NMR SLIC Sensing of Hydrogenation Reactions Using Parahydrogen in Low Magnetic Fields

Abstract

Parahydrogen-induced polarization (PHIP) is an NMR hyperpolarization technique that increases nuclear spin polarization by orders of magnitude, and it is particularly well-suited to study hydrogenation reactions. However, the use of high-field NMR spectroscopy is not always possible, especially in the context of potential industrial-scale reactor applications. On the other hand, the direct low-field NMR detection of reaction products with enhanced nuclear spin polarization is challenging due to near complete signal cancellation from nascent parahydrogen protons. We show that hydrogenation products prepared by PHIP can be irradiated with weak (on the order of spin–spin couplings of a few hertz) alternating magnetic field (called Spin-Lock Induced Crossing or SLIC) and consequently efficiently detected at low magnetic field (e.g., 0.05 T used here) using examples of several types of organic molecules containing a vinyl moiety. The detected hyperpolarized signals from several reaction products at tens of millimolar concentrations were enhanced by 10000-fold, producing NMR signals an order of magnitude greater than the background signal from protonated solvents.

NMR Spin-Lock Induced Crossing (SLIC) dispersion and long-lived spin states of gaseous propane at low magnetic field (0.05 T)

“…More than a factor of 14 difference in signal intensities may be partially explained by a non-ideal performance of the SLIC pulse, caused by inhomogeneity of B 1 and B 0 fields (most critical factor) and relaxation/decoherence during the long RF SLIC excitation pulse (0.5 s). 33 More importantly, these experiments were performed in a continuous flow mode, where gaseous propane was flowing through the NMR tube while the NMR signal was being acquired. As a result, a significant fraction of the produced HP propane might have migrated into or out of the RF coil during the SLIC pulse.…”

“…The first experimental setup was similar to that described earlier. 33 The magnet (∼2 MHz Magritek rock core analyzer, Halbach array, radial field direction) had homogeneity of ∼20 ppm over 4 cm diameter of spherical volume (DSV). A commercially available MR Kea2 spectrometer (Magritek, Wellington, New Zealand) was used for NMR detection as described by Waddell et al 51 The detection chamber (plastic, ∼2 mL) was placed in the home-built 1 H RF coil located in the magnet.…”

“…Furthermore, in addition to biomedical applications, the combination of hyperpolarization and low-field detection can be also potentially useful for spectroscopic and imaging analysis of industrial-scale processes, where overpopulated pseudo-singlet spin states are conveniently created using chemical reactions involving parahydrogen. 33 …”

“…35 The p -H 2 -induced singlet spin order can be converted into observable magnetization using various singlet-to-magnetization (S2M) transfer algorithms, 36 including spin-lock induced crossing, 37-39 and conveniently detected directly at low field. 33 …”

Coherent evolution of singlet spin states in PHOTO-PHIP and M2S experiments

“…However, one can transfer singlet spin population to magnetization (S2M) using techniques well described in the literature. [33][34][35] Here we choose the adiabatic passage spin-order conversion technique (APSOC) 23 because of its generality and robustness in application. The experimental photo-PHIP protocol with APSOC modification is shown in Fig.…”