Parahydrogen Allows Ultrasensitive Indirect NMR Detection of Catalytic Hydrogen Complexes

The ¹H NMR signal of dissolved molecular hydrogen enriched in parahydrogen (p-H₂) 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 T₀ level of orthohydrogen (o-H₂). 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-H₂. 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 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 and 700 MHz proton frequency show that the indirect detection has at least 3 orders of magnitude higher sensitivity than the normal NMR experiment. A theoretical model, including reversible binding and S - T₀ 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.