Intramolecular motion in dibenzobarrelenephosphinyl radical: a single crystal EPR study at variable temperature

The g, 31P and 1H hyperfine tensors of the dibenzobarrelene phosphinyl radical, trapped in an X-irradiated single crystal of dibenzobarrelene phosphine, were estimated at 45 and 300 K. They indicate that among the three locations of the phosphinyl hydrogen expected from DFT calculations, only two are occupied at 40 K and that the third one remains practically vacant, even at 300 K. The temperature dependence of the EPR spectrum was simulated by assuming jumps between two P–H bond orientations (energy barrier ≅0.5 kcal mol−1) which correspond to the conformation of the PH2 moiety in the only rotamer present in the dibenzobarrelene phosphine crystal.