- K. Saalwächter, H. W. Spiess.
Heteronuclear 1H-13C multiple-spin correlation in solid-state NMR: combining
REDOR recoupling and multiple-quantum spectroscopy.
J. Chem. Phys.
114, 5707-5728 (2001).
High-resolution magic-angle spinning (MAS) solid-state nuclear magnetic
resonance (NMR) spectroscopy exploiting the dipole-dipole coupling between
unlike spins is a powerful tool for the determination of internuclear distances.
In particular, the rotational-echo double-resonance
(REDOR) technique has established itself as a method for probing heteronuclear
dipole-dipole couplings in isotopically dilute systems of low-γ nuclei. In
organic substances it is, however, particularly advantageous to consider
heteronuclear spin-pairs such as 1H-13C, on account of the high natural abundance of 1H and thus a much wider range of possible applications. We
describe the possibility of performing 13C-observed REDOR in 1H-13C systems, where
very-fast MAS with spinning frequencies of up to 30 kHz is used to successfully
suppress the perturbing homonuclear couplings among the protons, which would
usually be expected to hamper a proper data analysis. Simple modifications of
the REDOR experiment are presented which lead to a two-dimensional experiment in
which heteronuclear multi-spin multiple-quantum modes are excited, the evolution
of which is monitored in the indirect frequency dimension. The existence of
higher quantum orders in the proton subspace of these heteronuclear coherences
is proven by performing a phase-incremented spin-counting experiment, while a
phase cycle can be implemented which allows the observation of specific selected
coherence orders in the indirect dimension of two-dimensional shift correlation
experiments. The significance of the heteronuclear approach to spin counting is
discussed by comparison with well-known homonuclear spin-counting strategies.
For the shift correlation, the high resolution of 1H chemical shifts in the indirect dimension is achieved by
the use of high B0 fields
(ωL,H/2π= 700.13 MHz) combined with
very-fast MAS, and dipolar coupling information can be extracted by analyzing
either peak intensities or spinning-sideband patterns in the indirect frequency
dimension. The method is termed dipolar heteronuclear
multiple-spin correlation (DIP-HMSC).