Rotational Barrier and Origin of Substituent Effect on Bond Dissociation Enthalpy of Para-substituted Anilines, Benzaldehydes, and Toluenes

Understanding the rotational barriers (RBs) and bond dissociation enthalpies (BDEt) of substituted aromatic compounds is crucial for predicting their chemical reactivity and stability. The RBs for 26 varying para-substituted anilines, benzaldehydes, and toluenes around the respective phenyl- NH2, -CHO, and -CH3 bonds, as well as around the corresponding radical phenyl-NH, -CO, and - CH2 bonds, were computed, based on the Density Functional Theory (DFT). The BDEt of the aminic N-H, CO-H, and methyl C-H bonds in the respective neutral molecules was also computed. The RBs and various geometric, molecular, and atomic properties were used to explain how the substituents influence the BDEt. The trends were rationalized by considering the relative stabilization/destabilization of the parent neutral molecules versus the corresponding radicals. This study is the first in which trends in the RBs and BDEts are rationalized by considering the effect of substituent, providing valuable information for understanding the fundamental behavior of substituted aromatics.