Design and application of robust rf pulses for toroid cavity NMR spectroscopy

TL;DR

This paper develops robust RF pulses for toroid cavity NMR spectroscopy, improving tolerance to B1 inhomogeneity and enabling advanced 2D NMR experiments with enhanced performance and shorter pulse durations.

Contribution

It introduces optimized RF pulses using GRAPE and OP algorithms, including a new class of non-adiabatic pulses, for improved NMR performance in toroid cavities.

Findings

Robust RF pulses tolerate sixfold B1 inhomogeneity.

First 2D NMR spectroscopy with a toroid probe demonstrated.

Ultra-short pulses achieved with acceptable performance.

Abstract

We present robust radio frequency (rf) pulses that tolerate a factor of six inhomogeneity in the B1 field, significantly enhancing the potential of toroid cavity resonators for NMR spectroscopic applications. Both point-to-point (PP) and unitary rotation (UR) pulses were optimized for excitation, inversion, and refocusing using the gradient ascent pulse engineering (GRAPE) algorithm based on optimal control theory. In addition, the optimized parameterization (OP) algorithm applied to the adiabatic BIR-4 UR pulse scheme enabled ultra-short (50 microsec) pulses with acceptable performance compared to standard implementations. OP also discovered a new class of non-adiabatic pulse shapes with improved performance within the BIR-4 framework. However, none of the OP-BIR4 pulses are competitive with the more generally optimized UR pulses. The advantages of the new pulses are demonstrated in simulations and experiments. In particular, the DQF COSY result presented here represents the first implementation of 2D NMR spectroscopy using a toroid probe.