Adiabatic Heteronuclear Isotropic Mixing in Low-Field Nuclear Magnetic Resonance.

Area of Science:

• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Physical Chemistry
• Spectroscopic Techniques

Background:

• Low-field NMR spectroscopy is expanding its chemical analysis capabilities through nuclear spin hyperpolarization techniques.
• Homonuclear and heteronuclear spin interactions are crucial for NMR signal enhancement and spectral resolution.
• Efficient polarization transfer is key to maximizing the utility of low-field NMR in diverse chemical environments.

Purpose of the Study:

• To demonstrate efficient heteronuclear isotropic mixing between proton (¹H) and fluorine-19 (¹⁹F) spins in low-field NMR.
• To evaluate the performance of adiabatic WURST pulses compared to DIPSI-2 for polarization transfer.
• To assess the robustness of these techniques under magnetic field (B₁) miscalibration for practical applications.

Main Methods:

• Heteronuclear two-dimensional correlation spectroscopy of 3-fluoropyridine was employed.
• Adiabatic WURST and DIPSI-2 pulse sequences were utilized for isotropic mixing.
• Density matrix simulations were performed to predict and validate experimental results.

Main Results:

• An adiabatic WURST pulse achieved 50% polarization transfer over a 2149 Hz frequency difference at 0.86 mT.
• Despite lower efficiency (63%), DIPSI-2 yielded a 26% lower signal-to-noise ratio than WURST.
• Adiabatic WURST showed superior robustness against B₁ miscalibration, with only a 1% efficiency drop versus DIPSI-2's 26% degradation.

Conclusions:

• Adiabatic WURST pulses offer efficient and robust heteronuclear spin polarization transfer in low-field NMR.
• These findings support the use of low-field NMR for cost-effective, ex-situ chemical analysis.
• Simulations accurately predict experimental outcomes, aiding the design of optimal mixing pulses for future applications.