Coherent D2 rotational tunneling and incoherent D2 dynamics in a solid non-classical RuD2 complex studied by 2H solid state NMR spectroscopy

Abstract

The ²H solid state NMR spectra and T₁ relaxation data of a transition metal η²-dideuterium complex, namely trans-[Ru(D₂)Cl(PPh₂CH₂CH₂PPh₂)₂]PF₆, have been measured in the temperature regime from 5.4 to 320 K. In the Ru–D₂ sample, coherent and incoherent exchange processes on the time scale of the quadrupolar interaction have been found, leading at low temperatures to a tunnel splitting of the ²H NMR spectrum. With increasing temperature a slight increase of the tunnel splitting is observed, in conjunction with a strong increase of the incoherent exchange process, which finally, at temperatures above 20 K, destroys the tunnel splitting and determines the spectral line shape. For the description of the experimental spectra a Liouville formulation of the Alexander–Binsch NMR line shape theory, adapted for exchanging deuterons, is employed. It is shown that the whole evaluation of the ²H magnetization takes place in four 2D and two 4D subspaces of the 81D Liouville space, leading to a drastic simplification of the numerical efforts in the simulation of the spectral line shapes. The height of the tunnel barrier calculated from the value of the tunnel splitting is 270 meV (6.22 kcal mol^-1). The incoherent exchange rates extracted from the spectra and from T₁ relaxation data are analyzed in terms of a Bell tunneling model, with a temperature dependent effective potential.