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 Oct 4 '19 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 Oct 4 '19 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 Oct 4 '19 at 12:37

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