Kinetics and thermodynamics of protonation reactions: H 3O + (H 2O) h + B = BH + (H 2O) b + ( h − b + 1) H 2O, where B is a nitrogen, oxygen or carbon base

Abstract

The extent of proton transfer from H 3O + (H 2O) h to compounds B due to proton transfer reactions (PT) H 3O + (H 2O) h + B = BH − (H 2O) b + ( h − b + 1)H 2O was studied with a pulsed-electron high-pressure mass spectrometer (PHPMS). The H 3O + (H 2O) h were equilibrium hydrate populations in a third gas (air or methane) containing water pressure in the torr range and B in the sub-millitorr range. When a limited reaction time was available ( t < 0.5 ms), the compounds B fell into three groups regarding the observed extent of PT. Compounds with gas-phase basicity (GB) higher than 200 kcal mol −1 experienced maximum proton transfer. For these B, which are mostly nitrogen bases, the extent of proton transfer was proportional to the rate constant k PTA (kinetic control) and the k PT were found close to the ion/molecule collision-rates. Compounds B with GB < 200 kcal mol −1 were found to reach PT equilibria (thermodynamic control). The oxygen bases in this group showed an extent of PT that decreased fairly regularly as GB(B) decreased. The extent of PT observed for a group of B under thermodynamic control was much less than for oxygen bases of the same GB. This group of compounds, which included the carbon bases pyrrole, furan, and thiophene, corresponds to BH + which form hydrates of very low stability. Hydration equilibria were measured for BH + of pyrrole, furan, and thiophene. The rate constants for PT involving the carbon base pyrrole were also measured and found to decrease as the PT exothermicity decreased.