Improved structure determinations when using riding-hydrogen model

A frequent assumption in refinement models used in conventional crystal-structure determination at cryogenic temperatures is that hydrogen-atom displacements change as much with temperature as the atom they are bonded to. Fixed multipliers of 1.2 and 1.5 of the ratio of hydrogen- and parent-atom displacements are used in most refinement programs in the so-called 'riding-hydrogen' description.

Scientists have found [Lübben et al. (2014). Acta Cryst. A70; doi:10.1107/S2053273314010626] that a physically better description of hydrogen-atom displacements in a structural model should take a temperature dependence of this ratio into account, thereby eliminating a model inaccuracy that potentially leads to small systematic errors in the results.

The researchers' findings have the potential to affect a large number of structure determinations, especially results based on diffraction data measured at cryogenic temperatures around and below 100 Kelvin (-173 °Celsius) which employ the riding-hydrogen model for refinement.

Since the actual effect is small, it can best be detected when 'invarioms', non-spherical scattering factors, are used in refinement of multi-temperature data. When conventional scattering factors are used as implemented in most refinement programs in wide distribution, e.g. SHELXL, OLEX2 or CRYSTALS, differences are within the experimental error. Temperature-dependent multipliers (e.g. 2.2/2.5 at 100 K and 3.2/3.5 below 50 K) are not the optimal solution.

Lübben and the team are currently testing the combination of segmented rigid-body (or 'TLS' for translation, vibration and screw) motion - as obtained from a fit to well determined non-hydrogen displacements - with displacements from frequency computations of model compounds for a particular chemical environment from the invariom database. This will allow assignment of the respective environment, takes temperature dependence of hydrogen atoms into account correctly and also allows an estimation of the anisotropy of hydrogen-atom motion of organic crystal structures.