Heteronuclear double-quantum MAS NMR spectroscopy in dipolar solids

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 isolated C-13-H-1 spin pair in deuterated ammonium formate with C-13 in natural abundance was chosen as a model system, and the perturbing influence of dipolar couplings to surrounding protons on the C-13-H-1 DQ 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 CH, bonded carbons. The elucidation of C-13-H-1 dipolar proximities is presented for a sample of bisphenol A polycarbonate with C-13 in natural abundance, recorded with a broadband version of the synchronized five-pulse sequence. (C) 1999 Academic Press.