AB-INITIO STUDY OF GAS-PHASE SULFURIC-ACID HYDRATES CONTAINING 1 TO 3WATER-MOLECULES

Sulphuric acid has a tendency to form hydrates, small clusters contain ing a few water molecules, in the gas phase. Hydrate formation has a s tabilising effect on the vapour as the pressure of sulphuric acid drop s (relative to unhydrated vapor), decreasing the nucleation rate. In c lassical nucleation theories the hydration energies and the hydrate di stribution are predicted assuming that hydrates can be described as li quid droplets having thermodynamic properties of bulk liquid. To obtai n a better understanding of the structures and formation energies of t he smallest clusters, we have performed nb initio density functional c alculations of the mono-, di-, and trihydrates. The hydrogen bonds bet ween the molecules are found to be strong. The more water molecules th e hydrate contains, the clearer ring-like structure is formed. Compari son to classical values for the hydration enthalpies confirms that the properties of bulk liquid do not describe the properties of the small est clusters too well. The energy barrier for proton transfer reaction H2SO4 . H2O --> HSO4-. H3O+ for mono- and dihydrate is high, and prot onisation is unlikely to occur, but in trihydrate the protonisation ha s almost occurred and the barrier is very low. We also studied the sin gly protonised monohydrate, and found that while sulphuric acid forms H bonds with the OH parts, the hydrogen sulphate ion tends to bind wit h the O (S=O) part, and the second proton stays tightly in the ion. (C ) 1998 American Institute of Physics. [S0021-9606(98)03103-1].