3 research outputs found
Capturing Entropic Contributions to Temperature-Mediated Polymorphic Transformations Through Molecular Modeling
Solid materials with
multiple observable phases can restructure
in response to a change in temperature, fundamentally altering the
materials’ properties. This temperature-mediated solid transformation
occurs primarily because of a difference in entropy between the two
crystal forms. In this study, we examine for the first time the ability
of classical point-charge molecular dynamics simulations to compute
entropy and enthalpy differences between solid forms of a range of
organic molecules and ultimately predict temperature-mediated restructuring
events. Twelve polymorphic organic small molecule systems with known
temperature-mediated transformations were modeled with the point-charge
OPLS-AA potential. Relative entropies and free energies between different
solid forms were estimated by computing the stability as a function
of temperature from 0 K up to ambient conditions using molecular dynamics
simulations. These simulations correctly found the experimental high
temperature solid form to have an entropy larger than that of the
low temperature form in all systems examined. The magnitude of the
temperature/entropy contributions to the free energy at ambient conditions
is generally larger than the change in enthalpy difference. We also
find that free energy differences between polymorphs computed with
a less expensive quasi-harmonic approximation are within 0.07 kcal·mol<sup>–1</sup> at all temperatures up to 300 K in the small rigid
molecules examined. However, the molecular dynamics free energies
deviate from the quasi-harmonic approximation in the more flexible
molecules and systems with disordered crystals by as much as 0.37
kcal·mol<sup>–1</sup>. Finally, we demonstrate that at
ambient conditions multiple lattice energy minima can convert into
the same crystal ensemble due to easily kinetically accessible transitions
between similar structures when thermal motions are present
Synthesis and Characterization of Two- and Three-Dimensional Calcium Coordination Polymers Built with Benzene-1,3,5-tricarboxylate and/or Pyrazine-2-carboxylate
Two new calcium coordination polymers, [Ca<sub>3</sub>(btc)<sub>2</sub>(H<sub>2</sub>O)<sub>12</sub>] (<b>1</b>)
and [Ca<sub>2</sub>(btc)Â(pzc)Â(H<sub>2</sub>O)<sub>3</sub>] (<b>2</b>) (btc
= benzene-1,3,5-tricarboxylate, pzc = pyrazine-2-carboxylate), have
been synthesized using the hydro/solvothermal method and have been
characterized using X-ray diffraction, IR, UV–vis, thermogravimetric
analysis, and fluorescence analysis. The structure of compound <b>1</b> is a three-dimensional framework consisting of helical chains
of calcium coordination polymers, while that of compound <b>2</b> is a double layered network in which the inorganic zigzag chains
of calcium coordination polyhedra are linked by organic ligands. Both
compounds show blue fluorescence when excited with UV light. Density
functional theory calculations on electronic absorption spectra of
organic ligands and calcium coordination polymers are discussed