The computational analysis enables a quantitative assessment of the effect of catalyst 1g on not only the ion-pair reorganization, but also the ion-pair collapse stage. Catalyst association leads to a higher barrier to ion-pair collapse relative to the uncatalyzed pathway (7.5 vs. 4.0 kcal/mol, Fig. ), consistent with the expected attenuating effect of H-bonding on the nucleophilicity of chloride. Yet inhibition of ion-pair collapse is more than compensated for by stabilization of phosphonium chloride ion pair in the reorganization phase. This results in overall rate acceleration by the HBD catalyst.