title: Weak coupling between magnetically inequivalent spins: The deceptively simple, complicated spectrum of a13C-labeled trimethylated amine.
abstract: Magnetic inequivalence of nuclear spins is well known to cause additional splittings that complicate spectral analysis. Here, we present an extraordinary case of magnetic inequivalence, manifested in the 13-spin system of a13C,15N-labeled trimethylated amine. All methyl group protons are chemically equivalent due to the molecular symmetry, but not all are magnetically equivalent as they have different1JCHand3JCHcouplings. In general, spectra of such a large spin system can be expected to be extremely complicated by the presence of hundreds if not thousands of extra lines, caused by the strong coupling between inequivalent nuclei. Surprisingly, the1H spectrum presented consists of very few lines, in a pattern of the utmost simplicity. Using sub-spectral analysis we show that this is due to weak coupling between the magnetically inequivalent nuclei, as a consequence of the particular combination of coupling constants. We find that the4JHHgeminal methyl coupling constant is 0.43Hz and2JCCis ∼0Hz. In addition, we demonstrate that homo-decoupling can be used to transform the spin system to a set of fully equivalent spins, resulting in disappearance of4JHH-splittings. We believe this curious case is a highly instructive example of magnetic inequivalence. The spectra may be considered deceptively simple, as fewer lines are observed than one would anticipate. At the same time, the spectra are deceptively complicated, as they can very well be approximated by intuitive reasoning.
authors.affiliation: Macromolecular Biochemistry, Leiden Institute of Chemistry, Leiden University, The Netherlands.
authors.forename: Ulric B
authors.initials: UB
authors.lastname: le Paige
authors.affiliation: Macromolecular Biochemistry, Leiden Institute of Chemistry, Leiden University, The Netherlands.
authors.forename: Bauke
authors.initials: B
authors.lastname: Smits
authors.affiliation: Department of Chemical Biology & Drug Discovery, Utrecht Institute for Pharmaceutical Sciences, University Utrecht, The Netherlands.
authors.forename: Peter
authors.initials: P
authors.lastname: 't Hart
authors.affiliation: NMR Facility, Leiden Institute of Chemistry, Leiden University, The Netherlands.
authors.forename: Fons
authors.initials: F
authors.lastname: Lefeber
authors.affiliation: Department of Chemical Biology & Drug Discovery, Utrecht Institute for Pharmaceutical Sciences, University Utrecht, The Netherlands.
authors.forename: Nathaniel I
authors.initials: NI
authors.lastname: Martin
authors.affiliation: Macromolecular Biochemistry, Leiden Institute of Chemistry, Leiden University, The Netherlands. Electronic address: h.van.ingen@lic.leidenuniv.nl.
authors.forename: Hugo
authors.initials: H
authors.lastname: van Ingen
keywords: (4)J(HH); Composite particle analysis; J-coupling; Magnetic equivalence; Sub-spectral analysis; Trimethylated amine