Tetrameric Fluorophosphazene, (NPF2)4, Planar or Puckered?

The results obtained in a comprehensive experimental study on the redetermination of the structure of N4P4F8 with single-crystal X-ray diffraction, gas electron diffraction (GED), and differential scanning calorimetry (DSC) establish clearly that, in contrast to the previous report, the eight-membered heterocycle is not planar. Above the phase transition temperature of −74 °C, the ring appears pseudoplanar. However, the N4P4 ring is disordered and is puckered above the phase transition when the disorder is modeled correctly. Below the phase transition the ring clearly resembles that of the saddle (K form) of N4P4Cl8. The unit cell of the low-temperature phase is derived from that of the higher temperature phase by doubling the c-axis and removing one-half of the symmetry elements. Full structure optimizations were performed at the HF/6-31G* and B3LYP/6-31G* levels and fully support the experimental diffraction data.