Hydrogen bond vs. proton transfer between neutral molecules in the gas phase
The possibility of observing a spontaneous proton transfer (PT) between a proton donor AH and a proton acceptor B has been analyzed using, as suitable model systems, the hydrogen bond complexes between HF, HCl, and HBr and a set of bases which covers a large range in the gas-phase basicity scale. This analysis was based on theoretical estimates obtained by means of high-level density functional theory calculations. We propose a model which permits us to predict that, when ΔacidH(AH) + PA(B) + 102 is ≥0 (in kcal mol-1), ΔacidH(AH) being the acidity of AH and PA(B) the proton affinity of B, a spontaneous PT from AH toward B should be observed. In contrast, if the value of this expression is negative, only a hydrogen-bonded cluster between the corresponding neutrals would be stable. We have also reached the conclusion that, to obtain a strong hydrogen bonded complex, a very polarized A−H bond and a sufficiently strong Lewis base, B, are necessary, whereas to observe a spontaneous PT the acidity of A−H and the basicity of B are determining factors, and to a much lesser extent the size of the interacting systems.