Rotational spectrum of phosphonium iodide vapour
The ground-state rotational spectrum of the associated species (PH3, HI) has been observed in the vapour above solid phosphonium iodide by means of pulsed-nozzle, Fourier-transform microwave spectroscopy. To fit the observed hyperfine frequencies of this symmetric-top isotopomer it is necessary to allow for the variation of the iodine nuclear quadrupole coupling constant with J and K as well as to take into account iodine spin–rotation coupling. Interpretation of the spectroscopic constants of the three isotopomers (PH3, Hl), (PH3, Dl) and (PH2D, Hl) leads to the conclusion that the observed species have nuclei in the order H3P ⋯ Hl and, for the first of the three, that r(P ⋯ l)= 4.3822 Å. For the series HX the order of the electrophilicity is F > Cl > CN > Br > l while the order of the nucleophilicity is HCN > HF > HCl ≈ Hl.