Selective ¹H-¹H recoupling via symmetry sequences in fully protonated samples at fast magic angle spinning

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy
• Molecular structure determination
• Advanced pulse sequence development

Background:

• Proton-detected solid-state NMR with fast Magic Angle Spinning (MAS) is standard for molecular characterization.
• Proton-proton (¹H-¹H) and proton-X (¹H-X) dipolar couplings are routinely used.
• Selective proton recoupling methods are emerging for distance estimation in structural characterization.

Purpose of the Study:

• To demonstrate that established CN^v_n sequences can be adapted for selective proton spin recoupling.
• To investigate the use of modified C elements for tailoring selectivity in proton recoupling.
• To explore the feasibility of measuring proton-proton distances in fully protonated samples.

Main Methods:

• Numerical simulations and experimental validation of tailored CN^v_n sequences.
• Employment of C elements of the type (β)_Φ(4β)_Φ+π(3β)_Φ for selective recoupling.
• Examination of several CN^v_n sequences, including C6¹₄ with a modified POST-element.

Main Results:

• The modified C6¹₄ sequence demonstrates selective polarization transfer efficiencies of 40-50% between proton pairs.
• These selective recoupling sequences, termed frequency-selective-CN^v_n, function effectively in fully protonated samples.
• Performance was assessed at radiofrequency (rf) amplitudes ranging from 0.3 to 0.8 times the MAS frequency.

Conclusions:

• Established CN^v_n sequences can be effectively modified for selective proton recoupling.
• Frequency-selective-CN^v_n sequences provide efficient polarization transfer and enable measurement of ¹H-¹H distances.
• These developments are crucial for structural characterization of molecules without isotopic enrichment or crystalline samples.