Basicities of Methyl-, Methylphenyl-, and Phenylphosphines in the Gas Phase

Article abstract of DOI:10.1021/ja00386a009

The relative gas-phase basicities of the methyl-, methylphenyl-, and phenylphosphines were measured with a pulsed electron high ion source pressure mass spectrometer.The kinetics leading to the equilibria and the equilibria B₀H⁺ + B = B₀ + BH⁺ were observed and the equilibrium constants K determined.These led to standard free energy changes ΔG^o.For a constant B₀ = Me3P, these were: B = Me2PhP, ΔG^o = -2.5 kcal/mol; B = MePh2P, ΔG^o = -3.2 kcal/mol; B = Ph3P, ΔG^o ca. -3.2 kcal/mol.Also the vertical ionization potentials of the phosphorus lone pairs were measured by photoelectron spectroscopy.These were found to decrease in the order Me3P -> Ph3P as expected basicity changes.The observed increase of basicity with phenyl substitution in the gas phase is opposite to the order in solution where the basicity decreases in the order Me3P -> Ph3P.The latter must be due to adverse solvation of the phenylphosphonium ions.The observation that the gas-phase basicities increase when methyl is replaced by phenyl is very interesting.A decrease is observed for the analogous amines where the basicity of Me2PhN is lower than that of Me3N.Part of this difference of behavior can be attributed to stabilization in the phenylphosphonium ions by ?-conjugation of the aromatic ring with empty orbitals of the phosphorus.Such a stabilization is not available to the nitrogen compounds.