31P nuclear magnetic resonance chemical shifts of elemental phosphorus in the gas phase

Abstract

The N.M.R. signals of phosphorus vapour, relative to liquid white phosphorus, have been measured in the temperature and vapour pressure ranges of 253 to 295°c and 440 to 990 mm of Hg. A linear relationship with a gradient of -3·3 × 10^-3 p.p.m./mm of Hg was found between the chemical shift of the vapour, relative to orthophosphoric acid, and the vapour pressure. Extrapolation to zero vapour pressure gives a value of 553·1 p.p.m. for the chemical shift of the isolated phosphorus molecule. The predicted gas phase chemical shift at zero pressure is found to be 11·0 p.p.m. to higher field than this extrapolated shift. A qualitative discussion about solvent effects indicates that this difference originates from the diamagnetic component of the shielding changes caused by the repulsive overlap forces. The spin-lattice relaxation time of phosphorus vapour in the investigated temperature range is estimated to be ⩾ 5 ms.