Torsion Angle Constraints Stereochemical Searching

In an earlier paper on the systematic analysis of structural data [1] we wrote There are now (1. 1. 82) more than 30,000 organo-carbon structures in the literature, a number which is likely to double in the next 5-7 years. Unfortunately, this wealth of information has not been exploited very much detailed discussions of individual structures are commonplace, but systematic studies of large numbers of related structures are rarely undertaken. The labour involved in locating and extracting relevant information from the literature has probably been a major barrier to such analyses. However, this barrier has now been virtually eliminated by the establishment of crystallographic databases.  

To any casual reader who scans a textbook on statistical methods, there appears to be a myriad of techniques that can be used in data analysis. Which ones are likely to prove useful in the chemical context The answer is that the choice of technique is dictated entirely by the type of data we wish to analyse and the type of problem we wish to solve. A wide variety of statistical methods has already been applied to the analysis of structural data and has generated results of some significance. In this chapter, we discuss these methods under three broad headings  

We conclude with a brief review of available software systems for statistical analysis, and with some remarks on future possibilities for knowledge acquisition from a large database of chemical structures. 

Our approach to this chapter is best described as experimental rather than mathematical. There are many excellent formal texts, some of which will be cited. Here, we wish to summarize the fundamental purpose of each technique, to explore its possible applications and limitations, and to illustrate (or provide references to) its use in systematic studies of molecular structure. Most of the examples have been drawn from the Cambridge Structural Database [2], using the methods of search and retrieval detailed in the preceding chapter. We preface the statistical content with two more general chemical sections in the first we discuss the selection of geometric parameters that are most appropriate for certain types of analysis, whilst in the second we discuss the possible sources of variation in crystallographic structural data. 

---