Low‐temperature dynamic nuclear polarization of gases in Frozen mixtures

Abstract

Purpose To present a new cryogenic technique for preparing gaseous compounds in solid mixtures for polarization using dynamic nuclear polarization (DNP). Methods 129Xe and 15N2O samples were prepared using the presented method. Samples were hyperpolarized at 1.42K at 5T. 129Xe was polarized at 1.65K and 1.42K to compare enhancement. Polarization levels for both samples and T1 relaxation times for the 129Xe sample were measured. Sample pulverization for the 129Xe and controlled annealing for both samples were introduced as additional steps in sample preparation. Results Enhancement increased by 15% due to a temperature drop from 1.65K to 1.42K for the 129Xe sample. A polarization level of 20±3% for the 129Xe sample was achieved, a 2-fold increase from 10±1% after pulverization of the sample at 1.42K. T1 of the 129Xe sample was increased by more than 3-fold via annealing. In the case of 15N2O, annealing led to a ~2-fold increase in the signal level after DNP. Conclusion The presented technique for producing and manipulating solid gas/glassing agent/radical mixtures for DNP led to high polarization levels in 129Xe and 15N2O samples. These methods show potential for polarizing other gases using DNP technology.

Polyolefin catalysis of propene, 1-butene and isobutene monitored using hyperpolarized NMR

“…For determining signal enhancements of hyperpolarized olefin carbons, 13 C NMR spectra of the same samples were acquired after hyperpolarization had decayed. The same pulse sequence as in the timeresolved hyperpolarized NMR measurements was used, except that n = 1.…”

“…Of the same samples, 1 H NMR spectra were further obtained. The intensity of the thermally polarized olefin 13 C signal was first estimated based on the intensity of thermally polarized 13 C signal of the solvent and the ratio of olefin to solvent concentration determined by 1 H NMR. Chemical shifts were referenced to the solvent resonance of toluene, which was calibrated against tetramethylsilane (TMS) using a separate sample.…”

“…2,3 This spectroscopic method is especially well suited for the determination of chemical structure. Chemical shifts such as of 13 C are sensitive indicators of motifs of stereo structure in polymers. 4,5 In combination with synthetic methods for isotope incorporation, NMR spectroscopy further presents a means for the determination of reaction mechanisms.…”

“…A significant drawback in the application of NMR spectroscopy in this context, where signal averaging is precluded, however arises from low sensitivity. Authors of this paper have previously shown that hyperpolarization of nuclear spins enables 13 C real-time NMR for the characterization of kinetics and mechanisms of chemical reactions including polymerization. [7][8][9] Dissolution dynamic nuclear polarization (D-DNP) enhances NMR signals of small molecules typically by several thousand-fold.…”

“…Previous studies have nevertheless demonstrated the DNP hyperpolarization of gases in frozen mixtures. These include 129 Xe and 15 N 2 O, [11][12][13] both of which have biomedical significance. Ethene was further hyperpolarized in the solid state in mesoporous silica matrices, however without subsequent dissolution and use of the hyperpolarized product.…”