Resumen de Elucidation of Molecular Structure Using NMR Long-Range Coupling:: Determination of the Single Isomer Formed in a Regiospecific Reaction

Samuel Delagrange, Françoise Nepveu

• The experiment presented in this article introduces students to the main concepts of two essential NMR techniques for investigating molecular structure. Heteronuclear multiple bond connectivity (HMBC) and proton-coupled 13C NMR are used to determine which isomer, from a possible two, is formed by a regiospecific reaction. The demonstration, based on long-range coupling between quaternary carbons and protons on neighboring carbons, is presented step by step. In the first step, ambiguous carbon signals were assigned with HMBC, which shows the long-range couplings between carbons and protons through two and three covalent bonds. The structure was finally resolved by recording a proton-coupled 13C spectrum (using gated decoupling) in which each carbon signal is split into a multiplet by coupling with surrounding protons. The study of both the multiplicity and the corresponding coupling constants gives information that allows the determination of the structure without ambiguity.