A high-pressure structural study of propionic acid and the application of CCD detectors in high-pressure single-crystal x-ray diffraction

We have determined the high-pressure crystal-structure of propionic acid, including the calculated positions of the hydrogen atoms, using a combination of single-crystal x-ray diffraction techniques and ab initio density functional calculations. We find that a previously unobserved triclinic P (1) over bar high-pressure phase and the low-temperature monoclinic P2(1)/c phase are both stable over a narrow pressure range just above the ambient temperature crystallization pressure. Both structures are characterized by the formation of isolated dimers and our calculations indicate that the rearrangement of the molecules between the two structures results in a very small reduction in the enthalpy (0.062 eV/molecule) for the triclinic phase. The high-pressure single-crystal x-ray diffraction work was partly undertaken on a diffractometer equipped with a CCD detector. We outline the techniques required for such studies and the considerable improvement in data quality that can be gained.