Proton Tunnelling Effects in Metal Hydride NMR

Abstract

The discovery of quantum exchange couplings in metal hydrides seems to be one of the most intriguing findings in the general area of liquid-phase nuclear magnetic resonance (NMR). This chapter describes a survey of experimental data, basic principles of exchange coupling, models of temperature effects on exchange coupling, ab initio calculations, and consistent treatment of coherent and incoherent effects. Quantum–chemical studies suggest that regardless of formal structure of the valence shell, a similar tunnelling mechanism is operative in trihydrides of various transition metals, and an extreme correlation was assumed among the individual hydrides; the spatial motions were considered in regard to the hydride pair as a whole. More studies are necessary to check whether the same tunnelling mechanism, involving a dihydrogen-like structure, is valid also for the adducts of ruthenium and niobium hydrides with Lewis acids. Numerical simulations of single-crystal NMR spectra of methyl-like quantum rotors reveal that for appropriate model systems, nonclassical stochastic behavior could be detected experimentally. The chapter indicates that verification of this theoretical inference may be a challenging task for future experimental studies on quantum tunnelling in systems of several like nuclei.