符号关系式在测定含重原子晶体结构中的应用——Ⅰ.一个符号修正方案

对于由序数相近的原子所构成的晶体,Sayre等式给出了衍射结构振幅之间的关系。在测定由碳、氮、氧等“轻”原子构成的中心对称晶体结构中,利用Sayrs等式曾获得不少结果。但是对于含“重原子’的晶体,Sayre等式即不再适用。此时通常都利用所谓“重原子法”来解结构。本文讨论了含重原子情况下,晶体的结构振幅符号、Sayre符号,以及重原子符号之间的关系,从而指出了将重原子法和Sayre法结合的可能性,并提出了一个结构振幅符号的循环修正方案,借此有可能从重原子符号或一套包含若干错误的结构振幅符号出发,不经过电子密度综合而最终获得正确的结构振幅符合。将此方法试用于一个假想的一维晶体结构,效果良好。本文还讨论了有关在实际的晶体结构测定中应用的若干问题。

THE USE OF SIGN RELATIONSHIP IN THE DETERMINATION OF HEAVY ATOM CONTAINING CRYSTAL STRUCTURES——I. A SIGN-REFINEMENT PROCEDURE

For crystals composed entirely of light atoms with atomic numbers close to each other, the Sayre equation gives the sign relation of the structure amplitudes. It has been successfully applied to the determination of crystal structures of organic compounds. Nevertheless, for crystals containing "heavy atoms", the Sayre equation is no longer valid; instead, the so-called "heavy atom method" is generally used. In the present work the interrelation between the respective signs of structure amplitudes, the Sayre equations, and heavy atoms has been considered, and the possibility of combining the heavy atom method and that of Sayre is pointed out. This leads to the suggestion of a sign-refinement procedure, with which the initial signs of heavy atoms can be refined to the correct signs of the structure amplitudes. This procedure has been verified with a hypothetical one-dimensional structure and proved to be efficient. Some problems concerning its application to the actual crystal-structure analysis are also discussed.