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★声明：本文仅代表个人观点，笔者学识有限，资料整理过程中难免存在疏漏谬误，请不吝指正。 孪晶拆分后是否平均Friedel反常散射点结果对比 最近，有小伙伴在清理数据时，发现一个很久之前的孪晶数据，因其结构已在《晶体学报》E辑上发表，故将其分享给笔者作为素材，如图1所示，在此感谢这位小伙伴提供的素材！ ▲图1 素材分享 该结构如图2所示（由ChemBioDraw [1]绘制），相当于胞嘧啶 [2]（Cytosine, CAS: 71-30-7, CCDC [3]: 1134840 [2a], 1134841 [2b], 1042861 [2c], 2212089 [2d]）与硝酸 [4]（HNO3, nitricacid, CAS: 7697-37-2, CCDC: 754934–754936）形成的硝酸盐（Cytosinium nitrate, CCDC: 2580371, DOI [5]: 10.5517/ccdc.csd.cc2sm2s9），该结构已发表在“Acta Cryst. 2003, E59, o983–o985.DOI: 10.1107/S160053680301287X.”。 ▲图2 案例结构 该数据衍射图如图3所示（由APEX6 [6]呈现），辐射光源为钼靶，曝光时间为2.60秒，共4轮数据，743帧衍射图，总大小为563 MB。 ▲图3 衍射图 该数据按常规步骤还原（参阅推文“布鲁克单晶数据还原操作步骤”），结果如图4所示（由Olex2[7]呈现），R因子（R1和wR2）、拟合优度（GooF）和残余峰（Max Peak）居高不下，其他参数则还不错。 ▲图4 案例数据常规还原结果 调用PLATON [8]运行TwinRotMat，有两个孪晶法则，如图5所示。 ▲图5 PLATON/TwinRotMat运行结果 在精修中添加上述孪晶法则，结果并无多大改善，如图6–7所示。 ▲图6 在精修中添加第一个孪晶法则结果 ▲图7 在精修中添加第二个孪晶法则结果 用PLATON生成的HKLF5格式文件精修，精修指标略有改善，但仍不理想，如图8所示。 ▲图8 用PLATON生成的HKLF5格式文件精修结果 ▲图9 Scale>>Finalize 按默认平均Friedel反常散射点处理结果如图10–11所示。 ▲图10 平均Friedel反常散射点HKLF4结果 ▲图11 平均Friedel反常散射点HKLF5结果 手动取消勾选“AverageFriedel opposites”，即不平均Friedel反常散射点处理结果如图12–13所示。 ▲图12 不平均Friedel反常散射点HKLF4结果 ▲图13 不平均Friedel反常散射点HKLF5结果 上述结果总结于下表 | 项目 | 常规还原 | 平均Friedel反常散射点 | 不平均Friedel反常散射点 | | | | | | 分辨率 | | | | | | | 信噪比 | | | | | | | Rint | | | | | | | 完整度 | | | | | | | Max Peak/e·Å–3 | | | | | | | Min Peak/e·Å–3 | | | | | | | GooF | | | | | | | R1 | | | | | | | wR2 | | | | | |
综合来看，对于本文案例来说，不平均Friedel反常散射点HKLF4结果最佳。 相关视频： 单晶结构解析练习786(数据还原-孪晶拆分)：https://www.bilibili.com/video/BV1Kvgj6SEYC 数据下载： 提取码: fmng 参考文献 [1] (a)Klein, F. M. CS ChemDraw Pro,1 Version 3.1 for Windows. J. Chem. Inf. Comput. Sci. 1995, 35, 166–167. DOI: 10.1021/ci00023a026. (b)Cousins, K. R. ChemDraw 6.0 Ultra CambridgeSoft Corporation, 100 Cambridge ParkDrive, Cambridge, MA 02140. http://www.camsoft.com. Commercial Price: $1395.Academic Price: $699. J. Am. Chem. Soc. 2000, 122, 10257–10258. DOI: 10.1021/ja0047572.(c) Buntrock, R. E. ChemOffice Ultra 7.0. J. Chem. Inf. Comput. Sci. 2002, 42, 1505–1506. DOI: 10.1021/ci025575p. (d) Li, Z.; Wan, H.; Shi, Y.;Ouyang, P. Personal Experience with Four Kinds of Chemical Structure DrawingSoftware: Review on ChemDraw, ChemWindow, ISIS/Draw, and ChemSketch. J. Chem. Inf. Comput. Sci. 2004, 44, 1886–1890. DOI: 10.1021/ci049794h.(e) Mendelsohn, L. D. ChemDraw8 Ultra, Windows and Macintosh Versions. J. Chem. Inf. Comput. Sci. 2004, 44, 2225–2226. 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