Measuring strong one-bond dipolar couplings using REDOR in magic-angle spinning solid-state NMR

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
• Quantum spin dynamics and magnetic resonance techniques.

Background:

• Rotational-Echo Double Resonance (REDOR) is a standard technique for measuring dipolar couplings between spin-1/2 nuclei in solid-state NMR.
• REDOR is valuable for estimating molecular motion and internuclear distances.
• Challenges arise with strongly coupled spin pairs (e.g., 13C-1H) due to fast dipolar dephasing, complicating measurements even at high Magic-Angle-Spinning (MAS) frequencies.

Purpose of the Study:

• To introduce a generalized REDOR pulse scheme for robust dipolar coupling measurements.
• To overcome the limitations of existing REDOR-based methods, such as Shifted-REDOR (S-REDOR).
• To achieve arbitrary scaling factors of dipolar couplings while maintaining experimental simplicity and robustness.

Main Methods:

• Development of a general REDOR pulse scheme involving the shifting of both recoupling pulses within a rotor period.
• Application of the new scheme to isolated 13C-15N and 1H-13C spin pairs.
• Experimental validation at high MAS frequencies (20 and 62.5 kHz).

Main Results:

• The generalized scheme allows for arbitrary scaling factors of dipolar couplings.
• This method avoids the disadvantages of S-REDOR, such as Fourier component mixing and the need for high radio-frequency fields.
• The technique proved effective for challenging spin pairs at relevant MAS rates.

Conclusions:

• The novel REDOR pulse scheme offers a versatile and robust approach for measuring dipolar couplings in solid-state NMR.
• It provides a significant improvement over existing methods for strongly coupled spin pairs.
• This advancement facilitates more accurate structural and dynamic studies in solid materials.