ja0659065_si_001.pdf (195.62 kB)
Solution-Phase Structural Characterization of Supramolecular Assemblies by Molecular Diffraction
journal contribution
posted on 2007-02-14, 00:00 authored by Jodi L. O'Donnell, Xiaobing Zuo, Andrew J. Goshe, Lev Sarkisov, Randall Q. Snurr, Joseph T. Hupp, David M. TiedeStructures of four molecular squares based on rhenium coordination chemistry have been
characterized in the solution phase using pair distribution function (PDF) analyses of wide-angle X-ray
scattering measured to better than 1 Å spatial resolution. In this report we have focused, in particular, on
a comparison of structures for pyrazine- and bipyridine-edged squares measured in solution with structures
determined for these molecules in the solid state using X-ray crystallography and models derived from
geometry optimization and molecular dynamics simulations using a classical force field. The wide-angle
scattering for these assemblies is dominated by pair correlations involving one or more rhenium atoms,
with both edge and diagonal Re−Re interactions appearing prominently in PDF plots. The pyrazine square
is characterized by a relatively rigid structure in solution, with PDF peak positions and linewidths
corresponding closely to those calculated from crystal structure data. For the bipyridine-edged square, the
experimental PDF peaks measured along the molecular sides match the positions and linewidths of the
PDF peaks calculated from static models. In contrast, PDF peaks measured across the diagonal distances
of the molecular square deviate significantly from those calculated from the static crystallographic and
energy minimized models. The experimental data are instead indicative of configurational broadening of
the diagonal distances. In this respect, molecular dynamics simulations point to the importance of butterfly
type motions that modulate the Re−Re diagonal distance. Indeed, the experimental data are reasonably
well fit by assuming a bimodal distribution of butterfly conformers differing by ∼25° in the Re−Re−Re−Re
torsion angle. Additionally, the measurements provide evidence for solvent ordering by the supramolecular
assemblies detected as regions of solvent association and exclusion.