Conformational Flexibility of the Methoxyphenyl Group Studied by Statistical Analysis of Crystal Structure Data

In a large sample of observed methoxyphenyl groups, the twist angle τ about the MeO-C_Ph bond measuring internal rotation of the MeO group shows a continuous distribution with maxima at (0°) (coplanar conformation) and (～90°) (perpendicular conformation). The preferred conformation of methoxyphenyl depends on the nature of the ortho--substituents: In general, it is coplanar in the case of one or two ortho-hydrogens, and perpendicular in the case of two substituents. The internal rotation of the MeO group is accompained by systematic variations in bond angles and bond distances: 1 if MeO is twisted out of plane, the bond angle CH₃? O? C_Ph decreases from 117.7°, until it reaches a minimum of 114.9° at τ = ±90°. The O? C? C angle which is syn to CH₃ for τ = 0° decreases from 124.6° to a minimum of 115.4° at τ = ±180°. These angles changes keep the nonbonded distance CH₃ …? ortho substituent maximal during internal rotation of MeO and tend to minimize the corresponding strain energy. (2) In the perpendicular conformation, the O-atom is ～ 0. 06 Å displaced from the Ph plane, O and CH₃ and being on opposite sides of this plane. In addition, small but systematic increases of bond lengths MeO? C_Ph and CH₃? O are observed. These variations indicate a decrease in conjugation with increasing twist angle. Their interdependence during twisting and the magnitudes of the changes are close values obtained by ab initio calculations.