Analysis of structural characteristics of chemical compounds in a large computer-based file. Part I. Non-cyclic fragments

The occurrence of elements in general collections of chemical structures is known to have a Poisson distribution, which must be compensated for in the design of screening systems for automatic substructure searching. Quantitative measures for the occurrence of elements and certain simple fragments are presented for a large sample of structures from the Chemical Abstracts Service Registry System.A computer technique is described which allows the distribution of fragments to be studied at differing levels of specificity. When selectively applied, the technique permits resolution of the commoner fragments into less frequent categories, producing a more even distribution. The method may be of value in the development of screening systems for substructure searching.