Resonance and solvent effects on absorption spectra. Part 5. Substituent and solvent effects on infrared spectra of 2- and 4-substituted aniline derivatives

Abstract

I.r. spectra of 2- and 4-substituted N-methyl- and N-acetylaniline derivatives in various solvents have been employed to examine intramolecular hydrogen bonding of amine hydrogen atoms with 2-substituents, or intermolecular hydrogen bonding with hydrogen-bond-acceptor (HBA) solvents. The N–H stretching frequencies, ν_NH′ in solvents having doubly bonded oxygen HBA base centres are well correlated according to the linear solvation energy relationship (LSER), XYZ = XYZ₀+sπ*+bβ, where π* and β are the solvatochromic parameters that measure solvent dipolarity-polarizability and HBA basicity. The order of susceptibilites, b, to solvent basicities of ν_NH for the compounds studied is: CO₂Me < COCH₃∼ CN < NO_2′ in contrast to susceptibilities of the corresponding u.v.-visible spectra, where the order is COCH₃∼ CN < NO₂ < CO₂Me.