Reducing the effects of weak homonuclear dipolar coupling with CPMG pulse sequences for static and spinning solids.

[Display omitted] • Weak homonuclear dipolar interactions affect CPMG echo trains in SSNMR. • Small flip angles for CPMG refocusing pulses attenuate dipolar dephasing. • Measured effective coherence lifetimes (T 2 eff) can be extended. • Significant signal enhancements can be achieved with modified CPMG pulse sequences for weak homonuclear dipolar coupled systems. • These effects manifest in spectra of spin-1/2 and quadrupolar nuclei under static and MAS conditions. The Carr-Purcell/Meiboom-Gill (CPMG) pulse sequence, initially introduced for measuring transverse relaxation time constants (T 2), can provide significant signal enhancements for solid-state NMR (SSNMR) spectra. The proper implementation of CPMG for acquiring spectra influenced by chemical shift anisotropies (CSAs), first and/or second order quadrupolar interactions, or paramagnetic broadening has been well documented to date, as have the effects of heteronuclear dipolar coupling on CPMG echo trains and T 2 lifetimes. Homonuclear dipolar coupling can also impact T 2 lifetimes and CPMG echo trains; these effects have been thoroughly investigated for spectra of homonuclear dipolar coupled spin-1/2 nuclei typically acquired under static conditions that are predominantly influenced by dipolar broadening (e.g. , 1H, 19F, etc.). In particular, it has been shown that short refocusing pulses with small flip angles can extend the effective T 2 (T 2 eff, the observed T 2 constant as impacted by experimental conditions) measured by CPMG sequences for strong homonuclear dipolar coupled spin-1/2 pairs under static conditions. To date, these effects have not been explored for (i) spin-1/2 nuclei that have significant CSAs and simultaneously feature weak homonuclear dipolar couplings, (ii) for quadrupolar nuclei that are also weakly homonuclear dipolar coupled, and (iii) for either of these cases under magic-angle spinning (MAS) conditions. Herein, we demonstrate that short refocusing pulses that cause small flip angles can reduce the attenuation of signal in CPMG echo trains resulting from dipolar dephasing caused by the weak homonuclear dipolar couplings. For both spin-1/2 and quadrupolar nuclei, this can lead to significant extensions in T 2 eff and signal enhancements of up to three times compared to conventional CPMG in favourable cases. These phenomena can occur under both static and magic-angle spinning (MAS) conditions, in the latter of which homonuclear couplings are reintroduced by rotational resonance (R2) recoupling. Experimental examples of 13C (I = 1/2), 2H (I = 1), 87Rb (I = 3/2), 23Na (I = 3/2), and 35Cl (I = 3/2) NMR under static and MAS conditions, as well as simulations of these phenomena, are shown and discussed. [ABSTRACT FROM AUTHOR]