ORTHOHYDROGEN AND PARAHYDROGEN INDUCED NUCLEAR-SPIN POLARIZATION

Emission and enhanced absorption lines occur in the NMR spectra record ed during homogeneous hydrogenation reactions, if enriched fractions o f the spin isomers of dihydrogen are used, namely either parahydrogen or orthohydrogen. As such the spectra resemble those resulting from th e Chemically Induced Dynamic Nuclear Polarization (CIDNP) phenomenon, which is caused by free radical intermediates. Here, however, a change of the symmetry due to the hydrogenation causes seemingly similar eff ects, even though no free radical intermediates occur. The phenomenon has been named PASADENA or PHIP (ParaHydrogen Induced Polarization). T he associated signal enhancement of PHIP over conventional NMR spectro scopy can reach values of up to 10(5). An apparatus is described, whic h provides for a continuous flow of enriched parahydrogen (51%) at pre ssures up to 20 bar, and for batches of enriched orthohydrogen (85%) a t ambient pressure. Liquid nitrogen is used as a coolant in either cas e. Integrated thermal conductivity cells allow a quantitative determin ation of the corresponding ortho/para ratios of the hydrogen. The enri ched ortho- or parahydrogen has been used for in situ NMR studies of h ydrogenation reactions using specially designed probes which allow inv estigations of gas/liquid reactions under continuous now conditions. T he same apparatus has also been used successfully to enrich ortho- and paradeuterium mixtures. The examples investigated include hydrogenati ons of various alkynes and alkenes at atmospheric and elevated pressur e using rhodium catalysts. The PHIP spectra of simple two-spin, reacti on products, for example those derived from diacetylenes, can easily b e 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 a ccount both the cross-relaxation processes based upon the Nuclear Over hauser Effect (NOE) and singlet/triplet mixing of the nuclear spin sta tes along the reaction coordinate. This option provides for a time sca le, which allows to determine the lifetime of intermediate dihydrogen complexes, and, furthermore, it is useful for both the analysis and th e simulation of the expected spectra in advance. The current version o f this computer program accommodates up to 9 spins on a personal compu ter and up to 10 on either a workstation or a main-frame computer.