Teaching Undergraduates X‑ray Crystallography with Porphyrins

The structures of
5,10,15,20-(tetra-p-tolyl)porphyrin
[H2TTP] and tetratolylphenylporphyrinato zinc(II) [ZnTTP]
were solved and discussed as a means to teach undergraduate students
X-ray crystallography. Single-crystal X-ray diffraction (XRD) provides
positive evidence for the metalation of Zn into the planar ZnTTP porphyrin
molecule as opposed to other geometric configurations and is thus
emphasized as a characterization tool in this laboratory experiment.
In teaching the limitations of XRD to detect low atomic scattering
factors, the structure of H2TTP was solved by manually
adding two hydrogen atoms in a trans configuration.
First-principles total energy calculations, molecular dynamics simulations,
and energy barrier computations of structural changes were used as
tools to elucidate the H2TTP structure in support of hydrogen
atoms assigned trans.