Remote substituent effects on N-X (X = H, F, Cl, CH3, Li) bond dissociation energies in para-substituted anilines

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 sigma(p)(+) constants. The slope (rho(+)) 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. p+ 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, alpha-substitution was found to be able to significantly change the substituent effects. rho(+) values for N-H BDEs of 4-C6H4NCH3-H and 4-C6H4NF-H are 2.5-4.0 and 1.7-1.9 kcal/mol, respectively.