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Conformational Flexibility of the Methoxyphenyl Group Studied by Statistical Analysis of Crystal Structure Data
Author(s) -
Hummel Wolfgang,
Huml Karel,
Bürgi HansBeat
Publication year - 1988
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19880710540
Subject(s) - chemistry , bond length , perpendicular , crystallography , substituent , dihedral angle , molecular geometry , group (periodic table) , rotation (mathematics) , twist , atom (system on chip) , conformational isomerism , maxima , crystal structure , plane (geometry) , ab initio , stereochemistry , molecule , geometry , hydrogen bond , art , mathematics , organic chemistry , performance art , computer science , embedded system , art history
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 CH 3 OC Ph decreases from 117.7°, until it reaches a minimum of 114.9° at τ = ±90°. The OCC angle which is syn to CH 3 for τ = 0° decreases from 124.6° to a minimum of 115.4° at τ = ±180°. These angles changes keep the nonbonded distance CH 3 … 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 CH 3 and being on opposite sides of this plane. In addition, small but systematic increases of bond lengths MeOC Ph and CH 3 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.