Infrared spectroscopic studies of the low temperature interconversion of sulfuric acid hydrates

IR spectra of single phase sulfuric acid hydrates have been measured and the conversion between sulfuric acid monohydrate (H3O+HSO4-, SAM) and sulfuric acid tetrahydrate ([H5O2+](2)SO42-, SAT) as a function of temperature and water partial pressure p(H2O) has been followed using the principal absorption bands of the four main ionic species present, H3O+, H5O2+, HSO4- and SO42-. Temperature-pressure variation studies of the phase transition show that intermediate structures are formed on hydration of SAM that are not formed in the dehydration of SAT. Spectra taken at regular time intervals during the conversion process have been used to monitor these intermediates which can be attributed to changes in the local coordination geometry of the sulfate ion as a function of the amount of available water. The sulfate ion core in SAT is nearly tetrahedral and principally shows a strong asymmetric S-O stretching fundamental at ca. 1070 cm(-1) in its mid-IR spectrum. The SO4 core of the bisulfate ion in SAM has pseudo-C-3v local symmetry, with 3 IR-active modes (2A(1)+E) which are observed to change markedly upon hydration. Slow hydration at 180 K results in the melting of SAM, with subsequent SAT crystallisation from this melt. At reduced temperatures (175 K), instead of melting, a sulfate ion is held in a solid matrix and successively coordinates to a second H3O+ ion in a structure with local C-3v symmetry. This change in coordination allows different vibrational modes to become IR active. The IR absorption bands in each of these configurations can be assigned by comparison with the vibrational modes of metal sulfates for which structures and spectra are known. The ultimate effect of hydration is to deprotonate the bisulfate core forming sulfate and hydrated protons, i.e. the formation of SAT. Isothermal dehydration of SAT in vacuo shows a simpler trend, through the loss of excess water from a SAT film until the overall stoichiometry reaches that of SAM: a smooth, direct conversion into SAM is observed. The rate of this process compared to the rate of hydration suggests that a barrier to SAT decomposition exists.