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The effect of substituents on I3C and 'H NMR chemical shifts in 2-and hubstituted furans, thiophenes, selenophenes and tellurophenes was studied by means of principal components analysis (PCA) and partial least squares (PLS) analysis. The analysis of the data by PCA showed that for both series the substituents are grouped into four different classes depending on the heteroatom present in the ring, and that the substituent-heteroatom interaction has the same nature for all the rings. PLS analysis of the data sets allowed the prediction of the chemical shift values of a different ring having the same substituents. KEY WORDS Principal components analysis Partial least squares analysis Pentaatomic heteroaromatics13C chemical shifts 'H NMR chemical shifts INTRODUCTIONDetailed studies of the 13C NMR chemical shifts of a series of 2-3-substituted furans, thiophenes, selenophenes and tellurophenes have been reported.'-' An important goal of these studies has been the understanding of the mechanism of the transmission of the substituent effects. Unfortunately, the problem was approached by considering one position at a time. Many different correlations have been proposed in this way, taking into consideration the 'H and 13C NMR chemical shifts,'-' but the conclusions were not generalizable to all heterocyclic nuclei and, in any case, it was impossible to obtain a clear and complete view of the substituent effect. For instance, a good correlation was found between H-3 and H-5 in 2-substituted derivatives or H-2 and H-4 in 3-substituted compounds for furans, thiophenes and selenophenes; in contrast, in 2-substituted selenophenes H-3 was correlated to C-3 and in the 3-substituted compounds H-2 was correlated to C-2, but the same relationships were not found in furans. Only a few 'H and I3C furan shifts caused by substituents have been linearly correlated with those of thiophenes and selenophenes.Many reactivity parameters have been tested in order to obtain a correlation. The best results have been obtained when the F and R Swain-Lupton reactivity parameters were used: but in all the series the bromo, iodo and methylthio derivatives were excluded, and good correlations were obtained only for a limited number of positions. The ips0 13C chemical shifts for the heterocyclic systems could be correlated only with the Q values: which is a semiempirical paramagnetic parameter. This parameter was introduced in order to * Author to whom correspondence should be addressed.correlate 'H chemical shifts in halosubstituted benzenes. ' The first aim of this investigation was to apply pattern recognition techniques to study the mechanism of the transmission of substituent effects in these ring systems, since with these methods the information obtained is based on all the available data and the models can explain the properties common to all the molecules.All previous studies were carried out with multiple regression analysis (MRA). Unfortunately, MRA includes a number of weak points: the ratio of objects to variables has to be as small as...

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