Interaction between aromaticity of rings and NMR chemical shifts of 1H and 13C in resonance-assisted hydrogen bond for 2-hydroxyl diaryl schiff bases

In this paper, forty-four schiff bases 2-OH-XArCH=NArY (2-OH-XBAY) compounds with different substituents were investigated by quantum mechanical calculations. The chemical shifts SH(OH) of H on OH and SC(CH=N) of C on CH=N in resonance-assisted hydrogen bond (RAHB) ring of compounds were determined, and the aromaticity of the rings of the compounds was evaluated using the para-delocalization index (PDI). The interaction between the aromaticity of the all rings in molecule and the chemical shift was studied. The results show: (1) Due to the electron transfer from the benzene ring A (at C of CH=N bond) to the quasi-aromatic ring Q (RAHB ring), the ff-electron delocalization of ring Q increases, resulting in a decrease in the ff-electron delocalization of ring A. (2) The ff-electron delocalization effect of the ring Q has a greater influence on the chemical shift SH(OH) of H on OH, followed by the ring A. The benzene ring B (at N of CH=N bond) has only little effect on SH(OH). (3) Only the aromaticity of ring Q in 2-OH-XBAY can be characterized by SH(OH) and SC(CH=N). (4) Through the structure and orbital analysis of parent molecule 2-OH-BA, it can be considered that the H on OH participates in the ff-electron circulation of the ring Q.