Heteronuclear Double-Quantum MAS NMR Spectroscopy in Dipolar Solids

Abstract

A new pulse sequence for high-resolution solid-state heteronuclear double-quantum MAS NMR spectroscopy of dipolar-coupled spin-12 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 13C–1H 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 13C–1H 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 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.