Revealing Signs and Hidden ¹H NMR Coupling Constants in Three-Spin Systems Using Long-Lived Coherences

Long-lived coherences (LLCs) in a pair of coupled protons have long lifetimes and hence decreased line width and increased spectral resolution. Fourier transformation of the damped oscillatory decay of the LLC also provides coupling information on the spin system. In a three-spin system, unlike in the two-spin case, the peaks in an LLC spectrum are observed at combinations of the coupling constants. This attribute is used to determine the relative signs of the coupling constants in weakly and strongly coupled model systems. In addition, it is shown that a coupling constant in a three-spin system that is unobservable in the ¹H NMR spectrum, as is the case in bispidinone, a molecule of significance in peptidomimetics, may be determined from the LLC spectrum.