Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides

Metal carbonyls react on metal oxide surfaces to give a wide range of structures analogous to those of known compounds. The reactions leading to formation of surface-bound metal carbonyls are explained by known molecular organometallic chemistry and the functional group chemistry of the surfaces. The reaction classes include formation of acid-base adducts as the oxygen of a carbonyl group donates an electron pair to a Lewis acidic center; nucleophilic attack at CO ligands by basic surface hydroxyl groups or O^2? ions; ion-pair formation by deprotonation of hydrido carbonyls to give carbonylate ions; interaction of bifunctional complexes with surface acid-base pair sites such as [Mg^2⊕O^2?]; and oxidative addition of surface hydroxyl groups to metal clusters. The reactions of surface-bound organometallic species include redox condensation and cluster formation on basic surfaces (paralleling the reactions in basic solution) as well as oxidation of mononuclear metal complexes and oxidative fragmentation of metal clusters by reaction with surface hydroxyl groups. Most supported metal carbonyls are unstable at high temperatures, but some, including osmium carbonyl cluster anions on the basic MgO surface, are strongly stabilized in the presence of CO and are precursors of catalysts for CO hydrogenation at 550 K.     

Electronic and geometric structure of acetylide carbon on the surface of lithium

The reaction products formed on clean evaporated lithium films exposed to acetylene gas under UHV conditions were examined with ultraviolet photoelectron and Auger electron spectroscopies. Good agreement was obtained between the observed ultraviolet photoelectron spectrum and a spectrum simulated from ab initio calculations for C₂Li₂ in D_2h symmetry. The components of the C(KW) Auger signal were also consistent with carbon being present as C₂^n? units (acetylide) rather than as carbonaceous aggregates or as monomeric carbon. The observed carbon AES structure was fit in good agreement by a simulation using experimental electronic binding energies and Auger transition intensities calculated for C₂Li₂. Saturation of the ultraviolet and Auger electron signals with increasing exposure to HC₂H was not found, although thin films of lithium were slowly consumed on long exposure to acetylene. A loss of the reaction product from the interfacial region was indicated. Cluster models of acetylide on the (100) face of lithium, C₂/Li₈(6, 2)-(100), were used to investigate geometric arrangement of C₂^n? relative to lithium. The results show that acetylide resident in the interface below the first layer of lithiums of the model was energetically favored compared to C₂^n? resident on top of the first layer of lithium. The stability of this conformation was calculated to be further increased by a small expansion of lithiums near neighbor to C₂Li₂. The restructuring calculated for the model complements the absent saturation and suggests that C₂^n? units diffuse into the actual film via a local expansion.     

Bounds for sorting by prefix reversal

For a permutation σ of the integers from 1 to n, let ?(σ) be the smallest number of prefix reversals that will transform σ to the identity permutation, and let ?(n) be the largest such ?(σ) for all σ in (the symmetric group) S_n. We show that $\text{?(n)?}\text{(5n+5)}\text{3}$, and that $\text{?(n)?}\text{17n}\text{16}$ for n a multiple of 16. If, furthermore, each integer is required to participate in an even number of reversed prefixes, the corresponding function g(n) is shown to obey $\text{3n}\text{2?1}\text{?g(n)?2n+3}$.     

DNA damage by fasicularin

Fasicularin is a structurally novel thiocyanate-containing alkaloid isolated from the ascidian Nephteis fasicularis. Early biological experiments suggested that this compound's cytotoxic properties may stem from its ability to damage cellular DNA. Sequence gel analysis reveals that treatment of a 5'-32P-labeled DNA duplex with fasicularin in pH 7.0 buffer causes strand cleavage selectively at guanine residues. Further experiments indicate that production of these base-labile lesions in DNA involves alkylation of guanine residues by a fasicularin-derived aziridinium ion. This work reveals fasicularin as the first natural product found to generate a DNA-alkylating aziridinium ion via a mechanism analogous to the clinically used anticancer drugs mechlorethamine, melphalan, and chlorambucil.     

