A C-13 NMR AND AM1 STUDY ON INTRAMOLECULAR INTERACTIONS DEFINING THE METHOXY GROUP CONFORMATION IN UNHINDERED ANISOLE DERIVATIVES

Interactions which define the planar conformation of the methoxy group with respect to the aryl ring in methyl aryl ethers are studied by C- 13 NMR spectroscopy and by MO calculations at the AM1 level in a famil y of anisole derivatives with unhindered ortho positions. Different me thoxy ortho substituent chemical shifts were found and they were ascri bed to different populations of the two coplanar conformers of the met hoxy group. Optimized geometries, total energies, total point-charge d ipole moments and II bond orders of the aromatic C-C bonds for both me thoxy rotamers of each of these compounds were calculated. Results sug gest that the different populations of both types of rotamer arise fro m an electrostatic interaction between the side-chain dipole moment an d an opposite dipole moment induced on the II electronic system of the aromatic C-ipso-C-ortho bond oriented s-cis to the methyl moiety of t he methoxy group. The strength of this interaction depends on the II b ond order of that bond. The failure of the AM1 method to describe quan titatively the energy of this interaction is discussed.