Catalytically Enhanced NMR of Heterogeneously Catalyzed Hydrogenations

Vladimir V. Zhivonitko, Kirill V. Kovtunov, Ivan V. Skovpin, Danila A. Barskiy, Oleg G. Salnikov, and Igor V. Koptyug 

Understanding OrganometalUc Reaction Mechanisms and Catalysis Computational and Experimental Tools, First Edition. Edited by Valentine P. Ananikov. 

In what follows, the use of the PHIP technique in the studies of homogeneous catalytic processes such as hydrogenation and activation of Hj by transition metal complexes and clusters is briefly reviewed first to demonstrate the potential information content of such studies. Then, applications of PHIP to the studies of heterogeneous hydrogenations catalyzed by metal complexes immobilized on solid supports and by supported metal catalysts are discussed. 

Simple as it may seem, molecular hydrogen (H2) is in fact a sophisticated quantum-mechanical construct. In most molecules, the effects of the nuclear spin on molecular properties are negligible and thus irrelevant. This is not the case, however, for H2 and some other small symmetric molecules such as H2O, H2CO, NH3, CH3F, and so on, because of their symmetry coupled with the fundamental laws of quantum mechanics [16,17]. 

the two nuclear spins of the H atoms can be combined in two different ways, resulting in the molecular species with the total nuclear spin of the two hydrogens of / = 1 or / = 0. In the ground electronic and vibrational state, each H2 molecule can be characterized by a combination of a certain rotational state with a certain nuclear spin state. As protons are fermions, Pauli s principle requires that the total wave function of a molecule should be antisymmetric with respect to the permutation of the two nuclei. The rotational states with even values of the rotational quantum number / are symmetric with respect to such permutations, including the rotational state with the lowest energy, which has / = 0. Such rotational states can be combined only with the antisymmetric nuclear spin state with 1 = 0, and such combinations correspond to parahydrogen (PH2). All rotational states with odd / values are antisymmetric and are only allowed in combination with the symmetric (/ = 1) nuclear spin states. These combinations correspond to orthohydrogen (oH2). At the same time, H2 molecules with even-even and odd-odd combinations of the two quantum numbers do not exist. As a result. 

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