The reaction force: Three key points along an intrinsic reaction coordinate

Abstract

The concept of the reaction force is presented and discussed in detail. For typical processes with energy barriers, it has a universal form which defines three key points along an intrinsic reaction coordinate: the force minimum, zero and maximum. We suggest that the resulting four zones be interpreted as involving preparation of reactants in the first, transition to products in the second and third, and relaxation in the fourth. This general picture is supported by the distinctive patterns of the variations in relevant electronic properties. Two important points that are brought out by the reaction force are that (a) the traditional activation energy comprises two separate contributions, and (b) the transition state corresponds to a balance between the driving and the retarding forces.

The mechanism of double proton transfer in dimers of uracil and 2‐thiouracil—The reaction force perspective

“…These critical points are used to define the so-called reaction regions in which different kind of processes might be taking place. 14,15 As shown in Figure 2b, the reactants region is defined from the position of the reactants at n R by the interval n R n n 1 , where the chemical species are expected to present most characteristics of the reactants and begins to be activated to reach the transition state. This region is characterized by a negative retarding force that becomes stronger reaching a minimum at n 1 .…”

“…In the product region (n 2 n n P ) the system relaxes and the driving force begins to diminish becoming zero at n P . 14,15 The main consequence of partitioning the reaction coordinate in reaction regions is that it provides a rational partition of the activation energy 8,12,[15][16][17][18] :…”

Insight into the informational-structure behavior of the Diels-Alder reaction of cyclopentadiene and maleic anhydride

“…According to Zewail [5,6], the passage from reactants to products is characterized not only by a TS [1,2], but also by different stages [7,8] that comprehend changes in geometrical structures associated with complex phenomena. Experiments at very short pulse levels will be required, which are limited by the fundamental laws of quantum mechanics through the Heisenberg principle.…”

“…This was achieved by the use of several of the density descriptors mentioned above. To the best of our knowledge, this is the first time that different descriptors of the evolution of a chemical process have been applied to a chemical reaction from concepts of the reaction force analysis [7,8], conceptual density functional theory (DFT) [18], cheminformatics, and IT in order to relate the molecular physicochemical properties with the informational content of their electronic distributions along the chemical path of a reaction. We have chosen the cycloaddition reaction of cyclopentadiene (CP) and maleic anhydride (MA) because it is a simple archetypal Diels-Alder reaction (Scheme 1).…”

“…Moreover, Tachibana [13] was able to visualize the formation of a chemical bond of selected model reactions by using the kinetic energy density to identify the intrinsic shape of the reactants, the TS and the reaction products along the course of the intrinsic reaction coordinate (IRC). The reaction force of a system's potential energy along the reaction coordinate has been employed to characterize changes in the structural and/or electronic properties in chemical reactions [7,8]. KullbackLeibler information deficiency has been evaluated by Geerlings et al [14] along the IRC for several SN2 reactions.…”