Centrifugal micro-fluidic biochip

In physical chemistry, the Bell–Evans–Polanyi principle (or Evans-Polanyi principle) observes that the difference in activation energy between two reactions of the same family is proportional to the difference of their enthalpy of reaction.

This relationship can be expressed as:

$E_{a}=E_{0}+\alpha \Delta H\,,$

where:

• E₀ is the activation energy of a reference reaction of the same class
• ΔH is the enthalpy of reaction
• α characterizes the position of the transition state along the reaction coordinate (such that $0\leq \alpha \leq 1$)

The Evans-Polanyi model is a linear energy relationship that serves as an efficient way to calculate activation energy of many reactions within a distinct family. The activation energy may be used to characterize the kinetic rate parameter of a given reaction through application of the Arrhenius equation.

The Evans-Polanyi model assumes the pre-exponential factor of the Arrhenius equation and the position of the transition state along the reaction coordinate are the same for all reactions belonging to a particular reaction family.

Derivation

The Evans-Polanyi model was developed in 1936 by MG Evans and M Polanyi to explain the apparent linear relationship between activation energy and free energy in acid disassociation, as described in the Brønsted catalysis equation.

Considering the reaction:

$AB+C\rightarrow A+BC$

The system is assumed to have two degrees of freedom: r_AB, the distance between atoms A and B, and r_BC, the distance between atoms B and C. The distance between A and C is assumed to be fixed such that

r = r_AB = constant - r_BC

As the A—B bond stretches, the energy of the system increases up to the activation energy associated with the transition state, whereupon the bond breaks. The energy then decreases as the B—C bond is formed. Evans and Polanyi approximated the two energy functions between reactants, the transition state, and the products by two straight lines (with slopes m₁ and m₂, respectively) that intersect at the transition state.

For the AB molecule, the energy is given as a function of bond distance, r:
Template:NumBlk At the transition state, r = r^‡ and E = E_a. Therefore we can write that
Template:NumBlk Which rearranges to give
Template:NumBlk For the BC molecule, a similar expression of energy as a function of r is given by Template:NumBlk
The overall enthalpy change of the system, ΔH, can thus be expressed as Template:NumBlk Plugging equation Template:EquationNote into equation Template:EquationNote and rearranging gives the following:
Template:NumBlk The constants in equation Template:EquationNote can be condensed into the common form of the Evans-Polanyi equation given above.

References

• Advanced Organic chemistry (part A: Structure and Mechanisms) FRANCIS A. CAREY
• Dill, Ken A., and Sarina Bromberg. Molecular Driving Forces. 2nd ed. New York: Garland Science, 2011.
• Vinu, R. and Broadbelt, L.J. "Unraveling reaction pathways and specifying reaction kinetics for complex systems," Annu. Rev. Chem. Biomol. Eng. 2012, 3, 29-54
• Bell, G. Models for the Specific Adhesion of Cells to Cells. Science 1978, 200, 618–627.