Barriers in Substituted Anilines

The effect of para substituents on nitrogen inversion barriers in anilines has been reported by Hehre et al. (9). We have extended those data to include additional substituents, as well as substituents at the meta position. Although the theoretical inversion barrier in aniline (2.72 kcal mole ) is somewhat higher than the experimental value (1.6 kcal mole ), the effect of the substituent on the inversion barrier might be more reliable as in the case of the rotational barriers in substituted phenols. The nitrogen inversion barriers, as well as optimized N bond angles, are summarized in Table 11. 

The results are similar to those obtained for rotational barriers in phenols although the barriers do, of course, move in opposite directions. Substituents that enhance conjugation between the NH2 group and the ring stabilize the planar transition state (4) more than the pyramidal ground state (5) leading to a reduced barrier. 

we observe that at the para position, tt effects are dominant, rr-Donors (CH3, NH2, OH, and F) increase the barrier, by opposing NH2-ring conjugation, and decrease the bond angles at N which open out as conjugation increases. For O , a powerful tt donor, a barrier of 7.62 kcal mole is observed. 

Calculated Total Energies and Dipole Moments for Substituted Cyanobenzenes 

Substituent (X) Conformation Dipole moment (debyes) Energy (hartrees) 

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Other problems dealt with by the Hehre-Pople method are internal rotation in vinylcyclopropane and vinylcyclobutane 153>, the structure of homoallyl cation 154> and ethylenebenzenium cation 155>, torsional barriers in -substituted phenols 156), inversion barriers in -substituted anilines 157>, the effects of a-substitution in keto-enol tautomerism 158> and the circumambulatory rearrangement in bicyclo [3.1.0] hex-3-en-2-yl cation 159> ... 

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