The purpose of experimental and theoretical studies in reaction dynamics is finding the connection between the observable dynamical properties and the potential energy surface governing the reaction. Some of the most instrumental among the general rules allowing one to understand and interpret the outcomes of chemical reactions are Polanyi’s rules. They establish a connection between the position of the potential barrier, the preferred form (vibrational or translational) of energy to promote a bimolecular reaction and the dominant form of the energy among the degrees of freedom of the products. The rules have been derived based on model potential surfaces and quasiclassical trajectory calculations and allowed the interpretation of experimental observations. They also turned out to be valid in quantum scattering calculations and in many reactions involving polyatomic reactant(s). There are, however, situations where the dynamical observations virtually contradict the rules. This can happen when along the minimum energy path, the breaking and forming binds are strongly coupled to other degrees of freedom. It also arises in reactions of vibrationally very highly excited molecules and in reactions involving the formation of a weakly bound complex and a submerged potential barrier. Using some case studies we demonstrate that in the latter two kinds of reactions the amplitude of the vibration of the breaking bond is so large that the reaction does not closely follow the minimum energy path, and the reactants are attracted when the vibration is near the outer turning point. One of the consequences is that in these reactions the cross sections diverge when the collision energy decreases toward zero. These are conditions that are out of the range of validity of Polanyi’s rules. As a consequence, comparison of the reactivity parameters for this kind of reaction at equal total energy but different contributions from reactant vibration and translation is not appropriate for the identification of the degrees of freedom favorable for reaction.

On the validity of Polanyi’s rules of reaction dynamics

ACS Spring Meeting 2021

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