Crystal structure of new formamidinium triiodide jointly refined by single-crystal XRD, Raman scattering spectroscopy and DFT assessment of hydrogen-bond network features

A novel triiodide phase of the formamidinium cation, CH5N2+center dot I-3(-), crystallizes in the triclinic space group P (1) over bar at a temperature of 110 K. The structure consists of two independent isolated triiodide ions located on inversion centers. The centrosymmetric character of I-3(-) was additionally confirmed by the observed pronounced peaks of symmetrical oscillations of I-3(-) at 115-116 cm(-1) in Raman scattering spectra. An additional structural feature is that each terminal iodine atom is connected with three neighboring planar formamidinium cations by N-H center dot center dot center dot I hydrogen bonding with the N-H center dot center dot center dot I bond length varying from 2.81 to 3.08 angstrom, forming a deformed two-dimensional framework of hydrogen bonds. A Mulliken population analysis showed that the calculated charges of hydrogen atoms correlate well with hydrogen-bond lengths. The crystal studied was refined as a three-component twin with domain ratios of 0.631 (1):0.211 (1): 0.158 (1).