Quantum exchange coupling

Thus for spin 1=1/2 nuclei, the quantum mechanical exchange interaction is formally equivalent to an indirect spin-spin interaction. Accordingly the exchange of spin-1/2 particle is usually treated in NMR like a J-coupling (quantum exchange coupling), employing the more simple Hamiltonian... 

For three other systems studied, the 2D model also gives excellent results that compare well with experiment, e.g., dm in [FeH(H2)(dppe)2]+ is 0.82 A by neutron diffraction and is 0.822 A with the 2D model. However, it is only useful if a sufficient number of INS transitions are observed. While instrumental limitations confine the observation of the tunneling transition to H2 complexes with a rotational barrier of less than 3 kcal/mol, transitions to the torsional states can in principle be observed for any such compound. The 2D methodology could then be applied to any experiment in which the dynamic process involves permutation of two identical particles, e.g., the quantum exchange coupling phenomenon (see Section 6.3.4). 

Chaudret, Limbach and coworkers [13] have recently shown that dihydrogen bonding effects can alter the quantum exchange coupling observed in the NMR spectrum of [Cp (PCy3)RuHj]. 

Quantum exchange coupling in transition-metal polyhydride... 

Abnormal phenomena have been observed afterwards in a number of transition metal polyhydrides, and most of them are collected in the review by Sabo-Etienne and Chaudret [22]. Extensive experimental work on transition metal hydride complexes showing the so-called quantum exchange coupling led to the systematisation of several trends. The magnitude of the coupling was found to be a sensitive function of the ligands attached to the metal, the nature of the metal and the charge of the complex. 

The advantage of the Landesmann s model is that it provides an analytical solution from which some trends of the quantum exchange coupling process can be analysed. However, the model is much too simple to describe correctly many related subtle effects like, for instance, the nature of the transition metal, the ligands or the isotopic effects. Moreover, the values of the parameters involved in Eqs 12 and 13 have to be assumed or fitted a posteriori to the experimental... 

In what follows we will analyse the effect of different factors on the size of the quantum exchange coupling ligand substitution, temperature, isotopic substitution, change of the transition metal and the addition of a Lewis acid. 

Figure 10. Dependence of the quantum exchange coupling on the barrier height...

Figure 12. Temperature dependence of the quantum exchange coupling for the [CpIr(CO)H3] complexes (a) L = PH. (b) L = CO...

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