Dual-parameter correlation analysis of UV spectral data for 1-methyl-2-formyl-5-Y-substituted pyrroles and their hydrazones. First observation of a dominant polar effect

Abstract

By using 1-methyl-2-formyl-5-Y-substituted pyrroles (1-Ys), 1-methyl-2-formyl-5-Y-substituted pyrrole phenylhydrazones (2-Ys) and 1-methyl-2-formyl-5-Y-substituted pyrrole (4-nitrophenyl)hydrazones (3-Ys) as models for nitrogen-containing heterocyclic aromatic compounds, the K-band ! max values of their UV spectra were measured. Correlation analysis of the # max values by a dual-parameter equation shows that the # max values of these compounds are affected, albeit to different degrees, by both the polar (s x ) and spin-delocalization effects (s Á ) of the substituents. Interestingly, the UV spectra of 3-Ys are mainly affected by the polar effects. The correlation results are in accord with a previous speculation that a higher degree of polarization of the substrate molecule would demand a higher degree of polar assistance from the substituent at the transition states.