Information about the unknown chemical structure of an organic compound can be obtained by comparing the infrared spectrum with the spectra of a spectral library. The resulting hitlist contains compounds exhibiting the most similar spectra. A method based on the maximum common substructure concept has been developed for an automatic extraction of common structural features from the hitlist structures. A set of substructures is derived that are characteristic for the query structure. Results can be used as structural restrictions in isomer generation.

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A substructure isomorphism matrix n x p contains binary elements describing which of the given p query structures (substructures) are part of the given n target structures (molecular structures). Such a matrix can be used to investigate the diversity of the target structures and allows the characterization and comparison of structural libraries. A quadratic substructure isomorphism matrix n x n is obtained if the same structures are used as molecular structures and as substructures; this matrix contains full information about the topological hierarchy of the n structures. A hierarchical arrangement of chemical structures is useful for the evaluation of results obtained from searches in structure databases.

Comparing the infrared spectrum of a compound whose chemical structure is unknown with the spectra of a library is a routinely used method to obtain information about the unknown structure. The resulting hitlist contains compounds exhibiting the most similar spectra. If the unknown is not contained in the library, a method based on the maximum common substructure concept can be applied to extract common structural features from the hitlist structures. The result is a set of substructures that are characteristic for the query structure. This approach has been applied to infrared spectra from a series of model compounds and has been compared with information obtained from mass spectra by the same procedure. A complementary chemometric method for evaluating spectral hitlists is principal component analysis of spectral and structural data. q

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