Difference between revisions of "Homolytic Bond Dissociation Energies"

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Homolytic bond dissociation energies or just bond dissociation energies (BDE) is a measure of a particular bond strength. BDE are also referred to as bond enthalpies.
 
Homolytic bond dissociation energies or just bond dissociation energies (BDE) is a measure of a particular bond strength. BDE are also referred to as bond enthalpies.
  
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==Example==
 
For example, the BDE for the C-C bond in
 
For example, the BDE for the C-C bond in
 
:H<sub>3</sub>C-CH<sub>3</sub> --> H<sub>3</sub>C• + •CH<sub>3</sub>
 
:H<sub>3</sub>C-CH<sub>3</sub> --> H<sub>3</sub>C• + •CH<sub>3</sub>
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can be determined using the calculated enthalpies of formation (ΔH<sub>f</sub>) using the following method:
 
can be determined using the calculated enthalpies of formation (ΔH<sub>f</sub>) using the following method:
  
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===HF/6-31(G)d===
 
Using WebMO/Gaussian to calculate the ΔH<sub>f</sub>(H<sub>3</sub>C-CH<sub>3</sub>), returns the following:  
 
Using WebMO/Gaussian to calculate the ΔH<sub>f</sub>(H<sub>3</sub>C-CH<sub>3</sub>), returns the following:  
 
:[[File:Screen Shot 2021-04-07 at 8.13.05 AM.png|400px]]
 
:[[File:Screen Shot 2021-04-07 at 8.13.05 AM.png|400px]]
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[[File:TB08_004.gif|400px]]
 
[[File:TB08_004.gif|400px]]
  
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===B3LYP/6-311+G(2d,p)===
 
Using a more accurate computational approach, B3LYP/6-311+G(2d,p):
 
Using a more accurate computational approach, B3LYP/6-311+G(2d,p):
 
:- B3LYP Energy -39.8561207613 Hartree
 
:- B3LYP Energy -39.8561207613 Hartree
 
:- B3LYP Energy -79.8583705307 Hartree
 
:- B3LYP Energy -79.8583705307 Hartree
 
::ΔH<sub>rxn</sub> = 0.14613 Hartree = 384 kJ/mol
 
::ΔH<sub>rxn</sub> = 0.14613 Hartree = 384 kJ/mol
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===GGG===
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===FFF===

Latest revision as of 15:45, 7 April 2021

Homolytic bond dissociation energies or just bond dissociation energies (BDE) is a measure of a particular bond strength. BDE are also referred to as bond enthalpies.

Example

For example, the BDE for the C-C bond in

H3C-CH3 --> H3C• + •CH3

can be determined using the calculated enthalpies of formation (ΔHf) using the following method:

HF/6-31(G)d

Using WebMO/Gaussian to calculate the ΔHf(H3C-CH3), returns the following:

Screen Shot 2021-04-07 at 8.13.05 AM.png

The RHF (Restricted HF) energy, ie. ΔHf(H3C-CH3) is reported in units of Hartree, -79.2287548119 Hartree

Using WebMO/Gaussian to calculate the ΔHf(CH3•), returns the following:

Screen Shot 2021-04-07 at 8.13.05 AM.png

The RHF (Restricted HF) energy, ie ΔHf(CH3•) is reported in units of Hartree, -39.5589916118 Hartree

ΔHrxn = ΔHf(Products) - ΔHf(Reactants)
ΔHrxn = 2*ΔHf(CH3•) - Hf(H3C-CH3)
ΔHrxn = 2*-39.5589916118 - (-79.2287548119)
ΔHrxn = 0.110771588 Hartree = 290.830826448 kJ/mol
According to Engel, 2nd, Chapter 15, table 15.2, the actual value is 406 kJ/mol)

As can bee seen in this table (shown in all Gen Chem textbooks), the "average bond enthalpy" for a C-C bond is 348 kJ/mol. TB08 004.gif

B3LYP/6-311+G(2d,p)

Using a more accurate computational approach, B3LYP/6-311+G(2d,p):

- B3LYP Energy -39.8561207613 Hartree
- B3LYP Energy -79.8583705307 Hartree
ΔHrxn = 0.14613 Hartree = 384 kJ/mol


GGG

FFF