Comparison of the high-pressure and low-temperature structures of sulfuric acid

We have determined the high-pressure crystal-structure of sulfuric acid, including the positions of the hydrogen atoms, using a combination of single-crystal X-ray diffraction techniques and ab initio density functional calculations. Just above the onset of crystallization, at 0.7 GPa, we find that a previously unobserved monoclinic structure, with P2(1)/c symmetry, is formed which is characterised by SO2 (OH)(2) tetrahedra interconnected by hydrogen bonds. In contrast to the low-temperature C2/c phase, the tetrahedra in the high-pressure crystal structure are no longer arranged in R-4(4)(16) hydrogen-bonded layers but, instead, they form chains where the hydrogen bonding adopts a R-3(3)(12) arrangement. A series of ab initio calculations indicates that this rearrangement of the molecules results in a relatively small reduction in the enthalpy (13.603 kJ mol(-1)) for the P2(1)/c structure at 0.7 GPa.