(428c) Thiohypoiodous Acid (HSI) Formation and Its Role As an Intermediate in H2 Production

In 1979, S. W. Benson observed a room temperature liquid formed by the reaction of Hydrogen Sulfide (H₂S) and Iodine (I₂) and postulated that this liquid was Thiohypoiodous acid (HSI), a never before observed molecule.[1] However, Benson had only a little bit of experimental evidence and no theoretical evidence to support this interpretation of this experiment. Aside from some proposed potential atmospheric relevance,[2] this molecule has attracted little attention because of the absence of a definitive characterization, and also because it was not obvious that it would be useful even if it did exist. No one has studied its reaction kinetics.

Very recently, a method for converting H₂S into H₂ + S has been proposed which would have significant environmental and industrial impact if successful. The first step in this process is the reaction of H₂S with I₂. We hypothesize that HSI is a key intermediate in the formation of HI from H₂S and I₂ in aqueous solutions. It is therefore important to quantitatively understand the thermochemistry and reaction kinetics of HSI.

Optimized geometries, energies and frequencies of HSI and the reactants and products in proposed HIS reactions have been calculated at the CCSD(T) level using Dunning’s correlation consistent triple zeta basis with added diffuse functions. The computed equilibrium constants are consistent with experimental observations of Benson et al. Also, the energetics of HSI's secondary reactions with H₂S, I₂, HSI and HI have been studied and will be reported.