2-Hydroxy-4-methylbenzenesulfonic Acid Dihydrate: Crystal Structure, Vibrational Spectra, Proton Conductivity, and Thermal Stability

The crystal and molecular structure of 2-hydroxy-4-methylbenzenesulfonic acid dihydrate C6H3(CH3)(OH)SO3-H5O2+ (I) was studied by X-ray diffraction and vibrational spectroscopy. The compound crystallized in the monoclinic crystal system; crystal data: a = 10.853(2) angstrom, b = 7.937(2) angstrom, c = 12.732(3) angstrom, beta = 112.13(3)degrees, V = 1015.9(4) angstrom(3), Z = 4, d(calc) = 1.466 g/cm(3), space group P2(1)/c, R-f = 0.0486, GOOF = 1.161. The S-O distances in the sulfonate group differed substantially (S1-O2 1.439(2) angstrom, S1-O3 1.455(2) angstrom, and S1-O4 1.464(2) angstrom. The symmetry of the H5O2+ cation decreased due to proton displacement toward one of the two water molecules. XRD data on the asymmetry of H5O2+ were confirmed by IR and Raman spectral data. The strong triplet at 2900, 3166, 3377 cm(-1) in the IR spectrum of I corresponds to different types of H-bond and shifted to 2185, 2363, 2553 cm(-1) after deuteration. The proton conductivity of the compound was mea- sured by impedance spectroscopy: 6 x 10(-7) S/cm at 298 K (32 rel %), E-act = 0.4 +/- 0.01 eV. The conductivity increased to 10(-3) S/cm, E-act = 0.1 eV when ambient humidity increased to 60 rel %.