Dimerization Systems

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

Salen ligands have also been used in the titanium-catalyzed trimethylsilyl-cyanation of benzaldehyde. The complexes were immobilized by substitution of a chloride with a surface silanol from the support. In the first study on this reaction [38], the most efficient ligand was the non-symmetrical salen Im (Fig. 11) (94% ee), whereas the selectivity obtained with the symmetrical ligand la was significantly lower (72% ee). In a recent paper, the immobilization of different titanium species, including monomeric and dimeric systems with... 

The potential of the dimeric system 7 is perhaps best exemplified by its ability to bring about the total hydrodechlorination of decachlorobiphenyl, a member of the notorious family of polychlorinated biphenyls or PCBs (Persistent Organic Pollutants, POPs), in the presence of the inexpensive base NaOH (Table 8.3). 

Metal hahdes in imidazolium ionic hquids offer unique enviromnents able to facihtate dehydration reactions. Under such conditions certain metal halides are able to catalyze formal hydride transfer reactions that otherwise do not occur in the ionic liquid media. We have now discovered two systems in which this transformation has been observed. The initial system involves the conversion of glucose to fractose followed by dehydration the second system involves the dehydration of glycedraldehyde dimer followed by isomerization to lactide. CrCls" anion is the only catalyst that has been effective for both systems. VCI3" is effective for the glyceraldehyde dimer system but not for glucose. 

Korchowiec J, Uchimaru T (2000) New energy partitioning scheme based on the self-consistent charge and configuration method for subsystems application to water dimer system. J Chem Phys 112(4) 1623—1633... 

In phenolic oxidative coupling reactions, these phenol-derived radicals do not propagate a radical chain reaction instead, they are quenched by coupling with other radicals. Thus, coupling of two of these resonance structures in various combinations gives a range of dimeric systems, as shown. The... 

Due to the complexity of MgP-P dimer systems, ah initio quantum mechanical studies designed to ascertain the importance of geometric relaxations must be delayed for future investigations. In this regard Warshel (36, rf. ) has applied a variety of semi-empirical and semi-... 

These contributions are not existing for the He atom, but as to the values for the SM and CP systems, they are systematically different (the contributions from the CP system are larger than those fi om the SM system). A quite different conclusion holds for the ghost contributions these values, calculated in the dimer system are much higher (for some functions with one or two magnitudes ) than in the CP system. This is contrary to common belief the lower total energy of a CP... 

A question arises, whether the various energy quantities derived by the SMO method could yield useful information on the monomer s role (proton donor or acceptor) in the weakly interacting water dimer system. 

The discussion of porphyrin complexes in the activation of dioxygen in solution can be readily subdivided into discussions of monomeric systems and dimeric systems. The dimeric systems (cofacial metallodi-porphyrins) have recently been reviewed by Collman, Wagenknecht, and Hutchison (73). That review highlights the significant amount of research stimulated by the initial discovery by Collman and co-workers (74), confirmed later by Chang and co-workers (75), that dicobalt cofacial diporphyrins can promote the direct four-electron reduction of dioxygen to water. 

B. Kohler 1 would like to ask two questions to Prof. Zewail. First, in your investigation of the electron transfer reaction in a benzene- complex, the sample trajectory calculations you showed appear to suggest that the charge transfer step may induce vibrationally coherent motion in h-. Have you tested this possibility experimentally My second question concerns your intriguing results on a tautomerization reaction in a model base-pair system. In many of the barrierless chemical reactions you have studied, you have been able to show that an initial coherence created in the reactant molecules is often observable in the products. In the case of the 7-azaindole dimer system your measurements indicate that reaction proceeds quite slowly on the time scale of vibrational motions (such as the N—H stretch) that are coupled to the reaction coordinate. What role do you think coherent motion might play in reactions such as this one that have a barrier ... 

Pulse radiolysis studies on unsymmetrical, chemically linked organic systems have shown the expected fall off in rate constant as AG becomes more favorable.81 In these experiments, advantage is taken of the fact that capture of electrons produced by pulse radiolysis is relatively indiscriminate and in some of the pulse events an electron is captured by the component in the dimeric systems which is the weaker oxidant. Following such an event, the experimental observation made is of the system relaxing by intramolecular electron transfer to the stable redox configuration, as shown for (I)->(2) where A is one of a series of polyaromatic or quinone electron acceptors.81... 

Computer simulation was carried out on an IBM 370/155 computer, using double-precision calculation and the double-precision version of the Bairstow method polynomial roots subroutine in the IBM scientific subroutines package. For the monomer-dimer case, the quadratic can be and was solved exactly—e.g., see Ref. 3. Estimates of propagated error are provided in the tables where needed. The analysis given here reduces to that given previously (3) when the system concerned is a monomer-dimer system. 

In polymetallic systems, the larger the number of coupled ions, the larger the spreading of the S levels. As a consequence, even relatively small / values give rise to large separations of the S levels and therefore to depopulation of the highest levels. The general theory is still the same. Analytical solutions, as in dimeric systems, are seldom possible. Often, numerical solutions are possible. 

Use of less basic triphenylsiloxide ligands also allowed the isolation of mononuclear complexes (Table 1) [46,47]. In addition to the silylamide route, anhydrous nitrates and isopropoxides were employed as synthetic precursors (Eqs. 1,2). The siloxide bridges in the solvent-free dimeric systems Ln2(OSiPh3)6 are readily disrupted by donor solvents like THF, OP Bu3 or DME. The m s-THF adducts of lanthanum, cerium (Fig. 5) and yttrium adopt an approximately /ac-octahedral geometry. 

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