Existence of a Preferred Orientation for the Methoxy Group on an Extended Aromatic System.

Abstract

AbstractThe conformations of a methoxy group introduced on naphthalene in position 2 has been investigated both experimentally (searches in crystallographic data, NOE measurements by NMR) and theoretically with ab initio calculations. While expected at first glance as being equivalent and predicted as such by force field methods, the two conformations where the methoxy is in the plane of the aromatic ring and turned either towards one or the other proton of naphthalene are actually very different in energy : ΔE=1.2 to 1.8 kcal mol−1, a surprisingly high value! The delocalization of the oxygen lone pair on the π‐system of the aromatic ring differs between the two conformers and is proposed to account predominantly for such unexpectedly large ΔE. It is our opinion that given the magnitude of the energies involved, force field methods should be corrected to take into account this conformational effect.