取代基效应对二芳基硝酮CH=N(O)桥基1H NMR化学位移的影响

合成一系列取代二芳基硝酮XArCH=N（O）ArY化合物，测定其核磁共振氢谱（¹H NMR），指认出桥基CH=N（O）上质子的化学位移δHCH=N（O）]，定量研究取代基效应对δHCH=N（O）]的影响.得到一个4参数定量方程，标准偏差（S）为0.020，较好地表达了δHCH=N（O）]的变化规律.结果表明，该类化合物的δHCH=N（O）]主要受4个因素影响：X基团的场/诱导效应σ_F（X）]；Y基团的共轭效应σ_R（X）]；基团X和Y之间的特殊交叉作用（Δσ²）；以及基团X和O^-之间的特殊交叉作用Δσ²_（X-O^-）].其中，Δσ²_（X-O^-）对δ_{HCH=N（O）]}变化的贡献超过70%.通过δHCH=N（O）]与二芳基希夫碱XArCH=NArY桥基CH=N上质子化学位移的δH（CH=N）比较发现，这两类化合物桥基上质子的化学位移之间没有良好的线性关系.因而，在应用NMR谱图解析有机化合物分子结构时，不能简单地用δH（CH=N）的变化去类比δ_{HCH=N（O）]}的变化.

Substituent Effects on the 1H NMR Chemical Shifts of CH=N(O) Bridge Group in Diaryl Nitrone

A series of substituted diaryl nitrones XArCH=N(O)ArY were synthesized, and their 1H NMR spectra were measured. The 1H chemical shift of the CH=N(O) bridge groups, δHCH=N(O)], in each compound was determined. The effects of substituents X and Y on δHCH=N(O)] were investigated quantitatively. A four parameters correlation equation was constructed to model the changes of δHCH=N(O)] with substituents, yielding a standard error of 0.020. The result indicated that δHCH=N(O)] in substituted diaryl nitrones are mainly affected by four factors: field/inductive effect (S) of substituent X σF(X)], conjugative effect of substituent Y σR(Y)], substituent specific cross-interaction effect between X and Y (Δσ2) and substituent specific crossinteraction effect between X and O- Δσ2(X-O-)]. Among these factors, the contribution of Δσ2(X-O-)to δHCH=N(O)] was more than 70%. A comparison of δHCH=N(O)] in substituted diaryl nitrones to δH(CH=N) of the CH=N bridge group in diaryl Schiff base XArCH=NArY revealed no good linear relationship between δHCH=N(O)] and δ H(CH=N). Therefore it should be noted that one cannot simply use the change of δH(CH=N) to deduce the change of δHCH=N(O)] when carrying out organic molecule structure elucidation with NMR spectra.