Heterolytic Dissociation of a P-H Bond. Gas-Phase Acidity

The standard Gibbs free enthalpy change, A G , for the heterolytic dissociation of a P-H bond acoordlng to 

The equilibrium constants K of a large number of proton transfer reactions, some of them involving PH3, were determined by pulsed ICR spectroscopy, and a scale of the relative acidities of a series of acids in the gas phase at 298 K, 5ArG298, was deduced from the relation 

6ArG298 = -RT InK. This experimental scale of relative acidities was converted to a scale of absolute acidities by including certain compounds as anchor points. Thus, the gas-phase acidity of PH3 was determined to be ArG29s = 363 2 kcal/mol. The entropy change for the deprotonation process was evaluated by procedures using statistical mechanics as ArS = 24.9 2 cal - mol K From these data the deprotonation enthalpy of PH3 at 298 K was calculated to be ArH298=PA(PHi) = 370.4 2 kcal/mol [1, 2]. 

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