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An Experiment to Demonstrate Magnetic Nonequivalence in Proton NMR

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Christopher J. Welch

Department of Pharmaceutical Chemistry, Organic Pharmaceutical Chemistry, Uppsala Biomedical Centre, Uppsala University, Box 574, S-751 23 Uppsala, Sweden

J. Chem. Educ., 1997, 74 (2), p 247

DOI: 10.1021/ed074p247

Publication Date (Web): February 1, 1997

Section:

History, Education, and Documentation

Abstract

The bicyclic compound, 3a,6a-diethoxycarbonyl-2,5-dimethyl-1,4-dioxo-octahydropyrrolo[3,4-c]pyrrole, prepared by a literature procedure is used to demonstrate magnetic nonequivalence for methylene protons in the proton NMR experiment. Two sets of nonequivalent protons are found, one for which constraints generated by inclusion in a ring cause large differences in magnetic environment giving rise to the nonequivalence, and a second where the magnetic environment experienced by two protons differs slightly during unrestricted rotation. In a first case the large difference in chemical shift gives an approximately first order coupling, whereas in the second case the chemical shifts differ only slightly giving overlap and the need for an algorithm for solution of the coupling constants.