Is the Depth of Lennard-Jones Potential Linked to Covalent Bond Strength?

Click For Summary

Homework Help Overview

The discussion revolves around the relationship between the depth of the Lennard-Jones potential and covalent bond strength, as observed through various molecular species and their corresponding parameters. Participants explore the implications of the Lennard-Jones potential in the context of intermolecular forces and bonding interactions.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the significance of the Lennard-Jones potential parameters, particularly ε, in relation to covalent bond strength. Questions arise about the contributions of different types of intermolecular interactions, such as dipole-dipole and induced dipole interactions, to the potential. There is also a query regarding the interpretation of the 1/r6 and 1/r12 terms in the context of bonding.

Discussion Status

The conversation is ongoing, with participants providing insights into the nature of the Lennard-Jones potential and its applicability to covalent bonds. Some participants have offered clarifications regarding the origins of the potential terms and their relevance to molecular interactions, while others are seeking further understanding of the concepts involved.

Contextual Notes

There is a noted uncertainty regarding the application of the Lennard-Jones potential to describe bonding within molecules versus intermolecular interactions. Additionally, participants express a need for clarification on the significance of the ε parameter in relation to bond energy.

terp.asessed
Messages
126
Reaction score
3

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?
 
Physics news on Phys.org
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.
 
  • Like
Likes   Reactions: terp.asessed
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.
 
terp.asessed said:
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?
 
I am looking at the LJ potential to describe a covalent bond strength...so, inside a molecule?
 
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.
 
  • Like
Likes   Reactions: terp.asessed