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Spin dynamics in the modulation frame: Application to homonuclear recoupling in magic angle spinning solid-state NMR

2008; American Institute of Physics; Volume: 128; Issue: 12 Linguagem: Inglês

10.1063/1.2834732

1520-9032

Gaël De Paëpe, Józef R. Lewandowski, Robert G. Griffin,

Muon and positron interactions and applications

We introduce a family of solid-state NMR pulse sequences that generalizes the concept of second averaging in the modulation frame and therefore provides a new approach to perform magic angle spinning dipolar recoupling experiments. Here, we focus on two particular recoupling mechanisms—cosine modulated rotary resonance (CMpRR) and cosine modulated recoupling with isotropic chemical shift reintroduction (COMICS). The first technique, CMpRR, is based on a cosine modulation of the rf phase and yields broadband double-quantum (DQ) C13 recoupling using >70kHz ω1,C∕2π rf field for the spinning frequency ωr∕2=10–30kHz and H1 Larmor frequency ω0,H∕2π up to 900MHz. Importantly, for p⩾5, CMpRR recouples efficiently in the absence of H1 decoupling. Extension to lower p values (3.5⩽p<5) and higher spinning frequencies is possible using low power H1 irradiation (<0.25ωr∕2π). This phenomenon is explained through higher order cross terms including a homonuclear third spin assisted recoupling mechanism among protons. CMpRR mitigates the heating effects of simultaneous high power C13 recoupling and H1 decoupling. The second technique, COMICS, involves low power C13 irradiation that induces simultaneous recoupling of the C13 DQ dipolar and isotropic chemical shift terms. In contrast to CMpRR, where the DQ bandwidth (∼30kHz at ω0,H∕2π=750MHz) covers the entire C13 spectral width, COMICS recoupling, through the reintroduction of the isotropic chemical shift, is selective with respect to the carrier frequency, having a typical bandwidth of ∼100Hz. This approach is intended as a general frequency selective method circumventing dipolar truncation (supplementary to R2 experiments). These new γ-encoded sequences with attenuated rf requirements extend the applicability of homonuclear recoupling techniques to new regimes—high spinning and Larmor frequencies—and therefore should be of major interest for high resolution biomolecular studies.