Very Short Single-Acceptor Hydrogen Bonding in Liquid Chlorosulfonic Acid Enables Long Chain Formation

Chlorosulfonic acid is a superacid widely used as a sulfonating and chlorosulfonating agent. Here, we have used total neutron and X-ray scattering to understand its intra- and intermolecular structure, and interactions in the liquid phase. The behavior is dominated by very short hydrogen bonds between the hydroxyl groups and the sulfonyl oxygens (H···Ooxo 1.43 Å and 1.66 Å), comparable in length to intramolecular hydrogen bonds, with the resulting coordination number of 0.96 of sulfonyl oxygen around hydrogen below 2 Å. The hydrogen bonding leads to formation of branched head-to-tail chains and the intramolecular structure distorts to give three near-equal S-O bond lengths, consistent with the superacid's low viscosity and high acidity.