Synthesis, structure and proton conductivity of 2,4,5-trimethylbenzenesulfonic acid dihydrate

Abstract

2,4,5-Trimethylbenzenesulfonic acid dihydrate was isolated in the monocrystalline state. Crystal and molecular structures of the compound were determined by X-ray diffraction analysis and vibrational spectroscopy. The compound crystal system is monoclinic: a =7.7445(3) Å, b = 7.5360(3) Å, c = 9.9477(4) Å, α = γ = 90°, β = 92.123(4)°, V = 580.17(4) ų, P2₁, Z = 2, R₁ = 0.0323, d_calc. = 1.353 g cm⁻³, T = 150 K. A 2D hydrogen bond network formed of a dioxonium ion and oxygen atom of a sulfogroup was identified in the crystal structure. IR absorption spectra, Raman spectra and ATR IR spectra of 2,4,5-trimethylbenzenesulfonic acid dihydrate and deuterodihydrate have been obtained in the region 50–4000 cm⁻¹. It was found that the proton and deuterium in the H₅O₂⁺ and D₅O₂⁺ cations shifted from the midpoint of the strong hydrogen bond toward one of two water molecules according to the scheme O⋯H⁺⋯O → O⋯H⁺–O and O⋯D⁺⋯O → O⋯D⁺–O. Spectroscopic data concerning dioxonium ion formation show satisfactory agreement with X-ray diffraction analysis. The effect of environment humidity on the state of 2,4,5-trimethylbenzenesulfonic acid dihydrate aggregation was investigated. It was shown that the number of water molecules increases up to 2.5 per sulfogroup in the 7–84 rel.% air humidity interval through additional water molecules adsorption. A further humidity increase led to compound deliquescence. The proton conductivity as measured by impedance spectroscopy is 10⁻⁹–10⁻⁵ S cm⁻¹ in the 7–58 rel.% humidity interval at 298 K with a [H₂O]/[SO₃⁻] = 2.2 ratio at maximum conductivity.