Study of substituent effects on conformational equilibrium (E⇆Z) of 2-(4-substituted anilino)-5-nitropyridines by infrared spectra, X-ray crystallography, and CNDO/2 calculations

Abstract

l.r. spectra of the amino stretching vibration bands (ν_NH) of 2-(4-substituted anilino)-5-nitropyridines with substituents NMe₂(1), NH₂, OH, OMe, Me, H (6), Cl (7), Br (8), CO₂Et, CN, and NO₂(11) were measured in CCl₄, CHCl₃, and CH₂Cl₂ solutions. The crystal structures of (1), (6), (7) and (11) were determined; (1) and (6) contain the E-isomer and (7) and (11) contain the Z-isomer. The ν_NH bands at lower and higher wavenumbers, ν_NH^I and ν_NH^h, were assigned respectively to ν_NH of the E- and Z-isomer coexisting at equilibrium (E⇆Z) in solution. For the respective solvents, the free energy difference (ΔG°) estimated from the intensity ratio ν_NH^h:ν_NH^l decreases linearly as the substituent σ constant increases in order from (1) to (11); the predominant form in solution of (1) is E and of (11) is Z. This agrees with the results of an i.r. study on the crystals of (1)–(11), i.e. that (1)–(6) are crystallized in the E-form (non-planar) and (7)–(11) are crystallized in the Z-form (planar). The differences between the total energies of the isomers (ΔE_tot) for (1)–(11), excluding (8), were calculated by the CNDO/2 method. The linear relation of ΔE_totversusσ was considered to be representative of that of ΔG° obtained for each solvent versusσ, taking into account the dielectric effect of the solvent on it. The energy terms governing the change in ΔE_tot were selected to reveal important factors controlling the equilibrium, and the resonance and Coulombic interaction energies were attended. The substituent effect on the ν_NH^l and ν_NH^h frequencies were also investigated.