Hydrogen bond vs proton transfer between neutral molecules in the gas phase

The possibility of observing a spontaneous proton transfer (PT) between a p roton donor AH and a protob acceptor B has been analyzed using, as suitable model systems, the hydrogen bond complexes between HF, HCl and HBr and a s et of bases which covers a large range in the gas-phase basicity scale. Thi s analysis was based on theoretical estimates obtained by means of high-lev el density functional theory calculations. We propose a model which permits us to predict that, when Delta H-acid(AH) + PA(B) + 102 is greater than or equal to 0 (in kcal mol(-1)), Delta H-acid(AH) being the acidity of AH and PA(B) the proton affinity of B, a spontaneous PT from AH toward B should b e observed. In contrast, if the value of this expression is negative, only a hydrogen-bonded cluster between the corresponding neutrals would be stabl e. We have also reached the conclusion that, to obtain a strong hydrogen bo nded complex, a very polarized A-H bond and a sufficiently strong Lewis bas e,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.