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

IR spectra of single phase sulfuric acid hydrates have been measured and th e conversion between sulfuric acid monohydrate (H3O+HSO4-, SAM) and sulfuri c acid tetrahydrate ([H5O2+](2)SO42-, SAT) as a function of temperature and water partial pressure p(H2O) has been followed using the principal absorp tion bands of the four main ionic species present, H3O+, H5O2+, HSO4- and S O42-. Temperature-pressure variation studies of the phase transition show t hat intermediate structures are formed on hydration of SAM that are not for med in the dehydration of SAT. Spectra taken at regular time intervals duri ng the conversion process have been used to monitor these intermediates whi ch can be attributed to changes in the local coordination geometry of the s ulfate 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 hydra tion. Slow hydration at 180 K results in the melting of SAM, with subsequen t SAT crystallisation from this melt. At reduced temperatures (175 K), inst ead of melting, a sulfate ion is held in a solid matrix and successively co ordinates to a second H3O+ ion in a structure with local C-3v symmetry. Thi s change in coordination allows different vibrational modes to become IR ac tive. The IR absorption bands in each of these configurations can be assign ed by comparison with the vibrational modes of metal sulfates for which str uctures and spectra are known. The ultimate effect of hydration is to depro tonate the bisulfate core forming sulfate and hydrated protons, i.e. the fo rmation of SAT. Isothermal dehydration of SAT in vacuo shows a simpler tren d, through the loss of excess water from a SAT film until the overall stoic hiometry reaches that of SAM: a smooth, direct conversion into SAM is obser ved. The rate of this process compared to the rate of hydration suggests th at a barrier to SAT decomposition exists.