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

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).