School of Resource Environment and Safety Engineering, School of Chemistry and Chemical Engineering, Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education, Hunan University of Science and Technology, Xiangtan, China.
Magn Reson Chem. 2021 Jul;59(7):701-712. doi: 10.1002/mrc.5131. Epub 2021 Feb 8.
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 H NMR and C NMR spectra were obtained; and the effects of the alkyl chain length and substituents on the chemical shifts (δ (CHN), δ (CHN), δ (OH), and δ (COH)) 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 δ (CHN) and δc(CHN) decrease, and the values of δ (OH) and δ (COH) increase. (2) The influence of substituent X attached to aromatic ring on the chemical shift is related to its position by taking OH or CHN as reference. As for the effect of substituent on the chemical shifts, the effect of Hammett constant σ(X) and excited-state substituent parameter with OH as reference are different from that ofσ(X) and with CHN 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.
席夫碱是一类重要的化合物，应用广泛。为了探讨烷基和芳环取代基对化学位移的影响，合成了 63 个标题化合物。得到了它们的 1H NMR 和 13C NMR 谱，并研究了烷基链长和取代基对化学位移（δ（CH=N）、δ（CH=N）、δ（OH）和 δ（C=O））的影响。结果表明：（1）烷基极化率效应指数（PEI）对上述四个原子的化学位移有重要影响，随着 PEI 的增加，δ（CH=N）和 δc（CH=N）的值减小，而 δ（OH）和 δ（C=O）的值增大。（2）芳环上取代基 X 的取代基对化学位移的影响与其位置有关，以 OH 或 CH=N 为参考。对于取代基对化学位移的影响，以 OH 为参考时，Hammett 常数σ（X）和激发态取代基参数 的影响与以 CH=N 为参考时不同，存在取代基的“同系竞争效应”。（3）由于 X 的位置不同，X 与 OH 之间的交叉相互作用对化学位移的影响也有显著差异。建立了四个原子的化学位移定量相关方程，并通过留一法交叉验证验证了所得到的方程的稳定性和预测能力。
