A dynamic model of reaction pathway effects on parahydrogen-induced nuclear spin polarization

A dynamic model for the calculation of parahydrogen (p-H-2) induced nuclear polarization (PHIP) of hydrogenation products is described which is based on the density matrix formalism. This formalism was proposed previously by Binsch for the calculation of NMR spectra broadened by chemical exchange between different sites. Using numerical simulations typical for actual experiments, it is shown that the PHIP patterns may depend not only on the type of experiment performed - e.g. ALTADENA (adiabatic longitudinal transport after dissociation engenders net alignment) in the absence and PASADENA (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) in the presence of a magnetic field - but also on the pathways of the hydrogenation reaction, in which generally transition metal catalysts are involved. Of particular importance are the properties of possible reaction intermediates where the reactants are complexed alone or together to the catalyst. Indirect information from the PHIP pattern of the hydrogenation products on the intermediate can be obtained, in particular its chemical shifts, exchange and magnetic couplings, and the incoherent dihydrogen self-exchange. In addition, the regioselectivity of the hydrogenation step is a factor influencing the PHIP patterns. The model and the results obtained here provide, therefore, a theoretical link between various phenomena concerning the hydrogen mobility in transition metal catalysts, and PHIP is shown to be a valuable tool for obtaining information on the reaction intermediates.