Mesitylenesulfonic acid dihydrate: structure and proton conductivity

Abstract

The molecular and crystal structure of single-crystalline mesitylenesulfonic acid dihydrate (1) was determined by X-ray diffraction and IR spectroscopy. According to X-ray diffraction data, water molecules in the crystal structure form H₅O₂ ⁺ cations stabilized by an intracationic hydrogen bond with a length of 2.45(1) Å. The formation of the asymmetric H₅O₂ ⁺ cation was confirmed by IR spectroscopy. The crystallographic nonequivalence of the water molecules results in a shift of the bridging proton from the midpoint of the strong hydrogen bond in the cation toward one of the water molecules. The proton conductivity of compound 1 was measured by impedance spectroscopy. Dihydrate 1 is completely dehydrated upon prolonged storage in a dry argon glove box and undergoes the transition to the dielectric state. Compound 1 is stable in the humidity range of 32–66 rel.%. The conductivity of dihydrate 1 is (2.4±0.3) · 10⁻⁵ Ohm⁻¹ cm⁻¹ at 298 K, E _a = 0.21±0.01 eV.