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The dipole moment of quinol is not zero because it can exist in two different energetically favourable conformations. The weighted average is calculated to arrive a dipole moment of 1.6 D.

According to Wikipedia, the dipole moment of terephthalic acid is zero. But few sources have a different opinion.

Can we say that terephthalic acid can also exist in these two conformations, leading to a high dipole moment?

Terephthalic acid conformations

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    $\begingroup$ Yes it has dipole moment. Only in one configuration, it's dipole moment is 0. But overall due to C-C bond rotation, it has non zero dipole moment. $\endgroup$
    – user600016
    Commented Oct 4, 2019 at 2:47
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    $\begingroup$ At room temperature the COOH groups are free to rotate (in sub nanoseconds ) so this will make the molecule symmetric and so zero dipole moment. In a crystal, just possibly, packing may cause a dipole to exist although this seems unlikely. $\endgroup$
    – porphyrin
    Commented Oct 4, 2019 at 7:22
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    $\begingroup$ It isn't even obvious to me that the Wikipedia value for quinol is right. The reference is from 1945 and the method is unclear. $\endgroup$
    – matt_black
    Commented Oct 4, 2019 at 12:37
  • $\begingroup$ The direction of the marked dipole moments seem to be incorrect. $\endgroup$
    – harry
    Commented May 26, 2021 at 1:37
  • $\begingroup$ @harry Different people follow different conventions, i.e., one towards more electronegative atom and the other is towards more electropositive atom. $\endgroup$
    – Apurvium
    Commented May 27, 2021 at 5:59

1 Answer 1

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You are indeed correct, a study by Karthikeyan et. al.[1] suggests that the Dipole moment of terephthalic acid is indeed around ~2.6D (in air), the same as your latter source.

From the paper,

The calculated dipole moment of the title compound is 2.57 Debye in gas and 3.14 in solvent

The same paper later mentions,

The calculated value of dipole moment is found to be 2.733 Debye. The highest value of dipole moment is observed in the component of µZ which is -2.733 D since the dipole vector is oriented negative Z direction. The dipole moment is zero in µX and µY component.

This is expected since terephthalic acid has only two stable rotamers with practically equal energies, both of which are planar.[2] Rotamers of terephthalic acid.

Now even though the paper does not mention this clearly anywhere, what this suggests to me is 2.5704 D is the average Dipole moment of the 3a rotamer (varying from 2.4078 to 2.733 due to COOH twisting vibrations? unclear from the paper), since the Dipole moment of 3b should definitely be zero.

Since both the rotamers exist in nearly equal concentrations, This could suggest that the average dipole moment of should be written as ~2.6/2 D? It's unclear.

References:

  1. Karthikeyan, N.; Joseph Prince, J.; Ramalingam, S.; Periandy, S. Electronic [UV–Visible] and vibrational [FT-IR, FT-Raman] investigation and NMR–mass spectroscopic analysis of terephthalic acid using quantum Gaussian calculations. Spectrochim. Acta, Part A 2015, 139, 229–242. DOI: 10.1016/j.saa.2014.11.112.

  2. Zoran, Marković; Dalibor, Bajduk; Ivan, Gutman ; Geometry and conformation of benzenecarboxylic acids, Journal of the Serbian Chemical Society 2004 69, 877-882. DOI: 10.2298/JSC0411877M

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    $\begingroup$ Welcome to Chem SE. I cannot access the journal. Can you summarize the reason in your answer? Is it because of two different conformers; one out of which has non-zero dipole moment (may be = 2 x 2.6D)? $\endgroup$
    – Apurvium
    Commented Aug 13, 2021 at 6:51
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    $\begingroup$ @Apurvium I added the summary from the paper, but further reading revealed several inconsistencies in the paper that weren't entirely explained. I added what I think might be the case. See I may have erred somewhere. $\endgroup$
    – Anunay
    Commented Aug 14, 2021 at 7:16

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