Driving and retarding forces in a chemical reaction.

The reaction force F(ξ) is the negative gradient of the potential energy of a chemical process along the intrinsic reaction coordinate ξ. We extend the rigorous concept of F(ξ) to the "activation strain model" of Bickelhaupt et al., to formulate the "strain" force Fstr(ξ) that retards a reaction and the "interaction" force Fint(ξ) that drives it. These are investigated for a group of Diels-Alder cycloadditions. The results fully support the interpretation of the minimum of F(ξ) as defining the beginning of the transition from deformed reactants to eventual products.