Abnormal effect of hydroxyl on the longest wavelength maximum in ultraviolet absorption spectra for bis‐aryl Schiff bases

Two sets of bis‐aryl Schiff bases that contain 4(or 4′)‐OH and 2(or 2′)‐OH were synthesized. The first set consists of 4‐HOArCH=NArY and XArCH=NArOH‐4′, and the second set consists of 2‐HOArCH=NArY and XArCH=NArOH‐2′. Their ultraviolet absorption spectra were measured and investigated. A very interesting phenomenon was observed by analyzing their wave number ν_max (cm^?1) of longest wavelength maximum λ_max (nm) of ultraviolet. Compared with the change regularity of the ν_max of XArCH=NArY (where the X and Y excluded OH), the 4′‐position hydroxyl (4′‐OH) and 2′‐position hydroxyl (2′‐OH) have abnormal performance. The details are the following: the 4′‐OH contributes an additional red shift to the ν_max of XArCH=NArOH‐4′ (λ_max increase), whereas the 2′‐OH contributes an additional blue shift to the ν_max of XArCH=NArOH‐2′ (λ_max decrease). In addition, there are ortho steric effects of all 2‐OH and 2′‐OH on the ν_max for 2‐HOArCH=NArY and XArCH=NArOH‐2′, and the ortho steric effect contributes a red shift to their ν_max. These experimental facts can provide an important theoretical reference for us using aryl Schiff base compounds as optical materials and performing the molecular design.     

Substituent effects on the 13C NMR chemical shifts of the imine carbon in N‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines

Long‐range electronic substituent effects were targeted using the substituent dependence of δ_C(C═N), and specific cross‐interactions were explored extendedly. A wide set of N‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines, p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y (X = NMe₂, OMe, Me, H, Cl, F, CN, or NO₂; Y = NMe₂, OMe, Me, H, Cl, or CN) were prepared for this study, and their ¹³C NMR chemical shifts δ_C(C═N) of C═N bonds were measured. The results show that both the inductive and resonance effects of the substituents Y on the δ_C(C═N) of p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y are less than those of the substituents Y in p‐X–C₆H₄CH═NC₆H₄–p‐Y. Moreover, the sensitivity of the electronic character of the C═N function to electron donation/electron withdrawal by the substituent X or Y attenuates as the length of the conjugated chain is elongated. It was confirmed that the substituent cross‐interaction is an important factor influencing δ_C(C═N), not only when both X and Y are varied but also when either X or Y is fixed. The long‐range transmission of the specific cross‐interaction effects on δ_C(C═N) decreases with increasing conjugated distance between X and Y. The results of this study suggest that there is a long‐range transmission of the substituent effects in p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of substituent on the UV–Vis spectra: an extension from disubstituted to multi‐substituted benzylideneanilines

For studying the substituent effects on the ν_max of substituted benzylideneanilines (XBAYs) systematically, 12 samples of 3,3′‐disubstituted XBAYs and 52 samples of multi‐substituted XBAYs were synthesized, and the substituent effects on their ν_max were investigated in this paper. A modified regression equation quantifying the ν_max of 4,4′/4,3′/3,4′/3,3′‐disubstituted and multi‐substituted XBAYs (shown as Eq. 3 ) was obtained. The results showed that the substituent effects on the ν_max of 3,3′‐substituted and multi‐substituted XBAYs became more complicated. In Eq. 3 , the contributions of the meta‐parameters to the ν_max of XBAYs were different from those of the corresponding para‐parameters. For the substituent cross‐interaction effects, there is no difference whatever the substituents are at meta‐position or para‐position. Compared with Eq. 1 , Eq. 3 obtained in this paper has a wider application and more accuracy in quantifying the ν_max of substituted XBAYs. Copyright © 2016 John Wiley & Sons, Ltd.     

Determining the excited‐state substituent constants of furyl and thienyl groups

Six series of styrene derivatives XCH═CHArY (total of 65) containing the styrene parent molecular skeleton were synthesized (here, Y is OMe, Me, H, F, Cl, CF₃, CN, and NO₂, and X is 2‐furyl, 3‐furyl, 2′‐methyl‐2‐furyl, 2‐thienyl, 3‐thienyl, and 2′‐methyl‐2‐theniyl). Their ultraviolet absorption spectra were measured in anhydrous ethanol, and their wavelength of absorption maximum λ_max was recorded. For the wavenumber ν_max (cm^?1, ν_max = 1/λ_max) of the obtained λ_max, a quantitative correlation analysis was performed, and 6 excited‐state substituent constants of groups X were obtained by means of curve‐fitting method. Taking the ν_max values of total 90 compounds of styrene derivatives as a data set (including 25 compounds from reference and 65 compounds of this work), a quantitative correlation analysis was performed, and the reliability of the obtained was verified. In addition, 12 samples of disubstituted Schiff bases (XCH═NArY) involving the above groups X were synthesized, and their ν_max values were recorded. Using these 12 ν_max together with the 14 ν_max values of Schiff bases taken from reference (total of 26 compounds), it was further verified that the values are reliable by means of quantitative correlation method.     

