Artificial intelligence-designed ultra-low-power broadband universal π2 and π pulses for NMR spectroscopy: Application to 19F NMR.

With the availability of high and ultra-high magnetic field strength, it becomes essential to develop innovative radiofrequency (RF) pulse shapes for NMR spectroscopy that can not only excite, but also efficiently invert and refocus nuclei with broad chemical-shift dispersions. Ideally, these new pulses should provide broad irradiation bandwidth, high operational fidelity, and superior inhomogeneity compensation. While classical hard pulses are inadequate to meet these requirements, broadband and ultra-broadband pulses are necessary for precise spin manipulations. By leveraging a combination of evolutionary algorithms and artificial intelligence, we have designed ultra-low-power RF class-B2 pulses capable of performing universal π/2 and π flipping operations with high fidelity across a 1 MHz bandwidth. Specifically, the bandwidth-to-RF amplitude (ΔΩ/νrf) ratio of these pulses is ∼40, with an average fidelity greater than 99% over the entire bandwidth. These values are significantly beyond those obtained with previously reported pulses designed for universal rotations. We tested these new pulses with a spin-echo sequence to observe the 1D 19F spectrum of a mixture of fluorinated organic compounds. The pulses developed here are particularly well-suited for NMR spectroscopy of nuclei exhibiting large chemical-shift dispersions and can be used to circumvent the power-handling limitations of current cryogenic probes.