Lennard-Jones potential .

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Homework Statement


Just my question from looking at a table of lennard-jones parameters (molecular species and accorded ε), it seems the depth of a Lennard-jones potential is strongly related to a covalent bond? Not hw-related, but course-related anyhow, so any suggestions would be welcome.

Homework Equations


..well, I am just going to include an equation from the same section I found the table in: u(r) = ε(r*/r)12 - 2ε(r*/r)6

The Attempt at a Solution


I understand that Lennard-jones potential is contributed by dipole-dipole, induced dipole and London-dispersion attraction.....and seeing how ε/kB increases dramatically from Hydrogen gas (37.0K) to CO (100K) to Carbon Dioxide (189K), I wonder dipole-dipole is the most significant in the potential? Also, the fact that including very polar atoms (for example Fluorine) plays an important role, for CF4 increases ε/kB to 152K? But, I don't understand why organic molecules, such as Methane, C2H4 and C3H8 have much higher ε/kB values?
 

Answers and Replies

  • #2
DrClaude
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The 1/r6 term originates from dipole-dipole interaction, but when modelling a molecule with a covalent bond (as opposed to, say, the van der Waals interaction in noble gases), then the LJ potential parameters becomes simply a "best fit" to the actual potential energy curve. The depth of the potential is then related to the dissociation energy of the bond.
 
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  • #3
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Wait, I was sure 1/r12 was repulsion and 1/r6 was a matter of attraction and distance between molecules/atoms, but is it simply matter of bonding, as in dipole-dipole, induced dipole and covalent? Please correct me, but I am new to the subject of 1/r12 and 1/r6, so forgive me if I am wrong.
 
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DrClaude
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Wait, I was sure 1/r12 was repulsion and 1/r6 was a matter of attraction and distance between molecules/atoms, but is it simply matter of bonding, as in dipole-dipole, induced dipole and covalent? Please correct me, but I am new to the subject of 1/r12 and 1/r6, so forgive me if I am wrong.
You are correct. Maybe it is me who is misunderstanding what you are asking about. Are you looking at the Lennard-Jones potential to describe bonding inside a molecule, or for intermolecular interactions?
 
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I am looking at the LJ potential to describe a covalent bond strength........so, inside a molecule?
 
  • #6
DrClaude
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Ok, so what I wrote was relevant. The origin of the 1/r6 term in the LJ potential is the long-range dipole-dipole (or better, induced dipole-induced dipole) interaction between two atoms or molecules, also known as the van der Waals interaction. This is completed by a repulsive short-range 1/r12 term that mimics the close-range interaction. One of the advantages of this potential is that it reproduces the long-range potential energy correctly.

Now you can use this potential to represent other situations, such as a covalent bond, but it is only an approximation to the real potential. Another common approximation is the Morse potential. To come back to your question, the parameter ε represents the energy scale of the potential, and so is related to the bond energy. When figuring out LJ parameters, people try to find the values of ε and r* that best reproduce a particular molecular bond, whatever "best" means (can be context specific). So yes, there is a link between ε and the strenght of covalent bonds.
 
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