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 1 H NMR and 13 C 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 Δ σ m 2 does influence δ C(C=N) , whereas the effect of Δ σ p 2 on δ C(C=N) is negligible. These findings offer a powerful new perspective for deeply understanding substituent effects in diaryl Schiff bases.