Molybdenum spectra simulations

Figure 8.36. A selection of Mo NMR spectra of molybdenum compounds. A. Observed MAS spectrum of M0O3 (lower) with simulation (upper). B. Powder lineshapes of the central transition of MoSct (upper) and M0S2 (lower). C. Observed powder lineshape of the central transition of M02C (lower) with the simulation (upper). From Bastow (1998), by permission...

Various kinds of the theoretical spectroscopies for the transition metal complexes were also reviewed. For the excitation spectrum of Cr02Cl2, the SAC-Cl method simulated accurate spectrum. For tetraoxo metal complexes, the systematic studies explained the spectral differences when the central metal was substituted. In the analysis of the NMR chemical shift, not only the optically allowed states but also the magnetically allowed states are important. In the molybdenum complexes, the inverse of the d-d excitation energy is proportional to the experimental chemical shift. The photofragmentation reaction of Ni(CO)4 was also studied and the reaction mechanism was clarified. 

The magnitude of the simulation problem becomes apparent when discussing the lineshape of the Mo7024 spectrum. In this modest cluster there are three distinct molybdenum sites in the ratio of 4 2 1, and each site requires eight parameters. Further, if the relative number of sites per cluster is also included in the analysis of the lineshape, the problem becomes a constrained 26-parameter fit. There is no doubt the lineshape can be fitted and the NMR parameters... 

These could consist of a whole range of Mo(VI) and Mo(IV) mixed oxygen/ sulfur compounds. There is no evidence for the presence of MoOi, except in one case. Simulation of the static powder lineshapes allowed the deconvolution of the various components, thus yielding values of quadrupole parameters as well as relative intensity data. From the spikelet experiments it was ascertained that all species are present as both static adsorbed and dynamically active phases. The Mo spectrum of the used catalyst shows the presence of the tetrahedral molybdenum-oxo species, along with a much reduced MoSt resonance (relative to the fresh system), perhaps suggesting that MoSt is the active site in the... 

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