H₂S dimer forms hydrogen bonds

Hydrogen bonds are one reason for water’s unique boiling and freezing behavior and other properties. But scientists have debated for decades whether dimers of hydrogen sulfide, water’s smelly triatomic cousin, also sport hydrogen bonds. Now, Elangannan Arunan of the Indian Institute of Science and colleagues report they have found experimental evidence that hydrogen bonds do indeed hold H₂S dimers together (Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201808162). Water’s four hydrogen bonds (via two H atoms and two lone electron pairs) are evident in ice’s tetrahedral structure, but in solid H₂S, each molecule is surrounded instead by a shell of 12 others. That suggested to some that H₂S does not hydrogen bond. Using microwave spectroscopy, Arunan’s group detected rotational energy barriers in the dimer below about 60 °C that they say indicate the presence of hydrogen bonds. Computational chemists have reached the same conclusion. Arunan explains that at higher temperatures, those bonds break and re-form too quickly to be detected via spectroscopy. He adds that the observation that the distance between the H and S atoms is smaller than their combined van der Waals radii and that the S–H–S angle is nearly 180° further confirms hydrogen bonding.