Parahydrogen and synthesis allow dramatically enhanced nuclear alignment

Abstract

The PASADENA effect is a method for transient high-sensitivity proton spin-labelling by molecular addition of dihydrogen. When the parahydrogen mole fraction differs from the high-temperature limit of 1/4, this population difference constitutes a form of spin order which can be converted to magnetization observable by NMR. Large NMR signals are observed, if subsequent to the hydrogen addition, the two protons experience magnetic inequivalence and spin-spin coupling and if observation is made before spin-lattice relaxation restores the equilibrium spin order. The analogous effect for D2 is also possible. The kinetic mechanisms of the homogeneous hydrogenation catalysts which permit the realization of the PASADENA effect have been the target of the experimental applications. The enhancement of the NMR transitions has facilitated the determination of true molecular rate constants. Ordinarily, the activity of a catalyst is assessed by dividing the observed rate by the total catalyst concentration. However, the question as to whether most of the catalytic rate is due to a tiny fraction of active species or a large fraction with a relatively low molecular rate is not clearly addressed by such an analysis. This ambiguity is entirely avoided in the PASADENA studies, since only active catalyst molecules can contribute to the enhanced signals from which all kinetic inferences are made. The sensitivity enhancement has also led to the identification of a novel intermediate in the mechanism for the Rh(DIPHOS)+ catalyzed hydrogenation of styrene. The rate of conversion of this species into product and starting material has been studied using two-dimensional NMR. The dramatically improved sensitivity should make it possible to observe key catalytic intermediates which do not build up in sufficient quantity to allow detection by conventional NMR arising from Curie-Law magnetization. The study of surface sites which bind pairwise with H2 is also a potentially fruitful area for future experimental work. The ambient temperature NMR spectroscopy of surfaces is not often feasible due to sensitivity limitations. Simulations have been performed using typical shift and coupling parameters in an effort to characterize the enhanced lineshapes which can be expected. The inverse of the PASADENA effect has also been proposed, whereby the spin order of a molecule containing hydrogen is probed by measuring the branching ratio to ortho and para dihydrogen. This RAYMOND phenomenon (radiowave application yields modulated ortho number desorbed) has the potential for measuring precursor NMR with extraordinary sensitivity, since it finesses the need for detection of radiowaves.