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

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Abstract
Ionization energies of the simple hydrogen-bonded dimers (AHn)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 (AHn)+ 2 are ‘proton-bonded’ complexes between the protonated monomer (AH+ n + 1) and the dehydrogenated radical (AHn–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 (τp) 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.

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