Gas-Phase NMR of Hyperpolarized Propane with H-1-to-C-13 Polarization Transfer by PH-INEPT

In this work we demonstrate the possibility to transfer parahydrogen-derived H-1 polarization to C-13 nuclei in the gas phase using PH-INEPT-based pulse sequences. The propane with hyperpolarized H-1 nuclei was produced via hydrogenation of propylene (at natural C-13 abundance) with parahydrogen over the heterogeneous 1 wt% Rh/TiO2 catalyst at 7.05 T magnetic field of a NMR spectrometer. The apparent proton polarization was estimated as 1.8 +/- 0.4%, taking into account the polarization losses caused by spin relaxation. The optimal inter-pulse delays for both the PH-INEPT and the PH-INEPT + sequences were determined via the numerical calculations considering the full spin system of propane which includes eight protons and one C-13 nucleus. The application of the optimized PH-INEPT polarization transfer sequence resulted in the C-13 polarization values of 0.07 +/- 0.01% and 0.030 +/- 0.006% for the methyl group of [1-C-13]propane and the methylene group of [2-C-13]propane, respectively. The experimental dependence of the C-13 polarization values for [1-C-13] propane and [2-C-13]propane on the inter-pulse delay tau(1) of the PH-INEPT sequence is in a good agreement with the simulation. The resulting C-13 polarization using PH-INEPT + sequence is similar to 2.5 times lower than that via PH-INEPT, which is also consistent with the numerical calculations.