A not so simple local multiplier algebra

We construct an AF-algebra A such that its local multiplier algebra Mloc(A) does not agree with Mloc(Mloc(A)), thus answering a question raised by G.K. Pedersen in 1978.     

The exchange property for purely infinite simple rings

It is proven that every purely infinite simple ring is an exchange ring. This result is applied to determine those Leavitt algebras that are exchange rings.

     

Theoretical study of the structure and vibrational spectra of the (H2O)2…HF and H2O…(HF)2 molecular complexes

Using the Hartree–Fock and MP 2 methods with bases of up to 6-31++G (2d, 2p) quality, the optimum geometry of the 1:2 and 2:1 (H₂O)_n… (HF)_m complexes of water and hydrogen fluoride is searched in a systematic way. Two minimum-energy conformations are found for the 1:2 complex connected through a low-energy transition state. For the 2:1 complex, only one minimum-energy structure is obtained. The analysis of the geometries of the minima and their vibrational frequencies shows that none of them can be used to explain the existence of the H …F? H reverse complex detected experimentally. © 1994 John Wiley & Sons, Inc.     

On the Existence of a Pyramidality Effect in d(8).d(8) Contacts. Theoretical Study and Structural Correlation

A qualitative molecular orbital study of d(8).d(8) contacts, using as models the dimers of [RhCl(CO)(3)] and cis-[PtCl(2)(CO)(2)], is presented, focusing on the effect of the pyramidalization of the metal atom on the strength of the metal-metal interaction. The existence of a pyramidality effect in dimers of d(8)-ML(4) complexes is supported also by the results of ab initio MP2 calculations on the dimer of cis-[PtCl(2)(CO)(2)]. A structural database analysis shows that a correlation between the pyramidality and the M.M distance exists for several families of dimers, oligomers, and polymers of square planar complexes of Ni(II), Pd(II), Pt(II), and Au(III). The importance of the pyramidality effect in those compounds is discussed in comparison with similar effects in binuclear complexes with metal-metal bonds of different multiplicities.     

Stereoselective aldol additions catalyzed by dihydroxyacetone phosphate-dependent aldolases in emulsion systems: preparation and structural characterization of linear and cyclic iminopolyols from aminoaldehydes

The potential of dihydroxyacetone phosphate (DHAP)-dependent aldolases to catalyze stereoselective aldol additions is, in many instances, limited by the solubility of the acceptor aldehyde in aqueous/co-solvent mixtures. Herein, we demonstrate the efficiency of emulsion systems as reaction media for the class I fructose-1,6-bisphosphate aldolase (RAMA) and class II recombinant rhamnulose-1-phosphate aldolase from E. coli (RhuA)-catalyzed aldol addition between DHAP and N-benzyloxycarbonyl (N-Cbz) aminoaldehydes. The use of emulsions improved the RAMA-catalyzed aldol conversions by three to tenfold relative to those in conventional DMF/water mixtures. RhuA was more reactive than RAMA towards the N-Cbz aminoaldehydes regardless of the reaction medium. With (S)- or (R)-Cbz-alaninal, RAMA exhibited preference for the R enantiomer, while RhuA had no enantiomeric discrimination. The linear N-Cbz aminopolyols thus obtained were submitted to catalytic intramolecular reductive amination to afford the corresponding iminocyclitols. This reaction was diastereoselective in all cases examined; the face selectivity was controlled by the stereochemistry of the newly formed hydroxyl group originating from the aldehyde. Characterization of the resulting iminocyclitols allowed the assessment of the diastereoselectivity of the enzymatic aldol reactions with respect to the N-protected aminoaldehyde. RAMA formed single diastereoisomers from N-Cbz-glycinal and from both enantiomers of N-Cbz-alaninal, while 14 % of the epimeric product was observed from N-Cbz-3-aminopropanal. Diastereoselectivity from RhuA was lower than that observed from RAMA. Interestingly, a single diastereoisomer was formed from (S)-Cbz-alaninal, whereas only a 34 % diastereomeric excess was observed from its enantiomer (i.e., (R)-Cbz-alaninal).     

Ligand effects and dimer formation in dicoordinated copper(I) complexes

The dimers of dicoordinated Cu(I) complexes of the type [CuXL], where X^? = Cl^?, Br^?, I^?, or CN^? and L is a neutral ligand such as NH₃, PH₃, or CNCH₃, have been studied by means of ab initio and density functional calculations. The performance of density functional calculations using the hybrid B3LYP method has been compared with MP2 results. Ligand and conformational effects are analyzed for two types of dimers, notably those in which the monomers retain their linear conformation and associate through Cu–Cu contacts and those in which the X ligands act as bridges resulting in tricoordination of each Cu atom and a rhombic Cu₂X₂ core. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001