Effects of single bond-ion and single bond-diradical form on the stretching vibration of CN bridging bond in 4,4′-disubstituted benzylidene anilines

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Vol. 163; pp. 96 - 101
• Main Authors: Cao, Chao-Tun, Bi, Yakun, Cao, Chenzhong
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.06.2016
• Subjects: Benzylidene aniline; Infrared absorption spectrum; Single bond-diradical form; Single bond-ion form; Stretching vibration of C[dbnd]N bond; Benzylidene aniline; Stretching vibration of CN bond; Single bond-diradical form; Single bond-ion form; Infrared absorption spectrum
• ISSN: 1386-1425
• 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 νCN of the CN bridging bond were determined. New stretching vibration mode was proposed by means of the analysis of the factors affecting νCN, that is there are mainly three modes in the stretching vibration of CN bond: (I) polar double bond form CN, (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 νCN 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). For the stretching vibration frequency νCN of the CN bridging bond in 4,4′-disubstituted benzylidene anilines, p-X-ArCH=NAr-p-Y, there are mainly three modes: polar double bond form CN, single bond-ion form C+–N− and single bond-diradical form C–N. The contribution of the form C–N is larger than that of the form C+–N− to the change of the νCN. [Display omitted] •New stretching vibration mode, single bond-diradical form C•-N• is proposed for the CN bond in 4,4’-disubstituted benzylidene anilines.•The form C•-N• make the most contribution to the CN stretching vibration frequency νCN.•The νCN can be quantified by using Hammett substituent constant and the excited-state substituent constant.