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

Abstract

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 ¹ H NMR and ¹³ C NMR spectra were obtained; and the effects of the alkyl chain length and substituents on the chemical shifts (δ_H (CHN), δ_C (CHN), δ_H (OH), and δ_C (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 δ_H (CHN) and δc(CHN) decrease, and the values of δ_H (OH) and δ_C (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 ${\sigma }_{\mathrm{CC}}^{\mathrm{ex}}{\left(X\right)}_{\mathrm{OH}}$ with OH as reference are different from that ofσ(X)_CHN and ${\sigma }_{\mathrm{CC}}^{\mathrm{ex}}{\left(X\right)}_{\mathrm{CH}N}$ 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.

Keywords: NMR chemical shift; alkyl polarizability effect; homoring competition effect; salen-type Schiff base; substituent effect.