An Introduction to Potential Functions and Surfaces—Bond Stretches

4 An Introduction to Potential Functions and Surfaces—Bond Stretches 

For a simple molecule, the homolytic cleavage of a bond that defines the BDE is a chemical reaction that can be related to a normal mode within the molecule. Normal modes are the vibrational degrees of freedom that a molecule has—namely, stretches, bends, wags, torsions, etc. Each has its own fundamental frequency. The structures of molecules are not 

The Half-Life for Homolysis of Ethane at Room Temperature The —90 kcal / mol C-C BDE of ethane sets a lower limit to the activation energy for the thermally induced homolysis of the molecule. In Chapter 7 we will introduce the Arrhenius equation, which can be used to calculate rate constants from activation energies If we assume an Arrhenius pre-exponential factor (A) of 10 (a common value for a unimolecular process), the half-life for homolysis of ethane at 25 °C would be approximately 10 years. Our universe is postulated to have been around for at most only 10 ° years. Thus, hydrocarbons are thermally very stable  

Morse potentials for bonds of varying strength. The rungs represent the quantized vibrational states. 

A to B to C represents increasingly weaker bonds, with correspondingly lower frequency ZPE vibrations. As the bond becomes weaker, the Morse potential becomes more shallow and the difference between the energies of the vibrational states decreases. 

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