| Abstract
| - High-level quantum chemical calculations [G3(MP2)-RAD//MP2/6-31+G(d,p)] have been employed to investigate the relationship between the binding energy (BE) of a substrate (X) and its protonatedform [H−X]+ with the proton affinity (PA) of the substrate (X) in several series of protonated homodimers([X···H−X]+). We find that for each series of closely related substrates, the binding energy (BE) is correlatedwith the proton affinity (PA) in an approximately quadratic manner. Thus, for a given series, the BE initiallyincreases in magnitude with increasing PA, reaches a point of maximum binding, and then becomes smalleras the PA increases further. This behavior can be attributed to the competing effects of the exothermicpartial protonation of the substrate and the endothermic partial deprotonation of the protonated substrate.As the PA increases, protonation of X contributes to increased binding but the penalty for partialdeprotonation of [H−X]+ also increases. Once the PA becomes sufficiently high, the penalty for the partialdeprotonation of [H−X]+ dominates, leading to maximum binding occurring at intermediate PA.
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