In this work, 13 compounds of 4,4′‐disubstituted stilbenes and 5 compounds of 3‐methyl‐4′‐substituted stilbenes were prepared and their UV spectra were measured. A new substituent effect constant, namely excited‐state substituent constant, was proposed, which was calculated directly from the UV absorption energy data of substituted benzenes. The investigation result shows that the proposed constant is different from the existing polar substituent constants and radical substituent constants in nature. The availability of the new constant was confirmed by the good correlations with the UV absorption energy of four kinds of compounds, 1,4‐disubstituted benzenes, 4,4′‐disubstituted stilbenes, substituted ethenes, and m‐Y‐substituted aromatic compounds. It is expected that the excited‐state substituent constant can be applied in QSPR study on organic compounds at the excited state. Copyright © 2008 John Wiley & Sons, Ltd.
The substituent effect on 13 C NMR of the C --N in benzylidene anilines XPhCH --NPhY was investigated, in which the substituents X and Y are in p-position or in m-position of the two aromatic rings. The substituent effects including the inductive effects of X and Y, the conjugative effects of X and Y, and the substituent specific cross-interaction effect were put into one model to quantify the 13 C NMR chemical shift d C (C --N) of the C --N in XPhCH --NPhY. A penta-parameter correlation equation with correlation coefficient 0.9975 and standard error 0.17 ppm was obtained for 80 samples of compounds. The result shows that the substituents X and Y have an opposite effect on the d C (C --N). The electron-withdrawing effects of X decrease the d C (C --N); while the electron-donating effects of X increase the d C (C --N). In contrast, the electron-withdrawing effects of Y increase the d C (C --N); while the electron-donating effects of Y decrease the d C (C --N). A new substituent specific cross-interaction effect parameter Ds 2 was proposed, which indicates that the most substituent specific cross-interaction effect exists in the pair of max electron-withdrawing group (EWG) and max electron-donating group (EDG) or the pair of max EDG and max EWG. Further to verify the obtained correlation equation, 15 samples of model compounds were prepared and their d C (C --N) was measured in this work. The predicted d C (C --N) values with the obtained equation are in good agreement with the measured ones for these prepared compounds, which confirmed the reliability of the obtained equation.By fixing the substituent X (or Y) at one end, Neuvonen [1,7] recently investigated the substituent cross-interaction effects on the electronic character of the C --N bridging group in substituted benzylidene anilines XPhCH --NPhY with the 13 C NMR spectra of C --N carbon. He obtained the valuable results:(1) for the X substituents: electron-withdrawing group (EWG) cause shielding, while electron-donating group (EDG) behave oppositely; (2) in contrast, for the Y substituents: EWG cause deshielding, while EDG cause shielding of the C --N carbon. Also, (wileyonlinelibrary.com)
In this paper, 61 samples of 3,4'-disubstituted stilbenes and 18 samples of 3,3'-disubstituted stilbenes were synthesized, and their UV data were measured in anhydrous ethanol. Based on the UV absorption energy (wavenumber) of 3,4'-disubstituted stilbenes, the excited-state substituent constants s ex CC m ð Þ of metasubstituent were determined by means of curve-fitting. The availability of s ex CC m ð Þ was confirmed by the good correlation with the UV absorption energy of 3,4'-disubstituted stilbenes and 4,4'-disubstituted stilbenes. Further, using the obtained constants s ex CC m ð Þ and the correlation equation, we calculated the UV wavenumbers of 3,3'-disubstituted stilbenes, and the calculated wavenumbers are in good agreement with the experimental values. These results verified that the excited-state substituent constants s ex CC m ð Þ of meta-substituent are reliable parameter to scale the effect of meta-substituent on the UV absorption energy.
In this paper, 72 samples of disubstituted benzylideneanilines were all synthesized, and their UV data were measured in anhydrous ethanol. In the study on the UV energy of the titled compounds with single substituent changed, for the effect of the aniline substituent Y on the UV wavenumbers, its UV data can be correlated with a dual‐parameter equation; for the effect of benzylidene substituent X on the UV data, its UV energy can be correlated with a single‐parameter equation (Y is an electron‐withdrawing group and H) or a dual‐parameter equation (Y is an electron‐donating group). In the study on the UV energy of model compounds with double substituents changed, a correlation equation between the UV absorption wavenumbers and substituent constants σCCex and σp, was obtained. For 72 samples of 4,4′‐disubstituted benzylideneanilines, the correlation coefficient was 0.9876, and the standard deviation s was only 358.46 cm–1. The equation can be used to predict well the UV energy of BA derivatives. It was found that Δσ2 is a better parameter than σXY to scale the substituent cross‐interaction effect on the UV wavenumbers of benzylideneanilines molecules. The results implied that the law of substituent effect on the UV energy of titled compounds is different from that of substituted stilbenes, and it is helpful to understand the effect of substituent effects on the chemical and physical properties of conjugated compounds with an imine bridging group (C = N) or a nonplanar parent. Copyright © 2011 John Wiley & Sons, Ltd.
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