We have explored the relationships between the reaction force F(xi), the reaction force constant kappa(xi) and the projected force constants of the intramolecular proton transfer HO-N = S -> O = N-SH along the intrinsic reaction coordinate xi. The structural changes and energetics associated with the reaction are analyzed in terms of the three regions defined by F(xi): reactant, transition and product. The significance of the similarity between kappa(xi) and the variation of the force constant associated to the reaction coordinate mode, kappa(xi)(xi), is discussed in detail. (c) 2008 Elsevier B. V. All rights reserved.