High-resolution solid state NMR experiments first came to fruition in John Waugh’s lab in 1968, and involved multiple pulse experiments for homonuclear decoupling, as exemplified by the famous WAHUHA four pulse method. Concurrently, theoretical methods to treat these cyclic, time dependent experiments lead to the development of average Hamiltonian theory (AHT). Today, 50 years after its inception AHT remains the preeminent theoretical approach to understanding time domain NMR. In addition, these initial efforts nucleated investigations of heteronuclear decoupling and of elegant methods to refocus dipolar interactions in solids, the magic echo. In a second set of pioneering papers, Waugh and his students, Alex Pines and Michael Gibby, described double resonance experiments for observation of dilute spins such as 13C and 15N. These dilute spin species were thought to have long T1’s, so it was proposed to circumvent this problem by cross-polarization (CP) methods introduced originally by Hartmann and Hahn. The addition of continuous wave 1H decoupling removed the dipolar broadening from the attached 1H’s. An additional benefit of this approach was that it enhanced the polarization of the dilute spins. Subsequently, Schaefer and Stejskal combined CP with magic angle spinning (MAS) to yield the CPMAS method which is the approach that is used universally today to observe many species of dilute spins. The NMR spectrometers that were in use in the late 60’s and early 70’s were single frequency resonance instruments that produced one or two pulses and were therefore not capable of performing the sophisticated experiments developed by Waugh and his colleagues. Accordingly, he proposed a new spectrometer architecture based on a heterodyne frequency generation system which could be used at multiple frequencies. Today, commercial spectrometers are based on this basic design as are pulse programmers, multiple channel MAS probes, etc.

John Stewart Waugh and high-resolution NMR in solids

2019 Spring National Meeting

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