An Introduction to the Nomenclature and Usage of the Reaction Path Concept

Our thinking about chemical reactions has benefitted greatly from the model of energy profiles as functions of so-called “reaction coordinates” (RC) . In textbooks, such profiles are often used to illustrate the main features of a chemical reaction, including its mechanism, in terms of transition state theory (TST) [1]. In his book “Potential Energy Hypersurfaces” (p. 86), P.G.Mezey [2] rightly describes the term “reaction coordinate” (RC) as a misleading one to characterize the reaction path (RP) . Confusion results from the term “coordinate” which suggests that only one dominant internal coordinate describes the reaction path. The use of one selected “distinguished” coordinate (or “leading” coordinate) in higher-dimensional systems (by minimizing the other independent internal coordinates at fixed values of the distinguished coordinate) generally does not produce cross-sections through a PES (potential energy hypersurface) that uniquely correspond to minimum energy paths (MEP), see below. The chemist must — nolens volens — understand what a curve in the high-dimensional coordinate space taken as RP actually means (cf. RC in Figure 1) ! The comparison of the RP to a normal mode resulting from Wilson’s FG matrix [3] method may be helpful.

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