Substituent effect on transitions in the UV-absorption spectra of 2,4-and 2,6-disubstituted pyridines

Analysis of UV-absorption spectra of several 2,4- and 2,6-disubstituted pyr idines confirmed their resemblance to the spectra of meta-disubstituted ben zenes. Depending on the resonance character of the substituents in the 2- a nd 4- positions, a varying importance of the resonance interactions between these substituents and the ring nitrogen atom was observed. Therefore, the se substituents fairly irregularly influenced the positions of the L-1(a)-b and in the spectra. It was shown that in some cases the substitution of the 2- and 4-monosubstituted pyridines in the 6- and 2-positions, respectively , produced hypsochromic shifts of the L-1(a)-band. Some rules governing the L-1(a)-band shifts, depending on the pattern of substitution and the reson ance character of the introduced substituent were attempted.