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‐CPh 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CPh 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CPh 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.