Recent progress in quantifying substituent effects

Substituent Effects on the N-H Bond Dissociation Enthalpies, Ionization Energies, Acidities, and Radical Scavenging Behavior of 3,7-Disubstituted Phenoxazines and 3,7-Disubstituted Phenothiazines

The substituent effects on the N-H bond dissociation enthalpies (BDE), ionization energies (IE), acidities (proton affinity, PA), and radical scavenging behavior of 3,7-disubstituted phenoxazines (PhozNHs) and 3,7-disubstituted phenothiazines (PhtzNHs) were determined using density functional theory, with the M05-2X functional in conjunction with the 6-311++G(d,p) basis set. These thermochemical parameters calculated in both gas phase and benzene solution with respect to the changes in several different substituents including halogen, electron-withdrawing, and electron-donating groups at both 3 and 7 positions in both PhozNHs and PhtzNHs systems were analyzed in terms of the inherent relationships between them with some quantitative substituent effect parameters. The kinetic rate constants of hydrogen-atom exchange reactions between PhozNH and PhtzNH derivatives with the HOO• radical were also calculated, and the effects of the substituents on the kinetic behaviors of these reactions were thereby quantitatively evaluated.

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

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).