In a large sample of observed methoxyphenyl groups, the twist angle 7 about the Me0 -Cph bond measuring internal rotation of the Me0 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 Me0 group is accompanied by systematic variations in bond angles and bond distances: 1 ) if Me0 is twisted out of plane, the bond angle CH,-O-Cph decreases from 117.7", until it reaches a minimum of 1 14.9" at t = f90". The 0-C-C angle which is syn to CH, for 7 = 0" decreases from 124.6" to a minimum of 115.4" at r = f180". These angle changes keep the nonbonded distance CH,. . 'ortho substituent maximal during internal rotation of Me0 and tend to minimize the corresponding strain energy. 2) In the perpendicular conformation, the 0-atom is -0.06 A displaced from the Ph plane, 0 and CH, being on opposite sides of this plane. In addition, small but systematic increases of bond lengths MeO-C,, and CH,-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 to values obtained by ab initio calculations.
Introduction.
~ In [ 11, the conformational flexibility of the acetoxyphenyl (AcOPh) group was studied by statistical analysis of crystal structure data. In the preferred conformation, the AcO group is perpendicular to the Ph ring. From an investigation of the methoxyphenyl (MeOPh) group, Nyburg and Faerman [2] found the coplanar conformation to be preferred, but they excluded molecules with non-hydrogen ortho-substituents from their survey. When such molecules are included, one may expect to find non-coplanar conformations as well. In this work, the conformational flexibility of the MeOPh fragment was studied using several methods of (multivariate) statistical analysis of crystal structure data. The results are discussed in terms of nonbonded interaction and conjugation effects. Data Retrieval. -Crystal structures containing MeOPh groups (Fig. I) were retrieved from the Cambridge Structural Database (CSD, version of May 1986 with 46750 entries in the connectivity file) [3]. The numbering scheme is adopted from the AcOPh fragment [l] in order to facilitate comparison between the two fragments. For the initial connectivity search, no restrictions were placed on the nature of the substituents at positions C(6) to C(10) (Fig. I ) . Atomic coordinates of 1244 structures were retrieved, and calculations of geometric parameters were performed. Names were assigned to geometrical parameters as follows: Dxy are bond distances (e.g. bond C(2)-0(4) is D24), Axyz are bond angles, and Txyzw are torsion angles. Twist deformation about the bond 0(4)-C( 5) is defined as in 141: