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New NMR Techniques for the Study of Catalysis
Published in Alexis T. Bell, Alexander Pines, NMR Techniques in Catalysis, 2020
Waclaw Kolodziejski, Jacek Klinowski
Bowers and Weitekamp predicted [169] and then experimentally demonstrated [170] a method, which they termed the PASADENA effect, of obtaining very large nuclear spin polarizations on molecules formed by addition of para-hydrogen such that the dihydrogen protons become magnetically inequivalent. The principle of the method is as follows. Para- and ortho- forms of molecular hydrogen differ from one another in their nuclear spin states. A consequence of the symmetrization postulate of quantum mechanics is that these two forms are associated only with specific states of molecular rotation. The nuclear spin order of parahydrogen can be converted by chemical reaction into large nonequlibrium NMR signals. The PASADENA effect was first demonstrated using the reaction of hydrogenation of acrylonitrile, CH2CHCN, to proprionitrile, CH3CH2CN, catalyzed by Wilkinson’s catalyst at ambient temperature and pressure [170]. Large transient 1H NMR signals were observed both in proprionitrile transitions and in the hydride region of the hydrogenated catalyst. The amplitude of these lines was at least two orders of magnitude greater than would result from the equilibrium magnetization of the product formed. The advantages of the technique, which is of obvious interest to heterogeneous catalysis, where the concentrations of the product are often small, are its great sensitivity and the fact that the products can be monitored in a time-resolved fashion. The PASADENA effect has been used to study the adsorption of H2 on the surface of zinc oxide [171].
Perspective on long-lived nuclear spin states
Published in Molecular Physics, 2020
Hyperpolarised spin order can be prepared efficiently and with comparatively simple instrumentation for dihydrogen gas. Dihydrogen exists as two spin isomers, ortho and para, corresponding to the three symmetric (triplet) states and the antisymmetric (singlet) state of the spin pair. The existence of spin isomers is due to the Pauli principle and is only observed for small and highly symmetric molecules. For dihydrogen, the ortho and para forms interconvert efficiently when the gas is in contact with a paramagnetic solid, but an out-of-equilibrium ortho/para distribution can otherwise be retained for several days. At room temperature, the four energy levels are nearly equally populated, resulting in a 75:25 ortho:para distribution. ‘Para-hydrogen gas’ (dihydrogen gas with a non equilibrium ortho:para distribution) is prepared by flowing dihydrogen gas at temperatures of 20 to 80 K, in the presence of a paramagnetic solid particles. Parahydrogen may be seen as a reservoir of long-lived singlet order. While parahydrogen is NMR silent and is unaffected by pulse sequences, its spin order can be released by a chemical addition. The addition is either directly onto a substrate of interest [58] (the PHIP mechanism, parahydrogen induced polarisation), in which case the enhanced spin order on the substrate is not renewable, or onto a metal centre on which a substrate is bound reversibly, in which case through-bond spin order transfer to the substrate spins can occur multiple times over several complex formations [59] (the SABRE mechanism, signal amplification by reversible exchange).