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.