Browsing by Author "Hathwar, VR"

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    Measurement of electric fields experienced by urea guest molecules in the 18-crown-6/urea (1:5) host–guest complex: an experimental reference point for electric-field-assisted catalysis
    (American Chemical Society, 2019-02-14) Shi, MW; Thomas, SP; Hathwar, VR; Edwards, AJ; Piltz, RO; Jayatilaka, D; Koutsantonis, GA; Overgaard, J; Nishibori, E; Iversen, BB; Spackman, MA
    High-resolution synchrotron and neutron single-crystal diffraction data of 18-crown-6/(pentakis)urea measured at 30 K are combined, with the aim of better appreciating the electrostatics associated with intermolecular interactions in condensed matter. With two 18-crown-6 molecules and five different urea molecules in the crystal, this represents the most ambitious combined X-ray/synchrotron and neutron experimental charge density analysis to date on a cocrystal or host-guest system incorporating such a large number of unique molecules. The dipole moments of the five urea guest molecules in the crystal are enhanced considerably compared to values determined for isolated molecules, and 2D maps of the electrostatic potential and electric field show clearly how the urea molecules are oriented with dipole moments aligned along the electric field exerted by their molecular neighbors. Experimental electric fields in the range of 10-19 GV m-1, obtained for the five different urea environments, corroborate independent measurements of electric fields in the active sites of enzymes and provide an important experimental reference point for recent discussions focused on electric-field-assisted catalysis. © 2019 American Chemical Society
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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.