{"title":"Approximate Symmetry in Low-Symmetry Space Groups: P2 and C2","authors":"Carolyn Pratt Brock","doi":"10.1021/acs.cgd.4c00361","DOIUrl":null,"url":null,"abstract":"This project determined the frequency of approximate periodic symmetry in well-determined (<i>R</i> ≤ 0.050) organic crystal structures refined in space groups (SGs) #3 (<i>P</i>2) and #5 (<i>C</i>2, <i>A</i>2, <i>I</i>2). The survey considered structures with <i>Z</i>′ > 1 (<i>Z</i>′ is the number of crystallographically independent molecules) and <i>Z</i>′ = 1 = 2(<sup>1</sup>/<sub>2</sub>) (two molecules, each on a 2-fold axis). The reasons that group #3 occurs rarely (20 structures) have been analyzed. Of the 570 SG #5 structures examined, 13 (2%) should have been refined in a higher symmetry group, leaving 557 reliable structures. The frequency of approximate symmetry in SG #5 is high (73%) but is somewhat lower than that in SG #1 (<i>P</i>1; 86%) and in SG #7 (<i>Pc</i>, <i>Pn</i>, <i>Pa</i>; 83%). As was found for SG #1, many of the enantiopure substances (95 of 471) mimic centrosymmetric packing; in another 17, the achiral parts of the molecule are segregated in layers described by non-Sohncke symmetry. Molecular layers well described by a layer group occur considerably less frequently than in SGs #1 and 7, but in SG #5, there are more structures in which symmetric layers are related by a good 2<i><sub>x</sub></i> axis perpendicular to the layer normal but with <i>x</i> not close to either 0 or 1. A guide to identifying and documenting approximate periodic symmetry has been deposited.","PeriodicalId":34,"journal":{"name":"Crystal Growth & Design","volume":null,"pages":null},"PeriodicalIF":3.2000,"publicationDate":"2024-06-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Crystal Growth & Design","FirstCategoryId":"92","ListUrlMain":"https://doi.org/10.1021/acs.cgd.4c00361","RegionNum":2,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"CHEMISTRY, MULTIDISCIPLINARY","Score":null,"Total":0}
引用次数: 0
Abstract
This project determined the frequency of approximate periodic symmetry in well-determined (R ≤ 0.050) organic crystal structures refined in space groups (SGs) #3 (P2) and #5 (C2, A2, I2). The survey considered structures with Z′ > 1 (Z′ is the number of crystallographically independent molecules) and Z′ = 1 = 2(1/2) (two molecules, each on a 2-fold axis). The reasons that group #3 occurs rarely (20 structures) have been analyzed. Of the 570 SG #5 structures examined, 13 (2%) should have been refined in a higher symmetry group, leaving 557 reliable structures. The frequency of approximate symmetry in SG #5 is high (73%) but is somewhat lower than that in SG #1 (P1; 86%) and in SG #7 (Pc, Pn, Pa; 83%). As was found for SG #1, many of the enantiopure substances (95 of 471) mimic centrosymmetric packing; in another 17, the achiral parts of the molecule are segregated in layers described by non-Sohncke symmetry. Molecular layers well described by a layer group occur considerably less frequently than in SGs #1 and 7, but in SG #5, there are more structures in which symmetric layers are related by a good 2x axis perpendicular to the layer normal but with x not close to either 0 or 1. A guide to identifying and documenting approximate periodic symmetry has been deposited.
期刊介绍:
The aim of Crystal Growth & Design is to stimulate crossfertilization of knowledge among scientists and engineers working in the fields of crystal growth, crystal engineering, and the industrial application of crystalline materials.
Crystal Growth & Design publishes theoretical and experimental studies of the physical, chemical, and biological phenomena and processes related to the design, growth, and application of crystalline materials. Synergistic approaches originating from different disciplines and technologies and integrating the fields of crystal growth, crystal engineering, intermolecular interactions, and industrial application are encouraged.