Aryl Planes Research Articles

Proton nuclear magnetic longitudinal relaxation in a series of AB systems

The proton longitudinal relaxation rates of the lines of three AB systems, the methyl ester of cinnamic acid-D5 (A), the trideuteromethyl ester of cinnamic acid-D5 (B) and benzalacetone (C), in solution were determined. The following dipolar contributions to the relaxation were separated : intermolecular ones ; intramolecular ones between the A and B proton and between the A or B proton and other magnetic nuclei in the molecules. From the data it can be concluded that in compounds A and B the aryl and ethenyl planes are not parallel, while information is obtained about the position of the methoxy group. Molecule C occurs in two conformers between which an energy difference of at least 2·9 kJ/mol exists. Due to the inaccuracy in the experimental relaxation rates not all information is obtained that might in principle be extracted from them.