NMR and INS line shapes of transition metal hydrides in the presence of coherent and incoherent dihydrogen exchange

Hans Heinrich Limbach, Stefan Ulrich, Stephan Gründemann, Gerd Buntkowsky, Sylviane Sabo-Etienne, Bruno Chaudret, Gregory J. Kubas, Jürgen Eckert

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Abstract

In this paper a unified description of the effects of the coherent and incoherent dihydrogen exchange on the NMR and INS spectra of transition metal hydrides based on the quantum-mechanical density matrix formalism of Alexander-Binsch is proposed. The dynamic parameters of the line shape analyses are the exchange couplings or rotational tunnel splittings J of the coherent exchange and the rate constants k of the incoherent exchange. As experimental examples, we report the temperature dependent values J and k for 1 ≡ Cp*RuH3(PCy3) (Cp* ≡ C5(CH3)5 and Cy ≡ cyclohexyl) including the kinetic HH/HD/DD isotope effects on the incoherent exchange, determined by NMR, and for the tungsten dihydrogen complex 2 ≡ W(PCy3)2 (CO)3-(η- H2), determined by INS. The temperature dependence of J and k is interpreted qualitatively in terms of a simple reaction scheme involving at each temperature a ground state and a dominant ro-vibrationally excited state. Using formal kinetics it is shown that a coherent exchange in the excited state contributes to J only if this exchange presents the rate limiting reaction step, i.e., if vibrational deactivation is fast. This is the case for levels located substantially below the top of the barrier. A very fast coherent exchange of levels located close to the top of the barrier contributes only to k. This result reproduces in a simple way the quantum- mechanical results of Szymanski, S. J. Chem. Phys. 1996, 104, 8216 and Scheurer, C.; Wiedenbruch, R.; Meyer, R.; Ernst, R. R. J. Chem. Phys. 1997, 106, 1. The results concerning the coherent and incoherent exchange processes in 1 and 2 are discussed in terms of the simplified reaction model.

Original languageEnglish
Pages (from-to)7929-7943
Number of pages15
JournalJournal of the American Chemical Society
Volume120
Issue number31
DOIs
StatePublished - Aug 12 1998

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