In 1959 Raymond Andrew showed line narrowing in NMR spectra of samples spinning at the magic angle (54.7o relative to the applied static magnetic field). Andrew adapted the ultracentrifuge air-bearing design of Jesse Beams to create nearly frictionless sample spinning. The same year, Irving Lowe, a graduate student in the Physics Department at Washington University, separated the air bearings (self-pumped by the Bernoulli effect) from the drive gas, and achieved comparable line narrowing by magic-angle spinning (MAS). Practical spinning speeds were of the order of 5 kHz which was insufficient to narrow spectra of samples that were proton rich. In 1972 Pines, Gibby, and Waugh showed proton-to-carbon cross polarization (CP) 13C NMR spectra of non-spinning organic samples in which the dipolar broadening due to 13C coupling to protons was removed by high-power 1H radio-frequency (rf) irradiation. However, the anticipated result of combining CP and MAS (for resolution and sensitivity enhancement) was 13C NMR spectra that would be missing contributions from non-protonated carbons. In 1976 we discovered, however, that the experimental CPMAS spectra had strong contributions from non-protonated carbons. The criterion for success was not that the spinning speed had to be less than the dipolar coupling between 1H and 13C ( which was weak for non-protonated carbons), but only less than the usually strong coupling between protons. In 1982, Ed Stejskal designed a stator-rotor combination with pumped air bearings and a separated drive gas. This combination resulted in highly reliable and stable controlled spinning and led to a variety of experiments in which rf pulses were synchronized with spinning. Today, MAS at 200 kHz is practical (for small-diameter cylindrical rotors and larger-diameter spherical rotors) so that proton decoupling is not needed to obtain liquid-like proton and rare-spin NMR spectra of solids.

Sixty years (and counting) of magic-angle spinning for NMR

2019 Spring National Meeting

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