Progress in Hyperpolarized Ultrafast 2D NMR Spectroscopy

Abstract

An important development in the field of NMR spectroscopy has been the advent of hyperpolarization approaches, capable of yielding nuclear spin states whose value exceeds by orders-of-magnitude what even the highest-field spectrometers can afford under Boltzmann equilibrium. Included among these methods is an ex situ dynamic nuclear polarization (DNP) approach, which yields liquid-phase samples possessing spin polarizations of up to 50 %. Although capable of providing an NMR sensitivity equivalent to the averaging of about 1 000 000 scans, this methodology is constrained to extract its "superspectrum" within a single-or at most a few-transients. This makes it a poor starting point for conventional 2D NMR acquisition experiments, which require a large number of scans that are identical to one another except for the increment of a certain t(1) delay. It has been recently suggested that by merging this ex situ DNP approach with spatially encoded "ultrafast" methods, a suitable starting point could arise for the acquisition of 2D spectra on hyperpolarized liquids. Herein, we describe the experimental principles, potential features, and current limitations of such integration between the two methodologies. For a variety of small molecules, these new hyperpolarized ultrafast experiments can, for equivalent overall durations, provide heteronuclear correlation spectra at significantly lower concentrations than those currently achievable by conventional 2D NMR acquisitions. A variety of challenges still remain to be solved before bringing the full potential of this new integrated 2D NMR approach to fruition; these outstanding issues are discussed.