Quantum Exchange Couplings in Metal Hydride NMR

In 1998, apart from the already quoted review, concentrated on the relevant NMR theories, a review focused on the electronic structure and chemistry of the metal hydrides exhibiting the QEC effect was published Developments in modelling of the effect by quantum chemical calculations were also reviewed In the present survey, only the aspects directly related to manifestations of the QEC effect in NMR spectra are covered. A review of the recent results will be preceded by a brief recapitulation of the main developments in the past research on the metal hydrides. 

The basic structures which can be of interest in the discussion of the QEC effect are shown in Fig. 1. In the trihydrides, the hydride nuclei are positioned at the vertices of an isosceles triangle the distance between the central and sideward positions are about 

Fig-1- Basic structures of the metal hydride complexes exhibiting the QEC effect, (a) Trihydride, (b) cA-dihydride, (c) dihydrogen, and (d) hydrido dihydrogen complexes. The symbol Cp denotes variously substituted cyclopentadiene anions. 

As was already mentioned, for a proton parr the term ttAP can be replaced with the /-coupling term for a pair of deuterons (spin-l), that term (no longer expressible 

For an iridium trihydride, the temperature behaviour of the long-lived quantum coherence described above was modelled theoretically under the simplifying assumption that the relevant dynamics of either of the two exchanging hydride pairs can be described in terms of a periodic two-well potential.Excellent agreement with the experimental k(T) and A(7) curves was obtained. 

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