Computational study of the vibrational spectra of alpha- and beta-Keggin polyoxometalates

The structures and vibrational frequencies of the alpha- and beta-isomers of the phosphomolybdate Keggin anion [PMo(12)O(40)](3-) have been calculated by using density functional theory. Good agreement between the calculated unscaled vibrational frequencies and those determined experimentally and between the calculated and observed IR traces has been obtained allowing the IR and Raman spectra to be assigned. For the alpha-isomer, the agreement with experiment using the current level of theory is superior to that obtained previously. For the beta-isomer, for which no non-empirical study has previously been reported, the agreement with experiment is slightly poorer but still allows the spectrum to be assigned unambiguously. To calculate the structure and vibrational spectra of these large molydate cluster ions requires large basis sets and a good treatment of electron correlation and relativistic effects. For the 53-atom [PMo(12)O(40)](3-) ions, the computational demands are very high, requiring several months computational time. The calculated IR spectral traces for the two isomers are quite similar due to the relative flexibility of the molybdates, where the slight weakening of the bonding of the rotated trimetallic unit to the rest of the cluster in the beta-isomer is compensated by contraction of the bonds within the unit, and the structure of the [MO(6)] and [PO(4)] units in the two isomers is nearly identical. The vibrations characteristic of the bridging Mo-O-Mo bonds involve both the "2-2" junctions between rotated [M(3)O(13)] units and the "1-2" junctions between rotated and unrotated units. The separation of "ligand" and "interligand" vibrations is not clear. The vibrational analyses confirm the high symmetry, namely T(d) and C(3v) for the alpha- and beta-isomers, respectively, assumed by previous workers in this field. The characteristic group frequencies for the Type I polyoxometalates containing both edge- and corner-sharing I octahedra have been identified.     

Molybdenum phosphide as an o-propylaniline hydrodenitrogenation catalyst: a first principles study

MoP has been shown experimentally to be an active catalyst in the hydrodenitrogenation of o-propylaniline. We investigate the structure and the energetics of the o-propylaniline adsorption on the Mo-terminated MoP(001) surface. Detailed information on the structure of the free MoP(001) surface and on the structure and adsorption energy of o-propylaniline on MoP(001) is obtained by using density functional theory. The transition state, reaction path, and the energy barrier are reported for one of the branches of the HDN reaction network that leads to the formation of propylbenzene by hydrogenolysis of the C-N bond.     

tBuP(NH2)2--a reactive synthon for the synthesis of molecular imidophosphinates of group 13 metals

Reactions of tBuP(NH(2))(2) with Group 13 trialkyls MR(3) (M=Al, Ga, In; R=Me, tBu) were investigated in detail. According to variable-temperature (VT) NMR investigations, the reaction proceeds stepwise with the initial formation of aminophosphane adducts, which subsequently react to give iminophosphorane adducts and finally the heterocyclic metallonitridophosphinates. BP86/TZVPP (DFT) calculations were performed to verify this reaction pathway, to elucidate the influence of the central Group 13 element on the stability of the reaction intermediates and the heterocycles, as well as to assess the thermodynamics of their formation. The relative stability of free and complexed aminophosphane RP(NH(2))(2) and iminophosphorane R(H(2)N)(H)P=NH (adducts) with P(III) and P(V) centers was studied in more detail with DFT and MP2 methods. In addition, the influence of the substituent R was investigated by variation of R from H to Me, tBu, F, and NH(2). In general, the aminophosphane form was found to be favored for the free ligand, however, upon complexation with MR(3) (M=Al, Ga; R=alkyl) both forms are almost equal in energy.