Rotational Barriers in Substituted Phenols

An experimentally accessible probe for studying x-electron interactions in substituted phenols is the barrier to rotation about the C—O bond (8). Interaction of the p-type lone pair on oxygen with the x orbitals of the ring in the planar conformation (1) is more effective than interaction of the sp -typc lone pair on oxygen in the orthogonal conformation (2) because of poorer overlap and lower orbital energy in the latter situation. 

As a consequence, phenol itself is planar, and the energy difference between the planar and orthogonal forms represents the barrier to internal rotation about the C—O bond. Substituents that increase conjugation between the OH group and the ring might be expected to increase the rotational barrier and vice versa (8). 

Theoretical rotational barriers for a number of para-substituted phenols have been previously reported (8). We present here results for additional substituents and consider meta substitution as well. To examine both meta and para 

Effect of Substituents on Rotational Barriers (AE2) and Overlap Populations in Substituted Phenols 

Theoretical values of AV2, the difference between the V2 value for the substituted phenol and phenol itself, are listed in Table 10, together with ir-overlap populations for the C—O bond. Although the theoretical 1 2 values are significantly different from those obtained experimentally (e.g. for phenol the calculated barrier is 5.16 kcal mole compared with the experimentally determined value of 3.56 kcal mole ) the theoretical A 2 values apprar generally to be in good agreement with the available experimental values. 

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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. 

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