Broadband-PISEMA solid-state NMR spectroscopy

The measurement of heteronuclear dipolar couplings using a 2D separated-local-field technique, Polarization Inversion Spin Exchange at the Magic Angle (PISEMA), is a unique way to determine the structure, dynamics and topology of molecules in solid-state. However, the resolution and sensitivity of PISEMA are highly dependent on the offset frequency of protons. To overcome this difficulty, in this study, a broadband-PISEMA pulse sequence is proposed. Experimental data from a single crystal and simulated results suggest that the new sequence compensates the offset effects. This is accomplished using a pair of 180°pulses that invert the spin-locked magnetization of I and S nuclei after certain number of SEMA cycles in the t₁ period. In addition, unlike PISEMA, BB-PISEMA provides offset-independent dipolar coupling line shapes even when low rf fields are applied.