Proton transfer in the ionic state of the simple hydrogen-bonded dimers (H2O) +2, (NH3) +2 and (HF) +2. An elementary process of protonation

Abstract

Ionization energies of the simple hydrogen-bonded dimers (AH_n)₂: (A = N, O and F; n= 3, 2 and 1), which are obtained by photoelectron spectroscopy and photoionization mass spectrometry, reflect sensitively the characteristic features of their ionic states. A theoretical analysis has shown that the experimental results can be interpreted consistently in terms of proton-transfer potential-energy surfaces of the dimer cations obtained by ab initio calculations. Several interesting results deduced from the analysis are summarized as follows. (1) The equilibrium dimer cations (AH_n)⁺₂ are ‘proton-bonded’ complexes between the protonated monomer (AH⁺_{n + 1}) and the dehydrogenated radical (AH_n–1). (2) The true adiabatic ionization energy of these dimers is not obtained from experiment, and must be considerably lower than the observed lowest values. (3) The potential-energy surfaces of the ionic states are characterized by their ‘auto-protonation’ process with no activation energy barrier from the vertically ionized point towards the dissociation limit into the protonation channel. (4) The characteristic time constant (τ_p) for this elementary proton transfer process is estimated to be of the order of 10^–14 s, giving theoretical evidence of an extremely fast reaction. (5) There are two significant dissociation channels for the equilibrium dimer cation, protonation and non-protonation. The reaction path connecting the two dissociation limits is closely related to the proton-transfer process in the ion–molecule reaction.