# What is a catemer?

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?

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.

(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)