θ-REDOR: an MAS NMR method to simplify multiple coupled heteronuclear spin systems

The measurement of heteronuclear dipolar couplings with rotational-echo, double-resonance NMR (REDOR) is straightforward for isolated S–I spin pairs. However, measuring the n dipolar couplings for an S spin coupled simultaneously to n I spins is not easy because of additional complications arising from the orientational dependencies between the n dipolar tensors. We present an important modification of REDOR, called θ-REDOR, that is useful for systems with multiple dipolar couplings. This new experiment recovers the n dipolar couplings while eliminating the complicating orientational dependence between the dipolar tensors of the REDOR experiment. A REDOR transform of data obtained by θ-REDOR provides the n dipolar frequencies.