PREFACE
Downloaded by 80.82.77.83 on May 14, 2018 | https://pubs.acs.org Publication Date: November 17, 1980 | doi: 10.1021/bk-1980-0141.pr001
T
his collection of papers was part of a unique symposium held during the 178th Meeting of the American Chemical Society. The symposium, Diffraction Methods for Structural Determination of Fibrous Polymers, had a pronounced international character, with scientists from 12 different countries. The speakers represented both the synthetic polymer and biopolymer fields, with contributions in each of the three classes of natural polymers: nucleic acids, proteins, and polysaccharides. Most important, the symposium centered on methods and techniques for studying fibrous polymers, methods that are usually taken for granted despite their inadequacies. In this volume, along with "method" papers, are contributions describing new structures that illustrate the methods and assumptions needed to determine the structure of a new polymer. Also included are reviews of classes of polymers for which investigation and methods development have coincided. The participants generally view fiber diffraction as the most useful method for determining the molecular arrangement of a polymer in the solid state, if the polymer is in the form of crystallites randomly ordered about a single axis. Other methods, such as IR spectroscopy, can provide information for evaluating a proposed structure. However, they are not usually as definitive as determining diffraction intensities, constructing a computer model of the polymer, and fitting the computer model to the diffraction data. Electron diffraction patterns often can supplement fiber diffraction patterns by providing information such as accurate cell dimensions and a confirmation of the space group. The sophistication of fiber diffraction has grown along with the development of digital computers. These techniques started with the calculation of diffraction intensities for a few proposed models for comparison with the diffraction pattern. At present, parameters of the models can be varied to produce the minimal variance for the observed and calculated diffraction intensities and simultaneously the minimal stereochemical or packing energy.
vii French and Gardner; Fiber Diffraction Methods ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Downloaded by 80.82.77.83 on May 14, 2018 | https://pubs.acs.org Publication Date: November 17, 1980 | doi: 10.1021/bk-1980-0141.pr001
Progress continues to depend on applying computers to several outstanding problems. Several chapters deal with automated data collection and reduction. Better computer models and more efficient computer programs are being developed to determine the ranges of stereochemically feasible models to be considered. Another application reverses the usual procedures of structure determination. Instead of essentially correcting the observed data for disorder and amorphous scattering, a pattern that includes effects of these conditions is calculated. In this way, the conditions become parameters of the structure determination. Although not usually considered as "structural" information, the kind of disorder and its magnitude often have physical consequences. One stumbling block is the limitation of our techniques. For example, are Hamilton's tests appropriate for current fiber diffraction studies? If applicable, these tests allow the calculation of the significance of a difference between R factors for two competing models. The tests are derived from analysis of variance, and the usual cautions for those analyses apply. But there are often large differences when different laboratories obtain data for the same substance. R factors between data sets range from 20 to 50% even though structures were refined for each set, giving R values (between observed data from one source and the model fitted to those data) of approximately 20%. Two factors, the standardization and distribution of refinement programs and the continuing effort to develop interactive graphics techniques to obtain and correct diffraction data, should soon bring added confidence to the fiber diffraction field. Also, what is the best means of reporting final results when the positions of all the nonhydrogen atoms can be determined directly? Is there a legitimate role for modeling methods if individual atomic positions can be determined? Surely we know bond lengths and valence angles from model compounds more accurately than we could calculate them from the atomic positions derived in a fiber study. To the end of accurately knowing such intramolecular features, it would be an unusual situation indeed that would justify reporting those parameters derived from fiber data. To understand intermolecular interactions, however, the derived atomic positions, with their standard deviations, might be more useful. Calculation of intermolecular effects based on a modeling technique might introduce cumulative errors. Future work should emphasize resolution of the above questions and continue the current strong emphasis on data collection and reduction.
viii French and Gardner; Fiber Diffraction Methods ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
The editors wish to thank the authors who participated in the sympo sium. In particular, we are grateful to Struther Arnott for his thorough treatment of fiber diffraction given in the first chapter.
Downloaded by 80.82.77.83 on May 14, 2018 | https://pubs.acs.org Publication Date: November 17, 1980 | doi: 10.1021/bk-1980-0141.pr001
Southern Regional Research Center USDA P.O. Box 19687 New Orleans, L A 70179
A L F R E D D. F R E N C H
Central Research & Development Department Experimental Station Ε. I. Du Pont de Nemours & Company Wilmington, D E 19898
K E N N C O R W I N H. GARDNER
May 21, 1980
ix French and Gardner; Fiber Diffraction Methods ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
