Strong coupling effects in 2D J, δ Spectra: An application for the determination of the conformational isomers of some 1,4 disubstituted dioxanes

Abstract

When strong proton–proton coupling is present, a projection of the tilted two-dimensional J, δ (J-resolved) spectrum frequently produces one or more extra lines, i.e., there are more lines in the projection than there are protons in the molecule. Contrary to popular belief, these lines are not “extraneous nuisances.” Instead, they may contain a wealth of information that can be of value for the determination of structural isomers. This article provides a brief introduction to the theory of these spectra and examines the origin as well as the practical properties of these extra transitions. Two important properties are: (a) in the untilted spectrum, the extra transitions have F2 frequencies that are equal to those of the normal transitions; and (b) they lie on a line that is midway between the lines that are formed by the normal transitions. As a practical application of the usefulness of these properties, this article presents the strategy used to identify several structural isomers of 2,5- and 2,6-disubstituted 1,4-dioxanes. All of the possible isomers were identified, including the separation of the “frozen” isomers from the equilibrating ones, and the separation of the 2,5- from the 2,6-disubstituted ones. The identification was achieved by measuring the coupling constants between the ring protons, and then using (a) the Karplus relationship to determine whether the substituent was axial or equatorial, and (b) the well-known planar-zigzag, or “W,” coupling to determine whether the substituents were both adjacent to only one of the ring oxygen atoms (the 2,6-isomer) or whether each was situated adjacent to only one ring oxygen atom (the 2,5-isomer). ©1997 John Wiley & Sons, Inc. Concepts Magn Reson 9: 359–384, 1997