The effect of intramolecular hydrogen bond on the ultraviolet absorption of bi-aryl Schiff bases

Two kinds of Schiff bases, 85 samples of N-(benzylidene)-anilines (ZBAY) and 83 samples of N-(phenyl-ethylene)-anilines (ZAPEY), were used as model compounds, in which the ZBAY contains 13 compounds with 2-OH and the ZAPEY contains 35 compounds with 2-OH (synthesized by this work). The quantitative correlation analysis ultraviolet absorption spectra of ZBAY and ZAPEY were performed, and the effect of intramolecular hydrogen bond on their wave number v(max) (cm(-1)) of the maximum absorption wavelength lambda(max) (nm) was investigated. The results show that (a) the factors affecting the v(max) of ZBAY and ZAPEY are roughly the same, but their intensities are different. (b) The v(max) move caused by intramolecular hydrogen bond is all red shift for both ZBAY and ZAPEY, but the red shift value (2,381) of the ZBAY is more than twice than that (850) of ZAPEY. (c) The effect of intramolecular hydrogen bond on v(max) is only dominated by the parent structure unit of Schiff base, rather than the substituents in the molecule. For those compounds with the same parent structure unit, their red shift values of v(max) are at fixed value, but their red shift values of lambda(max) are unequal. Generally, the red shift value caused by the intramolecular hydrogen bond is larger for the compound with a larger lambda(max) value. The above observed phenomena are discussed from the view of molecular coplanarity.