Small-molecule X-ray crystallography is the gold-standard analytical technique for determining three-dimensional structures at atomic resolution. Once a suitable crystal is obtained - either from the reaction process or through recrystallisation - it is carefully mounted on a pin and placed in the diffractometer. The crystal is exposed to X-rays, and diffraction images are collected on a detector as the crystal is rotated through a series of precise orientations.
The diffraction data is processed to extract the positions and intensities of reflections (spots observed on a diffraction pattern). The positions reveal the unit cell parameters and symmetry of the crystal, while the intensities are related to the electron density within the repeating unit. However, an electron density map cannot be generated directly at this stage because the relative phases of the reflections are not determined. By applying mathematical algorithms, such as direct methods or dual-space methods, initial phases are calculated, allowing an initial model of the atomic structure to be constructed.
This preliminary model is refined iteratively by comparing the calculated diffraction pattern from the model to the observed data, making adjustments to minimise discrepancies. The final refined model provides a highly accurate three-dimensional structure of the molecule, detailing atomic positions, bond lengths, bond angles, and any potential crystal disorder.
This technique is vital in fields such as chemistry, physics, and materials science, offering critical insights into molecular structures and their properties.