Influences of polarizability effect of alkyl group and homoring competition effect of substituents on the NMR spectra of salen‐type Schiff base

Salen‐type Schiff bases are a kind of important compounds and are widely used. In order to explore the effect of alkyl groups and substituents attached to aromatic ring on the chemical shifts, 63 title compounds were synthesized. Their 1H NMR and 13C NMR spectra were obtained; and the effects of the alkyl chain length and substituents on the chemical shifts (δH(CHN), δC(CHN), δH(OH), and δC(COH)) were studied. The results show that (1) the alkyl polarizability effect index (PEI) has an important influence on the chemical shifts of the above four atoms, with the increase of PEI, the values of δH(CHN) and δc(CHN) decrease, and the values of δH(OH) and δC(COH) increase. (2) The influence of substituent X attached to aromatic ring on the chemical shift is related to its position by taking OH or CHN as reference. As for the effect of substituent on the chemical shifts, the effect of Hammett constant σ(X)OH and excited‐state substituent parameter σCCexXOH with OH as reference are different from that ofσ(X)CHN and σCCexXCHN with CHN as reference, and there is a “homoring competition effect” of the substituent. (3) The effect of the cross‐interaction between X and OH on the chemical shift is also significantly different due to the different position of X. Quantitative correlation equations against chemical shifts were built for the four atoms, and the stability and prediction ability of the obtained equations were confirmed by leave‐one‐out cross validation. In this paper, the effect of alkyl chain length (number of carbon atoms n = 2–8) in diamine and substituents on NMR chemical shifts (δH(CHN), δC(CHN), δH(OH), and δC(COH)) were studied. The alkyl polarization effect index (PEI) has an important influence on the NMR chemical shifts of the above four atoms, and the “homoring competition effect” of substituent was proposed.