Does H₄SO₅ exist?

The possible existence of H₄SO₅ in aqueous sulfuric acid is analyzed in detail. For bare H₄SO₅, the computed free energy barrier for the exergonic transformation of H₄SO₅ into the H₂SO₄⋯H₂O complex is only 3.8 kcal mol^-1. The presence of water or sulfuric acid catalyzes the dehydration to such an extent that it becomes almost a barrierless process. In the gas phase, dehydration of H₄SO₅ is an autocatalytic reaction as the water molecule produced by the decomposition of one H₄SO₅ molecule induces further dissociation. Thus, in solution, the surrounding water molecules make the para-sulfuric acid a very vulnerable species to exist. The simulated Raman spectra also corroborate the absence of H₄SO₅ in solution.