A low-temperature IR spectroscopic investigation of the H₂O molecules in the zeolite mesolite

IR powder spectra of the zeolite mesolite (Na₁₆Ca₁₆[Al₄₈Si₇₂O₂₄₀]·64H₂O) were recorded at temperatures between 10 K and 300 K in order to better determine the local behavior of the H₂O molecules in the channel ways. The low-temperature IR spectra show well-resolved OH stretching vibrations of the H₂O molecules between 3100 and 3600 cm ⁻¹ and bending vibrations between 1500 cm⁻¹ and 1700 cm⁻¹. Five OH stretching modes are observed at room temperature and eight at 10 K. Three H₂O bending vibrations were observed at all temperatures. Using published crystal-structural data for mesolite and IR/Raman results on structurally similar natrolite, Na₁₆[Al₁₆Si₂₄O₈₀]·16H₂O, and scolecite, Ca₈[Al₁₆Si₂₄O₈₀]·24H₂O, a first attempt was made to assign the internal H₂O stretching and bending modes. The two OH stretching modes originating from H₂O in the natrolite-like units in mesolite appear to be red-shifted compared to their equivalents in the spectrum of natrolite. This could be due to slightly stronger H-bonding in mesolite. The OH modes related to H₂O in the scolecite-like units of mesolite appear to show less change in wavenumber compared to their equivalents in the IR spectrum of scolecite. The average OH stretching vibration wavenumbers for natrolite, scolecite and mesolite are approximately similar being around 3420 (±10) cm⁻¹. This is not far removed from the value of 3400 cm⁻¹ observed for liquid H₂O, while values for the various polymorphs of ice lie between 3220 and 3250 cm ⁻¹.