Quantum mechanical exchange of hydrides in solution: proton-proton exchange couplings in transition-metal polyhydrides

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Journal of the American Chemical Society




Several groups have recently reported observation of unusually large apparent J couplings between protons in transition-metal trihydrides. The observation that these couplings are not present between 'H and 'T nuclei in partially tritiated isotopomers leads us to propose that these couplings are not magnetic in origin. They are in fact exchange couplings between protons in direct analogy to exchange couplings observed between electrons in radical pairs or between 'He atoms in solid 'He. This is a rather unexpected manifestation of quantum mechanical motion in these hydrides as it has been observed in solution at ambient temperatures. This paper presents in detail a simple theory that quantitatively predicts the size and pronounced temperature dependence of these couplings. The theory also explains the extreme secondary isotope effects observed in these couplings. It is proposed that these couplings should be observed in systems other than trihydrides. Furthermore these couplings are likely to be responsible for the appearance of some polyhydrides to be fluxional on the NMR time scale even at the lowest attainable temperatures in solution.