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  Home > JCE Print > Journal of Chemical Education > Issues > 2000  > July  >
In the Laboratory
Elucidation of Molecular Structure Using NMR Long-Range Coupling: Determination of the Single Isomer Formed in a Regiospecific Reaction
Samuel Delagrange and Françoise Nepveu
Laboratoire Pharmacophores Redox, Phytochimie et Radiobiologie, 35 Chemin des Maraîchers, F-31062 Toulouse Cedex 4, France
Cover
July 2000
Vol. 77 No. 7
p. 895
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.
Supplement
NMR parameters for heteronuclear multiple bond connectivity and for proton-coupled 13C and figures are included.
*  Contents
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*  Citation
 Delagrange, Samuel; Nepveu, Françoise. J. Chem. Educ. 2000 77 895.
*  Keywords
 Molecular Properties / Structure; NMR Spectrometry; Organic Chemistry; Organic Synthesis
*  History
 Created:
Last Updated:		 June 2, 2000
August 31, 2005
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