Ortho- and Parahydrogen Induced Nuclear Spin Polarization


Emission and enhanced absorption lines occur in the NMR spectra recorded during homogeneous hydrogénation reactions, if enriched fractions of the spin isomers of dihydrogen are used, namely either parahydrogen or orthohydrogen. As such the spectra resemble those resulting from the Chemically Induced Dynamic Nuclear Polarization (CIDNP) phenomenon, which is caused by free radical intermediates. Here, however, a change of the symmetry due to the hydrogénation causes seemingly similar effects, even though no free radical intermediates occur. The phenomenon has been named PASADENA or PHIP (ParaHydrogen Induced Polarization).The associated signal enhancement of PHIP over conventional NMR spectroscopy can reach values of up to 105. An apparatus is described, which provides for a continuous flow of enriched parahydrogen (51%) at pressures up to 20 bar, and for batches of enriched orthohydrogen (85%) at ambient pressure. Liquid nitrogen is used as a coolant in either case. Integrated thermal conductivity cells allow a quantitative determination of the corresponding ortho/ para ratios of the hydrogen. The enriched ortho- or parahydrogen has been used for in situ NMR studies of hydrogénation reactions using specially designed probes which allow investigations of gas/liquid reactions under continuous flow conditions. The same apparatus has also been used successfully to enrich ortho-and paradeuterium mixtures. The examples investigated include hydrogénations of various alkynes and alkenes at atmospheric and elevated pressure using rhodium catalysts. The PHIP spectra of simple two-spin reaction products, for example those derived from diacetylenes, can easily be interpreted. Spectra of products containing three or more spins are frequently complicated by cross-relaxation phenomena; therefore, they are better analyzed using a computer based routine, which takes into account both the cross-relaxation processes based upon the Nuclear Overhauser Effect (NOE) and singlet/triplet mixing of the nuclear spin states along the reaction coordinate. This option provides for a time scale, which allows to determine the lifetime of intermediate dihydrogen complexes, and, furthermore, it is useful for both the analysis and the simulation of the expected spectra in advance. The current version of this computer program accommodates up to 9 spins on a personal computer and up to 10 on either a workstation or a main-frame computer.



Keywords and Phrases

CIDNP, PHIP, PASADENA; Hydrogénation reactions; Para- and orthohydrogen

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Article - Journal

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