chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, a reaction coordinate is an abstract one-dimensional coordinate which represents progress along a reaction pathway. It is usually a geometric parameter that changes during the conversion of one or more molecular entities. In molecular dynamics simulations, a reaction coordinate is called collective variable. These coordinates can sometimes represent a real coordinate system (such as bond length, bond angle...), although, for more complex reactions especially, this can be difficult (and non geometric parameters are used, e.g.,

bond order In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds. The bond order itself is the number of electron pairs (covalent bonds) between two atoms. For example, in diat ...

). Reaction coordinate is distinct from

extent of reaction In physical chemistry and chemical engineering, extent of reaction is a quantity that measures the extent to which the reaction has proceeded. Often, it refers specifically to the value of the extent of reaction when equilibrium has been reached. It ...

, a different parameter of reaction progress, which is a measure of the ''composition'' of the reaction system. (Free) energy is often plotted against reaction coordinate(s) to demonstrate in some schematic form the potential energy profile (an intersection of a potential energy surface) associated with the reaction. In the formalism of transition-state theory the reaction coordinate is that coordinate in set of curvilinear coordinates obtained from the conventional ones for the reactants which, for each reaction step, leads smoothly from the configuration of the reactants through that of the transition state to the configuration of the products. The reaction coordinate is typically chosen to follow the path along the gradient (path of shallowest ascent/deepest descent) of potential energy from reactants to products. For example, in the homolytic dissociation of molecular hydrogen, an apt coordinate system to choose would be the coordinate corresponding to the bond length.

Notes and references

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