Substituent effects on the physical properties and p K a of aniline

Abstract The aniline molecule is nonplanar, with its NH 2 group lying at an angle θ of approximately 42° to the plane of the benzene ring. Substituents on the phenyl ring alter this out‐of‐plane angle as well as other molecular properties such as the ring bond lengths and angles, the barrier to inversion E inv , and the p K a of the amino group. Ab initio 6‐311G** quantum chemical calculations have been employed to examine these substituent influences and the extent to which they are interrelated. Electron‐donating substituents increase the CN bond length R(CN), θ, E inv , and the p K a , whereas electron‐withdrawing substituents have the opposite effect. Among the molecular parameters that might serve as regression indicators for these changes, Hammett σ constants, which traditionally have been used to represent substituent electronic effects, yield fair to good correlations for R(CN) ( r 2 =0.797), θ ( r 2 =0.804), E inv ( r 2 =0.829), and the amino group p K a ( r 2 =0.931) for aniline and 18 substituted anilines. Of several measures of atomic charge, the Mulliken and electrostatic charges on the amino nitrogen atom show essentially no correlation with these properties. In contrast, the natural charge Q n on the amino nitrogen is well correlated with the bond length R(CN) ( r 2 =0.889), θ ( r 2 =0.932), E inv ( r 2 =0.839), and the amino group p K a ( r 2 =0.960). This latter result suggests that the natural charge, rather than either the Mulliken or electrostatic charges, may be the preferred charge descriptor for correlation purposes. Inclusion of electron correlation at the MP2 level increases the correlations of E inv with both σ ( r 2 =0.951) and Q n ( r 2 =0.892). © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 1107–1115, 2000