Superspace geometry and N = 1 non-minimal supergravity

We investigate superspace geometry for supergravity with non-minimal auxilliary fields. We find that the kinematic constraints and the superspace Bianchi identities are sufficient to obtain complete component expansions of all superspace quantities, including the vielbein and connection superfields. We include a detailed pedagogical discussion on the analysis of constrained superspace Bianchi identities, demonstrating how these are used to derive component field content and transformation laws. We also note that local, chiral supersymmetry representations which form arbitrary representations of the Lorentz group can exist only within the context of supergravity with non-minimal auxilliary fields.     

The influence of draw-induced structural changes on the activity of a semicrystalline polymer-supported catalyst

The activity of a solid, polymer-supported catalyst (a semicrystalline polyethylene film containing grafted, sulfonated styrene) was shown to be altered by cold drawing. The catalytic activity was measured by a test reaction, the dehydration of isopropyl alcohol to give propylene. Catalytic reaction rates were measured with variously drawn films clamped in a differential flow reactor operated at 100°C and 1 atm. The catalytic activity increased with the elongation of the polymer up to a draw ratio of 2.5; the activity decreased upon further drawing. The drawn films were characterized by x-ray diffraction, dynamic mechanical measurements, electron microscopy, birefringence, and density measurements. Since no evidence was found for the formation of surface sites by creation of microcracks, the changes in activity are attributed to modifications in the polymer structure induced by drawing. The kinetics of the catalytic reaction and the data giving percentage crystallinity and crystalline and amorphous orientation factors suggest that, in the undrawn polymer, the catalytically active ? SO₃H groups form a hydrogen-bonded network, which is excluded from the crystalline regions. Initial elongation partially breaks up the network, allowing more ? SO₃H groups to bond to alcohol and become catalytically engaged. Elongation beyond a draw ratio of 2.5 leads to a separation of ? SO₃H groups greater than that required for the formation of the reaction intermediate, which involves the alcohol hydrogen bonded to several ? SO₃H groups. The combined results of the catalytic kinetics experiments and structural characterizations imply that the preliminary deformation response of the semicrystalline polymer occurs predominantly within the noncrystalline regions and is accommodated by the rigid slip and tilting of crystal lamellae. The hydrogen bonding among the ? SO₃H groups in the noncrystalline regions hinders lamellar breakup and suppresses the formation of highly aligned fibrillar morphologies.     

Alpha-synuclein structures probed by 5-fluorotryptophan fluorescence and 19F NMR spectroscopy

Alpha-synuclein, the main protein component of fibrillar deposits found in Parkinson's disease, is intrinsically disordered in vitro. Site-specific information on the protein conformation has been obtained by biosynthetic incorporation of an unnatural amino acid, 5-fluorotryptophan (5FW), into the recombinant protein. Using fluorescence and 19F NMR spectroscopy, we have characterized three proteins with 5FW at positions 4, 39, and 94. Steady-state emission spectra (maxima at 353 nm; quantum yields approximately 0.2) indicate that all three indole side chains are exposed to the aqueous medium. Virtually identical single-exponential excited-state decays (tau approximately 3.4 ns) were observed in all three cases. Single 19F NMR resonances were measured for W4, W39, and W94 at -49.0 +/- 0.1 ppm. Our analysis of the spectroscopic data suggests that the protein conformations are very similar in the regions near the three sites.     

Mechanism for the elimination of aromatic molecules from polyenes in tandem mass spectrometry

We report here a general mechanism for the elimination of aromatic molecules from polyene containing natural products of several compound classes in tandem mass spectrometry.     

Oxide- and zeolite-supported molecular metal complexes and clusters: physical characterization and determination of structure, bonding, and metal oxidation state

This article is a review of the physical characterization of well-defined site-isolated molecular metal complexes and metal clusters supported on metal oxides and zeolites. These surface species are of interest primarily as catalysts; as a consequence of their relatively uniform structures, they can be characterized much more precisely than traditional supported catalysts. The properties discussed in this review include metal nuclearity, oxidation state, and ligand environment, as well as metal-support interactions. These properties are determined by complementary techniques, including transmission electron microscopy; X-ray absorption, infrared, Raman, and NMR spectroscopies; and density functional theory. The strengths and limitations of these techniques are assessed in the context of results characterizing samples that have been investigated thoroughly and with multiple techniques. The depth of understanding of well-defined metal complexes and metal clusters on supports is approaching that attainable for molecular analogues in solution. The results provide a foundation for understanding the more complex materials that are typical of industrial catalysts.