CHEMOMETRICS AND ITS ROOTS IN PHYSICAL ORGANIC-CHEMISTRY

Linear free energy relationships (LFERs) and extra-thermodynamic relat ionships (ETRs), i.e., similarity and analogy models of physical organ ic chemistry, are mathematically and statistically equivalent to the m odels much used in chemometrics and data analysis, i.e., PCA, PLS, and SIMCA. Examples of early LFERs and ETRs include the Bronsted, Hammett , Taft, and Hansch relationships. Much of the early development of che mometrics derives from this equivalence. Thus, the interpretation and derivation of LFERs and ETRs as the first terms of serial expansions o f perturbation theory applied to moderate structural change lead first to the SIMCA method for classification and discriminant analysis (pat tern recognition), then to the approach of principal properties for th e characterization of structural fragments, compounds, and materials, and finally also strongly influenced the development of PLS and its us e in structure-effect relationships such as quantitative structure-act ivity relationships (QSARs).The interpretation of chemical data by a c ombination of physical organic chemistry models and chemometric princi ples often leads to interesting conclusions as illustrated by some exa mples.