Browsing by Author "Thomas, SP"

Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    The ambiguous origin of thermochromism in molecular crystals of dichalcogenides: chalcogen conds versus dynamic Se−Se/Te−Te bonds
    (Wiley, 2023-11-06) Thomas, SP; Singh, A; Grosjean, A; Alhameedi, K; Grønbech, TBE; Piltz, RO; Edwards, AJ; Iversen, BB
    We report thermochromism in crystals of diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe), which is at variance with the commonly known mechanisms of thermochromism in molecular crystals. Variable temperature neutron diffraction studies indicated no conformational change, tautomerization or phase transition between 100 K and 295 K. High‐pressure crystallography studies indicated no associated piezochromism in dpdSe and dpdTe crystals. The evolution of the crystal structures and their electronic band structure with pressure and temperature reveal the contributions of intramolecular and intermolecular factors towards the origin of thermochromism—especially the intermolecular Se⋅⋅⋅Se and Te⋅⋅⋅Te chalcogen bonds and torsional modes of vibrations around the dynamic Se−Se and Te−Te bonds. Further, a co‐crystal of dpdSe with iodine (dpdSe‐I2) and an alloy crystal of dpdSe and dpdTe implied a predominantly intramolecular origin of the observed thermochromism associated with vibronic coupling. © 1999-2024 John Wiley & Sons
  • Thumbnail Image
    Item
    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