Title: IUCr Booklets no 21: Crystal Packing
Author:
Angelo Gavezzotti and Howard Flack revised edition 2005
Publisher: Electronic edition free to download and copy for research and educational purposes from:
The IUCr's useful set of monographs has now reached maturity, being 21 in number! This one is a good member of the series, and those who have heard the authors lecture will be able to hear their voices coming through the explanations. Thus, the coverage is a little idiosyncratic in parts, but this does not detract from the fact that the book provides a good introduction to the subject, and if readers are left with questions at the end, so much the better! No one could give a comprehensive coverage of this subject in 21 pages, and there is a substantial set of references and suggestions for further reading at the end, although a few more elementary texts might well have been mentioned.
The introduction, followed by a section on thermodynamics and kinetics is basically a clear and simple introduction to what a molecule is, and what differentiates a solid from a liquid. I would suggest, however, that SO 42- is not a good example of a species stable in the gas phase! Also, the sentence "... melting occurs because the entropy gain in your system by spatial randomisation of the molecules has overcome the enthalpy loss due to the breaking of crystal packing forces" should read "enthalpy gain ", and in the equation that follows, this is correctly stated.
The treatment of ionic, polar and dispersion forces is good, but it should be noted that high polarisability is also important in raising the melting point (e.g. naphthalene, C10 H8, and nonane C9H 20 ). Crystal symmetry is then treated, with a strong emphasis on the fact that small molecules do not like symmetry operations, other than the inversion centre, not involving translation. The space group frequency table is striking, although it probably does oversimplify the situation - all space groups now have at least one entry in the Cambridge database. A comparison is given with the ICSD to show how different the symmetries on non-molecular structures are from those of molecular.
Probably the weakest feature of the monograph are the diagrams, which are well conceived, but done by hand, and so are not immediately obvious. In distinguishing a screw axis from a glide plane, groups of small men catching rugby balls (?) are used, and it takes a bit of effort to tell which way they are facing, which is crucial to the difference being shown! Hopefully this can be improved in a second edition.
A valuable table shows how the symmetry of certain space groups leads to their symmetry elements tending to be involved in the formation of clusters, rows, layers and networks of molecules. The point is made that the "crystal structure descriptors" of density, packing coefficient and melting point should always be given with crystal structures.. This would be very good, but there should be a proviso that the melting point should be stated to be unknown if that is the case, since spurious values would probably be invented by those who considered them to be unimportant - as was indeed done with "measured" densities when these were required!
There is a very good and outspoken section on chirality - as one would
expect from these authors! In order to be complete, even the possibility of
an achiral crystal formed from chiral molecules is considered:
" in fact
[the failure of this to happen] is due to the individuality of the molecules
rather than to any underlying theorem of symmetry groups".
Indeed - if one allows the molecules not to retain their individuality,
anything may happen!
The pamphlet concludes with a concise introduction to polymorphism and some observations on experiments, including a plea that more notice should be taken by chemists of how their compounds do crystallize, since a huge amount of untapped information is lost daily by failyure to note and publish this in some way.
This is a stimulating booklet, and is available free. It deserves a wide readership and even its few flaws contribute to its readability! Why not start a reading group and discuss it?
Title:
Structure Determination by X-Ray Crystallography,
Author: Mark Ladd and Rex Palmer
Publisher: Klewer/Plenum 2003 4th edition
Price: £41 (paperback)
ISBN 0-30-647 4549 819 + xliii pages + CD
The fourth edition of this classic text has been extensively revised and expanded: it is over 200 pages longer than the 3rd edition with three completely new chapters on "X-ray Structure Determination with Powders", "Proteins and Macromolecular X-ray Analysis", and "Computer-Aided Crystallography".
It must emphasised from the outset that the 4th edition is not just the 3rd edition with new chapters: the original eight chapters have been extensively revised and sensibly re-organised with much new material being added. It is obvious that the authors have put a huge amount of work into creating this new edition. For example, chapter 3 now starts more logically (in my opinion) with the generation and properties of X-rays including those generated from a synchrotron source (this subject being relegated to an appendix in the earlier edition) before a discussion of how X-rays interact with crystals.
Starting at the beginning, the opening two chapters of the book begin with the timeless topic of crystal morphology and symmetry, a subject that is essential for a good understanding of crystallography. Chapter 3 then moves on to the subject of X-rays and how they interact with crystals. This leads on logically to the theme of the reciprocal lattice and how real-space symmetry is deduced from observations of the reciprocal lattice intensities. It would have been nice if the authors could have included lots of data sets on the CDrom for the new student to practice symmetry determination, but more on that subject later.
