RotationskristallDiffraction

RotationskristallDiffraction is a method in crystallography in which a single crystal is rotated relative to an incident beam to measure diffraction intensities as a function of orientation. By recording many diffraction spots while the crystal is rotated about a precise axis, the technique collects a three-dimensional set of reflections that map reciprocal space.

Principle: The incident beam (X-ray, neutron, or electron) interacts with the crystal lattice. As the crystal is rotated, Bragg reflections enter and exit the Ewald sphere, satisfying nλ = 2d sinθ at different orientations. The resulting pattern provides amplitudes of structure factors, which are later used to determine atomic positions after appropriate phase retrieval or refinement.

Experimental setup: A single-crystal diffractometer with a goniometer and rotation stage positions the crystal with high angular precision. An area detector or image plate records diffracted intensities as the crystal is rotated through chosen angle ranges. Data processing includes indexing, integration, scaling, and corrections for Lorentz, polarization, and absorption. Proper calibration yields accurate unit-cell parameters and crystal symmetry.

Applications: The method is central to X-ray and neutron crystallography for small molecules and macromolecules. It enables determination of unit-cell dimensions, space group, and full three-dimensional electron density or nuclear density maps. Rotational data can be combined with anomalous dispersion for phase information and refinement.

Limitations: Radiation damage and sample quality limit data quality. Electron diffraction requires different corrections due to dynamical scattering. Complex or disordered crystals pose challenges for indexing and refinement.

History and scope: RotationskristallDiffraction has been a standard technique since early crystallography, with advances in automated goniometry, area detectors, and synchrotron sources improving data completeness and accuracy.