title: Elucidating the mechanism responsible for anomalous thermal expansion in a metal-organic framework.
abstract: The previously reported anisotropic thermal expansion of a three-dimensional metal-organic framework (MOF) is examined by means of theoretical calculations. Inspection of the 100, 190, 280 and 370 K single crystal X-ray diffraction (SCD) structures indicated a concerted change in the coordination sphere of the zinc centre leading to elongation of the coordination helix in the crystallographic c direction (the Zn-O(H)-Zn angle expands), while the largely unaltered ligands (the ZnLZn distance remains constant) are pulled closer together in the ab plane. This study develops and evaluates a mechanistic model at the DFT level of theory that reproduces the convergent expansion of the coordination helix of the material. The linear increase in energy calculated for extension of a model consisting of six zinc centres and truncated ligands compares favourably with results obtained from a periodic DFT evaluation of the SCD structures. It was also found that the anisotropic thermal expansion trend could be reproduced qualitatively by Molecular Dynamics (MD) simulations in the NPT ensemble.
authors.affiliation: Department of Chemistry and Polymer Science, Stellenbosch University, South Africa. ljb@sun.ac.za ce@sun.ac.za.
authors.forename: Dewald P
authors.initials: DP
authors.lastname: van Heerden
authors.affiliation: Department of Chemistry and Polymer Science, Stellenbosch University, South Africa. ljb@sun.ac.za ce@sun.ac.za.
authors.forename: Catharine
authors.initials: C
authors.lastname: Esterhuysen
authors.affiliation: Department of Chemistry and Polymer Science, Stellenbosch University, South Africa. ljb@sun.ac.za ce@sun.ac.za.
authors.forename: Leonard J
authors.initials: LJ
authors.lastname: Barbour