Valence Structure from X-Ray Diffraction Data: An L-Shell Projection Method

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Abstract
Generalized x‐ray scattering factors, in a simplified form, have been applied to several organic molecular crystals. Atomic charge parameters for L‐shell scattering factors have been determined from x‐ray structure factors by the method of least squares. L‐shell scattering factors have been computed for products of SCF AO's and of standard molecular STO's (Slater‐type orbitals). Standard STO's are more satisfactory for point‐charge analysis than are SCF AO's. Bias from several sets of thermal parameters is rather small and does not alter general conclusions on valence structure. The experimental x‐ray charges are compared with Mulliken gross atomic populations from INDO and STO‐3G calculations. Agreement with theory is good for the molecular crystals of s‐triazine, cyanuric acid, and uracil. Within the point‐charge approximation, the experimental x‐ray dipole moment for uracil is 4.0 ± 1.3 D. The direction is at an angle 71° ± 12° from the N(1)–C(4) interatomic vector towards atom N(3).

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