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The current question is inspired by this existing question: At what frequency does a non-polar molecule acquire a dipole able to participate in London Dispersion forces?. In the comments and answers, it says that there is always a dipole (but the time-average of these dipoles is zero), so it does not make sense to ask how long the dipoles last, or how frequently they appear.

I am wondering how long it takes the neighboring molecule to adjust its electron density to the temporary dipole. Also, I wonder if this is the correct picture: that one molecule has the temporary dipole and the other responds with an induced dipole. Or is it more accurate to say that the neighboring atoms find some common groove of swaying their charge distribution.

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  • $\begingroup$ Try also Physics SE (if cross posting is fine). $\endgroup$
    – Alchimista
    Jan 31, 2020 at 8:14
  • $\begingroup$ What you describe in the last paragraph is what i would call a resonance condition and is probably what dictates the correlated behavior of the electrons. Speculating further, there is a transition lifetime of the electrons associated with the transition dipole moment that describes the resonance state. $\endgroup$
    – Buck Thorn ♦
    Jan 31, 2020 at 10:27
  • $\begingroup$ @BuckThorn But this is not a resonance phenomenon, is it? Dispersive forces do not chease to work at low temperatures, when everything is in its ground state. $\endgroup$
    – Karl
    Jan 31, 2020 at 17:05
  • $\begingroup$ @Karl I speculate but would not be surprised if it can be described using ideas very similar to resonance, but perhaps I use the word loosely (perhaps I should use the word correlation). Invoking resonance (as correlation) makes sense because electron clouds in the two molecules become correlated in dispersion. As for the low temperature behavior of the dispersion interaction, I don't know the answer. Is there a residual zero K entropy term associated with the dispersion interaction? Does its persistence imply that it is not a "resonance" phenomenon? $\endgroup$
    – Buck Thorn ♦
    Jan 31, 2020 at 18:31
  • $\begingroup$ @Karl the typical thermodynamics-based derivation of an expression for London dispersion starts from a Boltzmann equilibrium assumption, so I can see why there might be a T dependence and resonance is not considered. However that doesn't seem satisfactory when considering a question involving time-dependent behavior. $\endgroup$
    – Buck Thorn ♦
    Jan 31, 2020 at 19:26

1 Answer 1

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The upper limit are typically optical light frequencies. That is where the very last plateau is reached in dielectric spectroscopy. The electron shell becomes purely elastic.

I wrote a bit more about DES in my answer here: Difference between relaxation and resonance leading to an absorption spectral feature?

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  • $\begingroup$ Karl, I started reading and watching videos about dielectric spectroscopy. Right now I am pondering the limbo dance analogy in this video from Bend research: youtube.com/watch?v=qkpA0FwfBeI (15 min. in). $\endgroup$
    – Karsten ♦
    Feb 1, 2020 at 2:03
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    $\begingroup$ @KarstenTheis That limbo dance analogy is ludicrous at best. $\endgroup$
    – Karl
    Feb 2, 2020 at 9:36

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