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

Abstract

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.