Influence of Homoring Competition Effect of Substituents on the NMR Chemical Shifts in Resonance-Assisted Hydrogen Bond for 2-Hydroxyl Diaryl Schiff Bases

Abstract

In multi-substituent organic compounds, the interaction mechanism among groups is of great significance for understanding and predicting the properties of compounds. The homoring competition effect remains largely unexplored. This work rigorously investigates how this effect influences the chemical shift (δ_H(OH)) of hydrogen atom in resonance-assisted hydrogen bond (RAHB). Forty-four 2-hydroxyl diaryl Schiff bases (2-OH-XArCH=NArY, abbreviated as 2-OH-XBAY) bearing diverse substituents X and Y were selected as model molecules, and their ¹H NMR and ¹³C NMR spectra were measured. The results show the following: (1) Using the OH and CH=N groups of RAHB as references, a clear homoring competition effect of substituent X exists in 2-OH-XBAY compounds. (2) For δ_H(OH) of H in OH, the effects of σ_m(X) and σ_p(X) are not additive. The homoring competition effect of substituent X is pronounced: The influence of X on δ_H(OH) is transmitted primarily via path b (Figure 1), with its conjugation effect dominating over its inductive effect. (3) The effect of X on δ_C(C=N) is dominated mainly by σ_F(X), with σ_R(X) contributing negligibly. The cross-interaction effect Δσ_m² does influence δ_C(C=N), whereas the effect of Δσ_p² on δ_C(C=N) is negligible. These findings offer a powerful new perspective for deeply understanding substituent effects in diaryl Schiff bases.