Browsing by Author "Platts, JA"

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    Experimental and theoretical charge density distribution in two ternary cobalt(III) complexes of aromatic amino acids
    (American Chemical Society, 2007-10-11) Overgaard, J; Waller, MP; Piltz, RO; Platts, JA; Emseis, P; Leverett, P; Williams, PA; Hibbs, DE
    The experimental charge density distributions in two optically active isomers of a Co complex have been determined. The complexes are Δ-α-[Co(R,R-picchxn)(R-trp)](ClO4)(2)center dot H2O) (1) and Λ-β(1)-[Co(R,R-picchxn)(R-trp)](CF3SO3)(2)) (2), where picchxn is N,N '-bis(2-picolyl-1,2-diaminocyclohexane) and R-trp is the R-tryptophane anion. The molecular geometries of 1 and 2 are distinguished by the presence in complex 1 of intramolecular pi center dot center dot center dot pi stacking interactions and the presence in complex 2 of intramolecular hydrogen bonding. This pair of isomers therefore serves as an excellent model for studying noncovalent interactions and their effects on structure and electron density and the transferability of electron density properties between closely related molecules. For complex 2, a combination of X-ray and neutron diffraction data created the basis for a X-N charge density refinement. A topological analysis of the resulting density distribution using the atoms in molecules methodology is presented along with,d-orbital populations, showing that the metal-ligand bonds are relatively unaltered by the geometry changes between 1 and 2. The experimental density has been supplemented by quantum chemical calculations on the cobalt complex cations: close agreement between theory and experiment is found in all cases. The energetics of the weak interactions are analyzed using both theory and experiment showing excellent quantitative agreement. In particular it is found that both methods correctly predict the stability of 2 over 1. The transferability between isomers of the charge density and derived parameters is investigated and found to be invalid for these structurally related systems. © 2007, American Chemical Society
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    Investigation of steric influences on hydrogen-bonding motifs in cyclic rreas by using x-ray, neutron, and computational methods
    (Wiley-V C H Verlag GMBH, 2013-11-01) McCormick, LJ; McDonnell-Worth, C; Platts, JA; Edwards, AJ; Turner, DR
    A series of urea-derived heterocycles, 5N-substituted hexahydro-1,3,5-triazin-2-ones, has been prepared and their structures have been determined for the first time. This family of compounds only differ in their substituent at the 5-position (which is derived from the corresponding primary amine), that is, methyl (1), ethyl (2), isopropyl (3), tert-butyl (4), benzyl (5), N,N-(diethyl)ethylamine (6), and 2-hydroxyethyl (7). The common heterocyclic core of these molecules is a cyclic urea, which has the potential to form a hydrogen-bonding tape motif that consists of self-associative R-2(2)(8) dimers. The results from X-ray crystallography and, where possible, Laue neutron crystallography show that the hydrogen-bonding motifs that are observed and the planarity of the hydrogen bonds appear to depend on the steric hindrance at the -carbon atom of the Nsubstituent. With the less-hindered substituents, methyl and ethyl, the anticipated tape motif is observed. When additional methyl groups are added onto the -carbon atom, as in the isopropyl and tert-butyl derivatives, a different 2D hydrogen-bonding motif is observed. Despite the bulkiness of the substituents, the benzyl and N,N-(diethyl)ethylamine derivatives have methylene units at the -carbon atom and, therefore, display the tape motif. The introduction of a competing hydrogen-bond donor/acceptor in the 2-hydroxyethyl derivative disrupts the tape motif, with a hydroxy group interrupting the NHOC interactions. The geometry around the hydrogen-bearing nitrogen atoms, whether planar or non-planar, has been confirmed for compounds 2 and 5 by using Laue neutron diffraction and rationalized by using computational methods, thus demonstrating that distortion of O-C-N-H torsion angles occurs to maintain almost-linear hydrogen-bonding interactions. © 2014, Wiley-VCH Verlag GmbH & Co. KGaA
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    Relationships between electron density and magnetic properties in water-bridged dimetal complexes
    (ACS Publications, 2014) Overgaard, J; Walsh, JPS; Hathwar, VR; Jørgensen, MRV; Hoffman, C; Platts, JA; Piltz, RO; Winpenny, REP
    The experimental and theoretical electron density distributions in two structurally similar transition metal dimers (M = Ni, Co; see Figure) were analyzed using the atoms-in-molecules (AIM) approach, and selected properties related to the chemical bonding are compared to measured intramolecular magnetic exchange interaction parameters.The electron densities in two analogous dimetallic transition metal compounds, namely, [M2(μ-OH2)(tBuCOO)4(tBuCOOH)2(C5H5N)2] (M = Co(1), Ni(2)), were determined from combined X-ray and neutron single-crystal diffraction at 100 K. Excellent correspondence between the thermal parameters from X- and N-derived atomic displacement parameters is found, indicating high-quality X-ray data and a successful separation of thermal and electronic effects. Topological analysis of electron densities derived from high-resolution X-ray diffraction, as well as density functional theory calculations, shows no direct metal–metal bonding in either compound, while the total energy density at the bond critical points suggests stronger metal–oxygen interactions for the Ni system, in correspondence with its shorter bond distances. The analysis also allows for estimation of the relative strength of binding of terminal and bridging ligands to the metals, showing that the bridging water molecule is more strongly bound than terminal carboxylic acid, but less so than bridging carboxylates. Recently, modeling of magnetic and spectroscopic data in both of these systems has shown weak ferromagnetic interactions between the metal atoms. Factors related to large zero-field splitting effects complicate the magnetic analysis in both compounds, albeit to a much greater degree in 1. The current results support the conclusion drawn from previous magnetic and spectroscopic measurements that there is no appreciable direct communication between metal centers. © 2014, American Chemical Society.