Modified spherical scattering factor for the hydrogen atom

2 Modified Spherical Scattering Factor for the Hydrogen Atom 

It is not possible to determine k for a hydrogen atom directly from experimental X-ray data, because its value correlates strongly with the temperature parameter due to the absence of unperturbed inner-shell electrons. The use of neutron temperature parameters provides an alternative. Combined analysis of X-ray and neutron data on glycylglycine and sulfamic acid suggests that for X—H (X = C, N) groups, the H atom is more contracted than for the H2 molecule, with a k value as large as 1.4 for both C—H and N—H bonds (Coppens et al. 1979). 

1 Scattering Factors for the hydrogen atom. IAM, free atom SDS bonded atom as in H2, according to Stewart et al. (1965) k = 1.4, IAM density contracted with a kappa parameter equal to 1.4. 

The Khp = 1.4 value seems large, and possibly corresponds to an overestimate of the contraction, as the analysis depends on the reliability of the neutron temperature factors. Nevertheless, it has been used successfully. There is no doubt that the contraction of the hydrogen-atom density must be taken into account in accurate structure analysis. 

Application of the kappa formalism leads to net charges in good agreement with accepted electronegativity concepts, and molecular dipole moments close to those from other experimental and theoretical methods (Table 3.2). 

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