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Volume 10 - Materials Characterization Part 9

Tham khảo tài liệu 'volume 10 - materials characterization part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Single-Crystal X-Ray Diffraction Richard L. Harlow E.I. DuPont de Nemours and M.A. Paul Ed. National Academy of Sciences Washington 1974 Reviewing the Results A crystal structure analysis includes several assumptions. First atoms are spherical in shape. Core electrons ma overall spherical symmetry but bonding or nonbonding valence electrons do not. Second the nucleus of the ate located at the point of maximum electron density. For atoms with high atomic numbers this assumption is v However for light atoms in which the valence electrons account for most of the scattering the electron cloud is c distorted by the bonding environment. The worst deviations from this assumption occur for covalently bonded hydra atoms the maximum in the electron density is often 0.10 toA .2 m the nucleus toward the atom to which the hydrogen is bonded. Third the thermal motions of atoms can be described by simple spherical or elliptical models motions are more complex particularly if the atoms are covalently bonded such that they ride on each other. Accuracy denotes the measure of how close the structure is to the true structure. The above assumptions imply th positions of heavy atoms with many core electrons and little thermal motion can be determined with high accur Atoms as light as boron or carbon can be determined with reasonable accuracy if a structure contains light and I atoms the positions of the light atoms are in greater doubt because the heavy atoms dominate the scattering. Hy atom positions are almost always systematically in error. There are no indicators of accuracy that derive directly fra structural analysis. Precision denotes the measure of how well the structural model reproduces the diffraction intensiffesaliDbeis perhaps the most obvious indicator of the precision but the standard deviations in the positional and thermal paran as well as the bond distances and angles are also indicators. These standard deviations have nothing to do with ac However an accurate .