Couplings into Methylene Groups: a New Nuclear Magnetic Resonance Approach to Stereochemistry

A method of assigning dihedral angles to hydrogens adjacent to a methylene function by using computer analyzed n.m.r. coupling constants in a modified Karplus equation is presented. The "Dihedral Angle Estimation by the Ratio Method" (DAERM) is based on the assumption that, although the magnitudes of the Karplus constants, k1 and k2, vary, the ratio of k1 to k2 is a constant. Dihedral angles computed in this way seem not to be influenced by the effects of ring strain and substituent electronegativity. The conformational preferences of several four- to six-membered ring Systems, some containing oxygen and sulfur atoms, have been investigated by this method. Molecular features such as ring puckering and flattening are indicated by DAERM as are some subtle electrostatic influences.