In chemistry, a reaction coordinate is an abstract one-dimensional coordinate system which represents progress along a reaction pathway. Chemistry (in Greek: χημεία) is the science of matter and its interactions with energy and itself (see physics, biology). ... See Cartesian coordinate system or Coordinates (elementary mathematics) for a more elementary introduction to this topic. ...
These coordinates can sometimes represent a real coordinate system, although, for more complex reactions especially, this can be difficult. Reaction coordinates are often plotted against enthalpy (often but erroneously plotted against free energy) to demonstrate in some schematic form the energy landscape or potential energy surface associated to the reaction. Enthalpy (symbolized H, also called heat content) is the sum of the internal energy of matter and the product of its volume multiplied by the pressure. ... In thermodynamics, free energy is a measure of the amount of work that can be extracted from a system. ... A potential energy surface is generally used within the adiabatic or Born-Oppenheimer approximation in quantum mechanics and statistical mechanics to model reactions and interactions in simple chemical and physical systems. ...
For this reason, reaction coordinates usually represent continuous pathways which are locally minimal in terms of energy. Local maxima are associated to transition states They are determining the reaction rate within the RRKM theory. Frequently exaggerated diagrams are used, to accommodate superimposed diagrams or to make a discussion point about the enthalpy along the process. A collision between reactant molecules may or may not result in a successful reaction. ... The reaction rate for a reactant or product in a particular reaction is defined as the amount (in moles or mass units) per unit time per unit volume that is formed or removed. ... This theory of chemical reactivity was developed by Rice and Ramsperger in 1927 and Kassel in 1928 (the RRK theory). ...
For example, in the homolytic dissociation of hydrogen, an apt coordinate system to choose would be the coordinate corresponding to the interatomic distance.
The energies and reactioncoordinates could be plotted to produce a potential map such as that shown in Figure 4.1.
Also, the intermediate points generated along the reaction path are not defined in a strict mathematical sense because one of the degrees of freedom (the reactioncoordinate) is not variable.
The reaction gradient is computed for each value of the reactioncoordinate, and represents the slope of the energy profile along the reactioncoordinate.
The correction for quantized vibrational free energy in modes normal to the PMF reactioncoordinate is calculated from the average frequencies of the projected instantaneous normal mode analysis and is added to the classical potential of mean force.
The generalized free energy of activation as a function of the reactioncoordinate (which is the signed distance along the MEP) is called the free energy of activation profile, and it may be used to calculate reaction rate constants by variational transition state theory (VTST) as described in D. Truhlar and B. Garrett, Acc.
The second factor, kappa_i, is the semiclassical transmission coefficient that accounts mainly for tunneling, that is, the quantum mechanical effect on the reactioncoordinate, which is missing in the calculation of the QC rate constant.
Feeds |
Contact