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.