- K. Saalwächter, R. Graf, D. E. Demco, and
H. W. Spiess.
Heteronuclear Double-Quantum MAS NMR Spectroscopy in
J. Magn. Reson. 139, 287-301 (1999).
A new pulse sequence for high-resolution solid-state heteronuclear
double-quantum MAS NMR spectroscopy of dipolar-coupled spin-1/2 nuclei is
introduced. It is based on the five-pulse sequence known from solution-state
NMR, which is here applied synchronously to both spin species. The heteronuclear
double-quantum (HeDQ) spinning-sideband patterns produced by this experiment are
shown to be sensitive to the heteronuclear distance, as well as the relative
orientations of the chemical-shift and dipolar tensors. In particular, it is
shown that the HeDQ patterns exhibit an enhanced sensitivity to the chemical
shielding tensors as compared with the single-quantum spinning-sideband
patterns. The detection of HeDQ patterns via the I and S spins is discussed. The
spin pair in deuterated ammonium formate with 13C in natural abundance was chosen as a model system, and
the perturbing influence of dipolar couplings to surrounding protons on the
coherence is discussed. The pulse sequence can also be used as a heteronuclear
double-quantum filter, hence providing information about heteronuclear
couplings, and thus allowing the differentiation of quaternary and CHn bonded carbons. The elucidation of 13C-1H dipolar proximities
is presented for a sample of bisphenol A polycarbonate with 13C in natural abundance, recorded with a broadband version
of the synchronized five-pulse sequence.