1
$\begingroup$

A bonding potential like the Morse potential or a Lennard-Jones potential is characterized by a distance at which the potential is minimal, referred to as e.g. 'equilibrium bond distance'.

Is this distance the same as the bond length? Or is the bond length the average distance, depending on the energy level? Or the root mean square distance?

From what I've found, texts describing the potential don't use the term 'bond length', while texts about bond lengths (see e.g. wikipedia) give lists and experimental methods, but no calculations. I'm asking for the connection between these two approaches here (if possible with references).

$\endgroup$
1
$\begingroup$

Those approximate potential models do have a minimum of the energy when the atoms are of some distance apart. However, at non-zero temperature the distance will vary around this minimum. We know the potential well is not symmetric around the point, so the distance tends to fluctuate to larger distances. The result is that the average bond distance will actually increase at non-zero temperature. This may also be considered as a naive model of thermal expansion.

In addition, the concept of bond length is mostly used by chemists and thus the measured values would be more useful than theoretical predictions by some simple potentials.

$\endgroup$
  • 2
    $\begingroup$ I'd agree, and add that anyone using the word "bond length" should specify explicitly or by context what is meant. It could mean any of the things you suggest, except that I wouldn't think anyone would refer to the equilibrium position of an excited state as a bond length. But I could be wrong about that. Without further clarification, I'd take a guess: room temperature average separation in the ground state. $\endgroup$ – garyp Oct 30 '14 at 15:43
  • $\begingroup$ @garyp This comment is nearly as useful as the answer, sorry I can't upvote it (not enough reputation). $\endgroup$ – B Fuchs Oct 31 '14 at 18:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.