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What is a catemer and how does it differ from a dimer? What type of compounds could form catemers and how does their formation influence its crystal structure?

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In the crystal, carboxylic acids mostly form dimers through pairs of $\ce{OH\bond{~}O=C}$ hydrogen bonds.

In some cases, infinite chains of hydrogen bonds are formed instead: here, each carboxylic acid forms hydrogen bonds to two different neighbours. These structures are known as catemers.

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(Please note that the depiction of the catemer is very much simplified! In reality, the structure could also be a helix, stabilized by additional sets of interactions)

Actually, I have never used the term catemer in a crystallographic publication. It is very imprecise, almost meaningless. Instead, I have mostly used the a notation introduced by Margret C. Etter to describe hydrogen bonding pattern (dimers, stands, rings) in a crystal.

Two relevant articles in this context are:

  1. Encoding and decoding hydrogen-bond patterns of organic compounds, Acc. Chem. Res., 1990, 23, 120-126 (DOI)

  2. Graph-set analysis of hydrogen-bond patterns in organic crystals, Acta Cryst. B, 1990, 46, 256-262 (DOI)

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