Continuing with chapter 4, I liked the fact that it still includes a section on the optical examination of crystals. This is a very practical subject required by all students who have to pick out a suitable crystal for their X-ray structure analysis. This chapter then continues with single-crystal X-ray instrumentation. Given the huge changes that have occurred in instrumentation during the last decade, mainly resulting from the introduction of the now ubiquitous CCD detector, this chapter on experimental methods has been greatly revised to include modern instrumentation. However, it still includes long sections on traditional film methods and serial detector diffractometers: some aspects of the former may be relevant to modern single-crystal methods but much is of historical interest only as most high-throughput X-ray laboratories will now be equipped with CCD systems making a detailed discussion of, say, a CAD4 diffractometer somewhat irrelevant for the next student generation of X-ray crystallographers.
Chapters 5, 6, and 7 cover the classical subject areas related to X-ray structure solution, namely Fourier and Patterson maps plus direct methods. Although most of the underlying theory is now encoded in routinely-used black-box programs, e.g. those found in the SHELX software suite, these three chapters provide a useful " behind the scenes" view for the student who wants to know more about how structures are solved and refined by X-ray single crystal methods. These three chapters are then followed by chapter 8 that provides some practical examples. Although this chapter includes some pitfalls, personally, I feel it could be expanded further to include many more of the various problems that a new student to the field may encounter in their practical work, e.g. a discussion of how to cope with data from twinned crystals being an obvious one.
The new chapter 9 on structure determination from powders is a welcome addition to the book. In a single chapter, the authors can only provide the student with a useful overview as the subject could be expanded into a textbook by itself. While the subject is generally well-covered, a new student to the field may get the wrong impression as the chapter starts with a discussion of the now historical Guinier camera and completely misses out ultra high-resolution powder diffractometers at synchrotron sources, e.g. ID31 at the ESRF. However, the examples are very up-to-date and they cover all the various methods currently being used for structure determination by powder diffraction. The authors even include some of the latest work on polycrystalline protein samples, a highly-challenging area for powder diffraction analysis! It would be nice if some statistics had been given for structures solved by powder versus single-crystal: this would provide a new student to the field with some idea of the difficulties involved in this aspect of crystallography.
The second new chapter on macromolecular crystallography is aimed more at the student on the interface between chemistry and structural biology. As with the previous chapter, it can only provide an overview of a subject for which whole textbooks already exist. However, for students starting in small-molecule or materials crystallography and wishing to change field, this chapter provides a useful introduction to the subject.
The inclusion of the final chapter on computer-aided crystallography is certainly brave given how quickly this subjects dates. The chapter provides a good overview of some of the typical software packages available for both single-crystal and powder diffraction that should be useful to the novice student. This chapter is accompanied by the CDrom containing a small number of DOS-based programs and a few data sets to illustrate particular teaching points made in the chapter. Personally, I feel the authors missed a real opportun ity here: while software dates rapidly (especially with PC-based operating systems changing every few years as hardware changes), good-quality crystallographic data should last for an eternity. Thus a CDrom packed to the brim with example data sets (and corresponding solutions) collected by the authors during a lifetime' s experience in the field would have been a real bonus for the new PhD student trying to get to grips with unfamiliar software.
Overall the book contains quite a large amount of mathematics: this may be slightly off-putting for the current generation of synthetic chemist who may want to use crystallography as yet another analytical technique in the same way as they use, say, NMR. Also, given that colour printing has reduced dramatically in cost during the last decade or so, it is disappointing that academic publishers do not encourage it, as some sections of the book could benefit from colour illustrations. For example, colour photographs of the optical examination of crystals showing good and bad crystals for X-ray analysis (plus a few corresponding colour frames from a CCD detector too) would benefit new students. Other positive points to note in this textbook are the excellent index and the problems with worked tutorial solutions at the end of the book.
Finally, it is very pleasing to see that the book has appeared from the outset as a paperback edition aimed at students (in marked contrast to many IUCr textbooks) rather than the usual hardback intended mainly for libraries, though the price may still deter purchase by the students to whom the book is clearly aimed.
Jeremy Karl Cockcroft
Webeditor's notes:
Another book by Ladd was his 3rd edition of
'Introduction to Physical Chemistry'
reviewed in
Crystallography News June 1998 page 26, when the paperback edition cost
£22.95
.