Composite-pulse and partially dipolar dephased multiCP for improved quantitative solid-state ¹³C NMR

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Materials Science
• Organic Chemistry

Background:

• Quantitative 13C NMR spectroscopy of organic solids is crucial for material characterization.
• Conventional multiple cross-polarization (multiCP) sequences suffer from magnetization loss, limiting polarization enhancement, especially for weakly coupled 13C sites.
• This loss is primarily due to imperfect read-out and storage pulses.

Purpose of the Study:

• To develop and present improved multiCP pulse sequences for rapid quantitative 13C NMR acquisition.
• To address the limitations of existing multiCP techniques by minimizing magnetization loss.
• To enhance the accuracy and efficiency of NMR spectral analysis for organic solids.

Main Methods:

• Introduction of composite 90° pulses with non-orthogonal phases in the multiCP sequence (ComPmultiCP).
• These pulses flip magnetization onto the spin-lock field and back for 1H repolarization, reducing polarization loss.
• Optimization of amplitude ramp during cross-polarization (CP) and application of partial dipolar dephasing for specific samples.

Main Results:

• The ComPmultiCP sequence significantly reduces magnetization loss (<3% over ±10 kHz offset and 20% flip-angle error).
• ComPmultiCP demonstrates superior performance to conventional multiCP without trade-offs.
• Reduced sensitivity to Hartmann-Hahn mismatch and compensation for under-polarization of non-protonated carbons were achieved.
• Quantitative spectra were successfully obtained for branched polyethylene and soil organic matter.

Conclusions:

• The ComPmultiCP sequence offers a robust and efficient method for quantitative 13C NMR of organic solids.
• This technique overcomes previous limitations, providing enhanced polarization and accuracy.
• ComPmultiCP is applicable to diverse materials, including low-crystallinity polymers and complex organic matter.