The 1 H NMR signal of dissolved molecular hydrogen enriched in parahydrogen (p-H 2 ) exhibits in the presence of an organometallic hydrogenation catalyst an unusual, partially negative line shape (PNL). It results from a strongly enhanced two-spin order connected to the population of the 0 T level of orthohydrogen (o-H 2 ). This two-spin order is made visible by a slow asymmetric exchange process between free hydrogen and a transient catalyst-hydrogen complex. By Only Parahydrogen Spectroscopy (OPSY) it is possible to selectively detect the two-spin order and suppress the signal from the thermal o-H 2 . The intensity of the PNL can be strongly affected by the PArtially NEgative Line (PANEL) experiment, which irradiates a long narrow-band radio frequency (RF) pulse. When the RF-frequency is in resonance with the chemical shift values of the hydrogen bound to the elusive catalyst or of the free hydrogen, a strong intensity reduction of the PNL is observed. Numerical simulations of the experiments performed at 500 MHz and 700 MHz proton frequency show that the indirect detection has at least three orders of magnitude higher sensitivity than the normal NMR experiment. A theoretical model, including reversible binding and 0 S T − evolution, is developed, which reproduces the NMR line-shape, the nutation angle dependence and the dependence on the frequency of the irradiation field of the PNL and permits the determination of the proton chemical shift values and the sign of the scalar coupling in the transient NMR invisible complex where singlet-triplet conversion take place.

For the first time, chemical exchange saturation transfer (CEST) nuclear magnetic resonance (NMR) is utilized to study short-lived hydride intermediates in the catalytic cycle of an organometallic complex [Ir(IMes)(Py) 3 (H) 2 ]Cl. These complexes are typically not observable by other NMR techniques because they are low concentrated and undergo reversible ligand exchange with the main complex. The intermediate complexes [Ir(Cl)(IMes)(Py) 2 (H) 2 ] and [Ir(CD 3 OD)(IMes)(Py) 2 (H) 2 ] are detected, assigned, and characterized in solution, in situ and at room temperature. Understanding the spin dynamics in these complexes is necessary for enhancing the performance of the nuclear spin hyperpolarization technique signal amplification by reversible exchange. By eliminating [Ir(Cl)(IMes)(Py) 2 (H) 2 ] and manipulating the spin system by radiofrequency irradiation, the nuclear spin singlet lifetime of the hydride protons was increased by more than an order of magnitude, from 2.2 ± 0.1 to 27.2 ± 1.2 s. Because of its simplicity and ability to unravel unobservable chemical species, the utilized CEST NMR approach has a large application potential for studying short-lived hydride intermediates in catalytic reactions.

A series of novel bioactive derivatives of the sunflower trypsin inhibitor-1 (SFTI-1) suitable for hyperpolarization by parahydrogen-induced polarization (PHIP) was developed. The PHIP activity was achieved by labeling with L-propargylglycine, O-propargyl-L-tyrosine, or 4-pentynoic acid. (1) H NMR signal enhancements (SE) of up to a factor of 70 were achieved in aqueous solution. We found that an isolated spatial location of the triple bond within the respective label and its accessibility for the hydrogenation catalyst are essential factors for the degree of signal enhancement.

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Fluorinated substances are important in chemistry, industry, and the life sciences. In a new approach, parahydrogen-induced polarization (PHIP) is applied to enhance (19)F MR signals of (perfluoro-n-hexyl)ethene and (perfluoro-n-hexyl)ethane. Unexpectedly, the end-standing CF3 group exhibits the highest amount of polarization despite the negligible coupling to the added protons. To clarify this non-intuitive distribution of polarization, signal enhancements in deuterated chloroform and acetone were compared and (19)F-(19)F NOESY spectra, as well as (19)F T1 values were measured by NMR spectroscopy. By using the well separated and enhanced signal of the CF3 group, first (19)F MR images of hyperpolarized linear semifluorinated alkenes were recorded.