ABSTRACT:This paper describes the first synthesis of a new class of topological macromolecules which we refer to as "starburst polymers." The fundamental building blocks to this new polymer class are referred to as "dendrimers." These dendrimers differ from classical monomers/ oligomers by their extraordinary symmetry, high branching and maximized (telechelic) terminal functionality density. The dendrimers possess "reactive end groups" which allow (a) controlled moelcular weight building (monodispersity), (b) controlled branching (topology), and (c) versatility in design and modification of the terminal end groups. Dendrimer synthesis is accomplished by a variety of strategies involving "time sequenced propagation" techniques. The resulting dendrimers grow in a geometrically progressive fashion as shown: Chemically bridging these dendrimers leads to the new class of macromolecules-"starburst polymers" (e.g.,
_Although the (n + l)p orbital is unoccupied in transition-metal ground-state configurations which are all ndX(n + I)sY, these (n + l)p functions playa crucial role in the structure of transition metal complexes. As we show here, the usual solution, adding one or more diffuse functions, can be insufficient to create an orbital of the correct energy. The major problem appears to be due to the incorrect placement of the Cn + 1) P orbital's node. Even splitting the most diffuse component of the np orbital and adding a second diffuse function does not completely solve this problem. Although one can usually solve this deficiency by further uncontracting of the np function, here we offer a set of properly optimized (n + 1) P functions that offer a more compact and satisfactory solution to the proper placements of the node. We show an example of the common deficiencies seen in typical basis sets, including standard basis sets in GAUSSIAN94, and show that the new optimized (n + I)p function performs well compared to a fully uncontracted basis set.
The catalytic cycle for H2 oxidation in [NiFe] D. gigas hydrogenase has been investigated through
density functional theory (DFT) calculations on a wide variety of redox and protonated structures of the active
site model, (CO)(CN)2Fe(μ-SMe)2Ni(SMe)2. DFT calculations on a series of known LFe(CO)(CN)(L‘)
= Cp or Cp*, L‘ = CN, CO, CNCH3; n = 0, 1, 2) complexes are used to calibrate the calculated CO bond
distances with the measured IR stretching frequency. By combining this calibration curve with the energy and
CO bond distance of the DFT calculations on the active site model and the experimental IR frequencies on the
enzyme, the redox states and structures of active site species have been determined: Ni-B is a Ni(III)−Fe(II)
species, Ni-SI(a) is a Ni(II)−Fe(II) species, Ni-SI(b) has a protonated terminal sulfur (Ni bound), Ni-R is a
Ni(II)−Fe(II) dihydrogen complex with H2 bound at Fe, and Ni-C is a Ni(III)−Fe(II) species with an Fe−H−Ni bridge. The latter species returns to Ni-SI through a Ni(I)−Fe(II) intermediate, which is potentially
observable. Protonation of the Ni bound terminal sulfur results in a folding of the Fe(μ-S)2Ni framework.
Dihydrogen activation is more exothermic on the Ni(III) species than on the corresponding Ni(II) or Ni(I)
species. Our final set of proposed structures are consistent with IR, EPR, ENDOR, and XAS measurements
for these species, and the correlation coefficient between the measured CO frequency in the enzyme and the
CO distance calculated for the model species is 0.905.
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