Hydrogenation structural study

The isomerization of isopentenyl diphosphate to dimethylally diphos phate is catalyzed by JPP isomerase and occurs through a carbocation pathway Protonation of the IPP double bond by a hydrogen-bonded cysteine residue ir the enzyme gives a tertiary carbocation intermediate, which is deprotonated b a glutamate residue as base to yield DMAPP. X-ray structural studies on the enzyme show that it holds the substrate in an unusually deep, well-protectec pocket to shield the highly reactive carbocation from reaction with solvent 01 other external substances. 

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

Casas, J.M., Diosdado, B.E., Falvello, L.R., Fornies, J., Martin, A. and Rueda, A.J. (2004) Hydrogen-bond mediation of supramolecular aggregation in neutral bis-(C5F5)Pt complexes with aromatic H-bond donating ligands. A synthetic and structural study. Dalton Transactions, (17), 2733-2740. 

The properties of these complexes are well studied 126,132). Although monomeric in solution 126,133), they are dimeric in the solid state and a structural study of Cu(Et2copper atom lies 0.26 A out of the plane formed by four sulfur atoms at a distance of 2.30(1) A. A fifth, long Cu—S bond (2.85 A) is approximately perpendicular to this plane, whereas a hydrogen atom of an ethyl group is situated at the other side of the S4 plane at a distance of 2.86 A from the copper atom. 

Other Cg hydrocarbons. The dehydrogenation of normal hexane and 2,3-di methyl butane also proceeds but not as voraciously on small platinum clusters. Figure 8 is a plot of the hydrogen content in the first adduct as a function of the size of the platinum metal cluster. The metal atom reacts via dihydrogen elimination to produce PtC6Hi2 products. The platinum trimer is now the smallest cluster that will produce a C H near one. The similarity of size dependent dehydrogenation of the normal hexane and the branched molecule suggest that these systems may not readily aromatize these alkanes. Further structural studies are needed to identify the reaction products. 

Huot, J., G. Liang, S. Boily, A.V. Neste, and R. Schulz, Structural study and hydrogen sorption kinetics of ball-milled magnesium hydride, /. Alloys Compd., 293-295, 495-500, 1999. 

The previous EXAFS studies were restricted to supported catalysts. Furthermore, the structural properties determined by MES and EXAFS were mainly related to the HDS activity and not to the other catalytic functions. Presently, we will report EXAFS (both Mo and Co), MES, HDS and hydrogenation activity studies of unsupported Co-Mo catalysts. These catalysts have been prepared by the homogeneous sulfide precipitation method (l8) which permits large amounts of Co to be present as Co-Mo-S. The choice of unsupported catalysts allows one to avoid some of the effects which inherently will be present in alumina supported catalysts, where support interactions may result in both structural and catalytic complexities. 

FIGURE 27.1 Heteronuclear and homonuclear mono- and disubstituted hydrogen bonded structures studied. (Reprinted from Ozen, A.S., Aviyente, V., De Proft, F., and Geerlings, P., J. Phys. Chem., A110, 5860, 2006. With permission.)... 

The self-organization of polysilanols in the presence of other hydrogen bond acceptors has been studied by several groups.512-516 Several other publications have dealt with the stepwise synthesis of siloxane and siloxanol chains.450,517-522 Recent work on fully condensed siloxane rings and silsesquioxane cages involves the non-aqueous hydrolysis of chlorosilanes - as well as mechanistic and structural studies. 

P. G. Jonsson, Hydrogen bond studies. CXIII. The crystal structure of ethanol at 87 K. Acta Crystallogr. B32, 232 235 (1976). 

---