMR studies that show CO2 and H2 production at 70 °C from solutions containing dppm, AgBF4, NaO2CH and HO2CH.
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ItemMultiple P450s and Variation in Neuronal Genes Underpins the Response to the Insecticide Imidacloprid in a Population of Drosophila melanogaster(NATURE PORTFOLIO, 2017-09-12)Insecticide resistance is an economically important example of evolution in response to intense selection pressure. Here, the genetics of resistance to the neonicotinoid insecticide imidacloprid is explored using the Drosophila Genetic Reference Panel, a collection of inbred Drosophila melanogaster genotypes derived from a single population in North Carolina. Imidacloprid resistance varied substantially among genotypes, and more resistant genotypes tended to show increased capacity to metabolize and excrete imidacloprid. Variation in resistance level was then associated with genomic and transcriptomic variation, implicating several candidate genes involved in central nervous system function and the cytochrome P450s Cyp6g1 and Cyp6g2. CRISPR-Cas9 mediated removal of Cyp6g1 suggested that it contributed to imidacloprid resistance only in backgrounds where it was already highly expressed. Cyp6g2, previously implicated in juvenile hormone synthesis via expression in the ring gland, was shown to be expressed in metabolically relevant tissues of resistant genotypes. Cyp6g2 overexpression was shown to both metabolize imidacloprid and confer resistance. These data collectively suggest that imidacloprid resistance is influenced by a variety of previously known and unknown genetic factors.
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ItemPartitioning the roles of CYP6G1 and gut microbes in the metabolism of the insecticide imidacloprid in Drosophila melanogaster(NATURE PORTFOLIO, 2017-09-12)Resistance to insecticides through enhanced metabolism is a worldwide problem. The Cyp6g1 gene of the vinegar fly, Drosophila melanogaster, is a paradigm for the study of metabolic resistance. Constitutive overexpression of this gene confers resistance to several classes of insecticides, including the neonicotinoid imidacloprid (IMI). The metabolism of IMI in this species has been previously shown to yield oxidative and nitro-reduced metabolites. While levels of the oxidative metabolites are correlated with CYP6G1 expression, nitro-reduced metabolites are not, raising the question of how these metabolites are produced. Some IMI metabolites are known to be toxic, making their fate within the insect a second question of interest. These questions have been addressed by coupling the genetic tools of gene overexpression and CRISPR gene knock-out with the mass spectrometric technique, the Twin-Ion Method (TIM). Analysing axenic larvae indicated that microbes living within D. melanogaster are largely responsible for the production of the nitro-reduced metabolites. Knock-out of Cyp6g1 revealed functional redundancy, with some metabolites produced by CYP6G1 still detected. IMI metabolism was shown to produce toxic products that are not further metabolized but readily excreted, even when produced in the Central Nervous System (CNS), highlighting the significance of transport and excretion in metabolic resistance.
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ItemEnvironmental Polymer Degradation: Using the Distonic Radical Ion Approach to Study the Gas-Phase Reactions of Model Polyester Radicals(AMER CHEMICAL SOC, 2017-07-20)A novel precursor to the distonic O- and C-centered radical cations Oxo+O• and Oxo+C• was designed and synthesized, which represents model systems for radicals produced during polyester degradation. The precursor is equipped with a nitrate functional group, which serves as a masked site for these alkoxyl and carbon radicals that are unleashed through collision-induced dissociation (CID). Oxo+O• and Oxo+C• feature a cyclic carboxonium ion as permanent charge tag to enable monitoring their ion-molecule reactions on the millisecond to second time scale in the ion trap of the mass spectrometer. The reactions of Oxo+O• and Oxo+C• with cyclohexene, cyclohexadiene, ethyl acetate, 1,1-dimethoxyethane, and 1,2-dimethoxyethane, which exhibit structural features present in both intact and defective polyesters, were explored through product and kinetic studies to identify "hot spots" for radical-induced damage in polyesters. All reactions with Oxo+O• were extremely fast and proceeded predominantly through HAT. Oxo+C• was about two orders of magnitude less reactive and did not noticeably damage aliphatic ester moieties through hydrogen abstraction on the time scale of our experiments. Radical addition to alkene π systems was identified as an important pathway for C-radicals, which needs to be included in polymer degradation mechanisms.
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ItemNichtwässrige mikrowellengestützte Synthesen von Deca‐ und Hexamolybdovanadaten(Wiley, 2017-07-10)Abstract Ein neuer Ansatz zur Synthese von Heterohexa‐ und Heterodecametallaten unter nichtwässrigen, mikrowellengestützten Reaktionsbedingungen wurde entwickelt. Die zwei neuartigen Molybdovanadate wurden isoliert und im festen sowie gelösten Zustand mittels Einkristall‐Röntgenbeugung, FT‐IR‐, UV/Vis‐ und multinuklearer NMR‐Spektroskopie sowie ESI‐MS charakterisiert. Die relativen Stabilitäten der möglichen Strukturisomere beider Polyoxometallatsysteme wurden mithilfe von dispersionskorrigierten DFT‐Rechnungen untersucht.