Raman spectroscopy of 3‐(pent‐1‐enyl) methyl ether and selected deuterium‐labelled analogues

The Raman spectra of 3‐(pent‐1‐enyl) methyl ether (3‐methoxypent‐1‐ene) and four deuterium‐labelled analogues are reported and discussed. Correlations between specific structural features and the associated Raman bands are developed, with a view to enhancing the analytical application of Raman spectroscopy in investigating materials containing an alkenyl group. Particular attention is given to developing means of distinguishing the methyl group attached to the carbon skeleton from that of the methoxy group, to maximize the analytical utility of the signals associated with ν(sp² CH), ν(sp² CH₂) and ν(CC) stretching vibrations, and to interpreting in more detail certain δ(sp² CH) and δ(sp² CH₂) vibrations of the atoms of the double bond. These results establish a definitive spectroscopic protocol for differentiating a methoxy group from a methyl substituent attached directly to a carbon atom in unsaturated ethers. Copyright © 2007 John Wiley & Sons, Ltd.     

Investigation of the substituent specific cross‐interaction effects on 13C NMR of the CN bridging group in substituted benzylidene anilines

The substituent effect on ¹³C 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 ¹³C NMR chemical shift δ_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 δ_C(C?N). The electron‐withdrawing effects of X decrease the δ_C(C?N); while the electron‐donating effects of X increase the δ_C(C?N). In contrast, the electron‐withdrawing effects of Y increase the δ_C(C?N); while the electron‐donating effects of Y decrease the δ_C(C?N). A new substituent specific cross‐interaction effect parameter Δσ² 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 δ_C(C?N) was measured in this work. The predicted δ_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. Copyright © 2010 John Wiley & Sons, Ltd.     

An attempt of molecular design and synthesis of 3,4′/4,3′‐disubstituted benzylideneanilines with specified UV–Vis absorption maximum wavelength

Thirty‐one samples of 3,4′/4,3′‐disubstituted benzylideneanilines (XBAY) with specified UV–Vis absorption maximum wavelength (λ_max) were designed and synthesized by applying the equation (Eqn 1 ) which was abstracted from the UV–Vis absorption maximum wavelength energy (ν_max = 1/λ_max) of 4,4′‐disubstituted benzylideneanilines. Then, the UV–Vis data (λ_max) of the designed compounds were measured in anhydrous ethanol. The predicted UV–Vis data of designed compounds are in agreement with the experimental ones, in which the mean absolute error is 2.9 nm. The results show that Eqn 1 is applicative for the prediction of UV–Vis absorption λ_max values of both 4,4′‐disubstituted benzylideneanilines and 3,4′/4,3′‐disubstituted benzylideneanilines. For a same pair of groups (X and Y), one can at least get four disubstituted benzylideneaniline compounds which have different λ_max values. It perhaps provides a convenient method to design an optical material for benzylideneaniline compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

The intramolecular charge transfer in a donor–π–acceptor dianion probed by resonance Raman spectroscopy and quantum chemical calculations

The dideprotonation of 4‐(4‐nitrophenylazo)resorcinol generates an anionic species with substantial electronic π delocalization. As compared to the parent neutral species, the anionic first excited electronic transition, characterized as an intramolecular charge transfer (ICT) from the CO⁻ groups to the NO₂ moiety, shows a drastic red shift of ca. 200 nm in the λ_max in the UV‐vis spectrum, leading to one of the lowest ICT energies observed (λ_max = 630 nm in dimethyl sulfoxide (DMSO)) in this class of push‐pull molecular systems. Concomitantly, a threefold increase in the molar absorptivity (ε_max) in comparison to the neutral species is observed. The resonance Raman enhancement profiles reveal that in the neutral species the chromophore involves several modes, as ν(C N), ν(NN), ν(CC) and ν_s(NO₂), whereas in the dianion, there is a selective enhancement of the NO₂ vibrational modes. The quantum chemical calculations of the electronic transitions and vibrational wavenumbers led to a consistent analysis of the enhancement patterns observed in the resonance Raman spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

The nature of the electronic factor governing diastereofacial selectivity in remotely substituted (X) 2‐adamantyl cations: 5‐X versus 4‐X substitution

A limited series of 4^eq‐substituted (X) 2‐methyleneadamantanes ( 6 , Y?CH₂, X?F, Cl, Br, I, and SnMe₃) has been synthesized and diastereoselectivities for their hydrochlorination (HCl/CH₂Cl₂) have been determined. Diastereoselectivities for the fluorination (DAST/CH₂Cl₂) of secondary alcohol mixtures, obtained from the hydride reduction of the precursor ketones ( 6 ,Y?O) to the alkenes, have also been measured. A comparison of this selectivity data for nucleophilic trapping of 4^eq‐substituted (X) 2‐adamantyl cations ( 4 , R?H and Me) with the corresponding information for 5‐substituted (X) 2‐adamantyl cations ( 1 , R?H and Me) has revealed important distinctions between the two series. In particular, whereas extended hyperconjugative effects appear to be the predominant electronic effect governing facial selectivity in the 5,2‐series, electrostatic influences prevail in the 4,2‐disposition. Copyright © 2007 John Wiley & Sons, Ltd.