Trimethylphosphine: anion–molecule reactions and acidity in the gas phase

Abstract

Gas-phase acidity of trimethylphosphine has been investigated at ambient temperature by examining proton-transfer reactions in 40 Pa of helium buffer gas in a Flowing Afterglow instrument. On the basis of the occurrence–non-occurrence of a number of proton-transfer reactions and the observation of rapid H–D exchange between D₂O and the conjugate base of trimethylphosphine, it has been determined that trimethylphosphine is more acidic than water. Quantitative measurements are reported for the reaction of trimethylphosphine with atomic oxygen anion and methoxide. These latter two anions exhibit multiple reaction pathways, one of which is proton transfer. From measurements of the rate coefficients of these two reactions and the relative product yields, it is concluded that the gas-phase acidity of trimethylphosphine is very similar to that of the hydroxyl radical; the recommended value is ΔG°_acid(PMe₃) 1 577 ± 13 kJ mol^–1. The derived acidity measurement is in slight contrast to recent theoretical and experimental estimates. Furthermore, it is found that anions which react only slowly with trimethylphosphine by proton transfer can undergo an alternative reaction which corresponds to addition of the neucleophilic anion to trimethylphosphine followed by loss of methane.