Remote Substituent Effects on N−X (X = H, F, Cl, CH3, Li) Bond Dissociation Energies in Para-Substituted Anilines

Abstract

UB3LYP/6-311++g**//UB3LYP/6-31+g* and ROMP2/6-311++g**//UB3LYP/6-31+g* methods were used to calculate (i) N−X bond dissociation energies (BDE) in 4-YC₆H₄NH−X and (ii) N−H BDEs in 4-YC₆H₄NU−H, where Y = H, Me, OCH₃, SMe, NH₂, NMe₂, SiMe₃, F, Cl, CN, COOH, CF₃, and NO₂, X = H, CH₃, F, Cl, and Li, and U = H, F, and CH₃. It was found that N−H BDEs of 4-YC₆H₄NH₂ have a positive correlation with the substituent σ_p⁺ constants. The slope (ρ⁺) is about 3.0−4.3 kcal/mol, which is in good agreement with the experimental results. It was also found that the substituent effects on N−X BDEs of 4-YC₆H₄NH−X change considerably when X changes. ρ⁺ values for N−CH₃, N−F, N−Cl, and N−Li BDEs were calculated to be 3.1−4.6, 1.3−1.9, 1.8−2.6, and 4.9−6.8 kcal/mol, respectively. The reason for the variation of substituent effects was proposed to be the ground-state effect, i.e., the interaction between the intact NH−X moiety and the para substituents. Finally, α-substitution was found to be able to significantly change the substituent effects. ρ⁺ values for N−H BDEs of 4-C₆H₄NCH_3-H and 4-C₆H₄NF−H are 2.5−4.0 and 1.7−1.9 kcal/mol, respectively.