Showing items related by title, author, creator and subject.

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  Electronic structure investigations of multiple bonding between atoms: From metal-nitrogen triple bonds to metal-metal triple and quadruple bonds

  Lichtenberger, Dennis L.; English, Jason B. (The University of Arizona., 2002)

  The nature of multiple bonding involving transition metal atoms has been explored via photoelectron spectroscopic and computational studies of molecules containing metal-metal quadruple and triple bonds as well as of molecules containing formal metal-nitrogen triple bonds. The principles governing the nature of the multiple bonding in these systems are similar whether the multiple bonding occurs between two transition metals or between a transition metal and a nitrogen atom. First, the electronic structures of the R₃M≡N molecules, where R = ᵗBuO (Cr, Mo, W); iPrO (Mo); (CH₃)₂CF₃CO (Mo); and Cl (Mo), are examined by photoelectron spectroscopy in conjunction with density functional calculations. To assign the features seen in the photoelectron spectra, close attention is paid to the effects of (1) metal substitution and (2) alkoxide (or Cl) substitution. Examination of the photoelectron spectra of the full series of alkoxide-substituted molecules allows the relative positions of the ionizations from the M≡N σ and π orbitals to be identified. Of great importance to the electronic structure of these molecules are the alkoxide orbital combinations that mix strongly with the M≡N σ and π orbitals. The importance of the ancillary ligand combinations is clearly demonstrated by the photoelectron spectroscopic and computational studies of Cl₃Mo≡N. The replacement of the alkoxide ligand with chlorides greatly simplifies the resultant photoelectron spectrum, allowing all of the valence ionizations to be assigned. Next, the bonding in the M₂X₄(PMe₃)₄ molecules, where M = Mo (X = Cl, Br); W (X = Cl); and Re (X = Cl, Br, I), is explored by photoelectron spectroscopic investigations in conjunction with electronic structure calculations. From these investigations, the ionizations from the metal-based orbitals as well as several ligand-based orbitals have been assigned. The first ionization energies of both the molybdenum (δ) and rhenium (δ*) molecules decrease as the electronegativity of the halide increases. The origin of this inverse halide effect is explored. Finally, the nature of the quadruple metal-metal bond in the M₂(chp)₄ molecules (M = Cr, Mo, W; chp = 2-chloro-6-oxo-pyridinate) is probed. For all three metal systems, an ionization from the M₂ δ orbital can be seen. This is only the second time a distinct ionization feature has been noted for ionization of the delta orbital from a dichromium molecule. Comparisons with the previously studied M₂(mhp)₄ molecules (mhp = 6-methyl-2-oxo-pyridinate) allow for a better understanding of the electronic structure of these molecules.
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  Metal, ligand, and symmetry influences on metal-metal bonds: Photoelectron spectroscopy and theory

  Lichtenberger, Dennis L.; Lynn, Matthew Allen (The University of Arizona., 2000)

  Three sets of metal-metal bonded systems of the form M₂(L ͡ L)₄ have been studied by gas-phase ultraviolet photoelectron spectroscopy and electronic structure calculations to understand the electronic structures of and bonding in these molecules. The ligand sets range from the relatively poor electron donor trifluoroacetate ligand, to hydroxymethylpyridinate (mhp), and finally to the relatively strong electron donor N,N'-diphenylformamidinate (form) ligand. Not only does this study elucidate the methods by which metal and ligand interact throughout a series of differing electron donor ligand sets, but it also presents a cohesive understanding of the electronic structures of these systems in terms of overall molecular symmetry. In particular, the relative stabilities and orbital characters of the metal-metal bonding and antibonding orbitals are probed to understand the ability of a particular ligand set to affect the ability of two metal atoms to bind together. First, compounds of the form M₂(form)₄ (M = Cr, Mo, W, Ru, Rh, Pd) are examined. The spectra of the metal-metal quadruple bond-containing systems (i.e., M₂(form)₄ where M = Cr, Mo, W) are used to identify several metal- and ligand-based ionization features, which can then be used to identify the additional metal-based features in the spectra of the remaining systems. Given the ease with which functional groups can be added to the formamidinate ligand, a series of substituted Mo₂(form)₄ systems have been prepared and their ionization data have been compared with solution-phase electrochemical results. Next, the electronic structures of M₂(O₂CCF₃)₄ (M = Mo, Rh) are studied. Variable energy photon experiments reveal a predominance of ligand character in the M-M σ and π orbitals, despite the relatively poor overall electron donor ability of the ligand. The means by which such a ligand can interact by symmetry with these metal orbitals are studied by computational methods. Finally, the bonding in M₂(mhp)₄ (M = Cr, Mo, W, Ru, Rh) systems is probed. The lower symmetry of these molecules and the intermediate donor properties of this ligand set allow for correlation with the electronic structures of M₂(form)₄ and M₂(O₂CCF₃)₄. Unlike for the higher symmetry systems, ligand involvement in the M-M δ bond is observed and can be understood in terms of molecular symmetry arguments.
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  The Leoncino Dwarf Galaxy: Exploring the Low-metallicity End of the Luminosity–Metallicity and Mass–Metallicity Relations

  McQuinn, Kristen. B. W.; Berg, Danielle A.; Skillman, Evan D.; Adams, Elizabeth A. K.; Cannon, John M.; Dolphin, Andrew E.; Salzer, John J.; Giovanelli, Riccardo; Haynes, Martha P.; Hirschauer, Alec S.; et al. (IOP PUBLISHING LTD, 2020-03-18)

  Extremely metal-poor (XMP) galaxies are low-mass, star-forming galaxies with gas-phase oxygen abundances below 12 + log(O/H) = 7.35 (similar to 1/20Z(circle dot)). Galaxy evolution scenarios suggest three pathways to form an XMP: (1) secular evolution at low galaxy masses, (2) slow evolution in voids, or (3) dilution of measured abundances from infall of pristine gas. The recently discovered XMP galaxy Leoncino, with an oxygen abundance below 3% Z(circle dot), provides an opportunity to explore these different scenarios. Using Hubble Space Telescope imaging of the resolved stellar populations of Leoncino, we measure the distance to the galaxy to be D = 12.1(3.4)(+1.7) Mpc and find that Leoncino is located in an underdense environment. Leoncino has a compact morphology, hosts a population of young, massive stars, has a high gas-to-star mass ratio, and shows signs of interaction with a galaxy nearby on the sky, UGC 5186. Similar to nearly all XMP galaxies known in the nearby universe, Leoncino is offset from the Luminosity-Metallicity (LZ) relation. However, Leoncino is consistent with the stellar Mass-Metallicity (MZ) relation defined by Local Volume galaxies. Thus, our results suggest that the offset from the LZ relation is due to higher recent star formation, likely triggered by a minor interaction, while the low oxygen abundance is consistent with the expectation that low-mass galaxies will undergo secular evolution marked by inefficient star formation and metal loss via galactic winds. This is in contrast to XMP galaxies that are outliers in both the LZ and MZ relations; in such cases, the low oxygen abundances are best explained by dilution due to the infall of pristine gas. We also discuss why quiescent XMP galaxies are underrepresented in current surveys.