Effects of single bond-ion and single bond-diradical form on the stretching vibration of C=N bridging bond in 4,4'-disubstituted benzylidene anilines

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Vol. 163; p. 96
• Main Authors: Cao, Chao-Tun, Bi, Yakun, Cao, Chenzhong
• Format: Journal Article
• Language: English
• Published: England 15.06.2016
• Subjects: Benzylidene aniline; Stretching vibration of CN bond; Single bond-diradical form; Single bond-ion form; Infrared absorption spectrum
• ISSN: 1873-3557
• DOI: 10.1016/j.saa.2016.03.021

Fifty-seven samples of model compounds, 4,4'-disubstituted benzylidene anilines, p-X-ArCH=NAr-p-Y were synthesized. Their infrared absorption spectra were recorded, and the stretching vibration frequencies νC=N of the C=N bridging bond were determined. New stretching vibration mode was proposed by means of the analysis of the factors affecting νC=N, that is there are mainly three modes in the stretching vibration of C=N bond: (I) polar double bond form C=N, (II) single bond-ion form C(+)-N(-) and (III) single bond-diradical form C-N. The contributions of the forms (I) and (II) to the change of νC=N can be quantified by using Hammett substituent constant (including substituent cross-interaction effects between X and Y groups), whereas the contribution of the form (III) can be quantified by employing the excited-state substituent constant. The most contribution of these three forms is the form (III), the next is the form (II), whose contribution difference was discussed with the viewpoint of energy requirements in vibration with the form (III) and form (II).