Study on the Properties of Resonance-Assisted Hydrogen Bonding in ortho-Hydroxy-α-methyldiaryl Schiff Bases

Resonance-assisted hydrogen bonding (RAHB), a special type of intramolecular hydrogen bond (IMHB), plays a crucial role in biological systems and exhibits significant potential in material design, biomolecular chemistryand catalytic chemistry. Due to its importance, RAHB has been extensively studied. Accurate estimation of RAHB energy is essential for understanding molecular interactions and conformational behavior. In this study, we synthesized thirty-three N-(phenylethylene)anilines (2-OH-R1PEAR2) with varying substituents and measured their ¹H NMR chemical shift δH(OH) of the OH and the ¹³C NMR chemical shift δC(CMe=N) of the CMe=N. Using these model compounds, we calculated the hydrogen bond energy EHB, bond lengths dO–H and dN···H, and the aromaticity of the phenyl rings (PDI). We investigated the relationship between the EHB and the bond lengths (dO–H, dN···H) in 2-OH-R1PEAR2, the correlation between NMR chemical shifts δH(OH), δC(CMe=N), EHB and the aromaticity of the hydrogen-bonded ring (PDIQ). A comparative analysis was performed with structurally similar ortho-hydroxy diaryl Schiff bases (2-OH-R1BAR2). In 2-OH-R1PEAR2, the calculated dO–H and dN···H values show a strong correlation. The EHB also correlates well with dO–H, suggesting that dO–H can serve a reliable indicator of hydrogen bond strength. In compound 2-OH-R1PEAR2, EHB has good correlations with δH(OH), δC(CMe=N) and PDIQ. The introduction of α-CH3 on the bridge bond causes the two benzene rings in 2-OH-R1PEAR2 to deviate from the coplanar more severely than in 2-OH-R1BAR2, rendering the aromaticity of ring B negligible in influencing δH(OH) and δC(CMe=N).