Treatment of Hydrogen Atoms in Neutron Diffraction Studies

3 Treatment of Hydrogen Atoms in Neutron Diffraction Studies 

Hydrogen atoms have special properties in neutron diffraction crystal structure 

Deuterium atoms are indistinguishable from carbon atoms and the difference from oxygen is not large. This is seldom a problem, however, since neutron diffraction crystal structural studies are always preceded by X-ray studies, and the configuration of the molecules is sufficiently well known to avoid any ambiguity. 

In protein neutron crystal structure analyses, complete H/D exchange is rarely obtained and the partial deuterium substitution has become a standard method of exploring the relative accessibility of hydrogen-bonding functional groups to solvent exposure. 

In neutron diffraction structure analyses, the correction for thermal motion is much more complex for hydrogen atoms than for nonbydrogen atoms. It has long been recognized that vibrational thermal motion causes an apparent reduction in interatomic distances relative to those for the atoms at rest, and the methods for thermal motion analysis and geometry correction are well developed [190—192]. 

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