Thermodynamic sink

The guanine radical cations (G +) are detected by their reactions with water, which leads after treatment with piperidine or ammonia to selective strand cleavage [14]. A similar charge detection method was used by J.K. Barton, G.B. Schuster and I. Saito as described in their articles in this volume. The cleavage products were separated and quantified by gel electrophoresis. A typical example is shown in Fig. 7 where the GGG unit acts as a thermodynamic sink for the positive charge, and the efficiency of the charge transfer can be measured by the product ratio Pggg/Pg-... 

In order to determine the influence of the bridge length on the efficiency of the hole transfer between guanines, we studied double strands where the number of (A T)n base pairs between the guanines were varied (Fig. 9). In these experiments the GGG unit acts as a thermodynamic sink for the charge [17]. 

The G doublet and triplet effectively functions as a thermodynamic sink in DNA-mediated hole transport. However, the rate determining step of hole trapping at guanine clusters and the rate of hole trapping are not well understood. Furthermore, hole transfer between the donor and acceptor should compete with the hole trapping reaction the relative rate of hole trapping versus the hole transfer rate determines the overall efficiency for the hole transfer. 

Independent studies of the reduction of C=C and C=C bonds indicate that the latter is kinetically favored. Thus, in the absence of phenylacetylene, the rate of hydrogenation of styrene to ethylbenzene is about one order of magnitude faster than those for C=C bond reduction, indicating that the origin of the selectivity cannot be kinetic. The styryl compound represents a thermodynamic sink that causes virtually all the osmium present in solution to be tied up in this form, and therefore the kinetically unfavorable pathway becomes essentially the only one available in the presence of alkyne.31... 

As reported in the previous section, the reduction of 6 with alkali metals in THF led to metal-metal bonded species.22 Suitable substrates can intercept the d2 [ j9-But-calix[4]-(0)4 W] fragment, preventing the formation of such dimers, which represent a thermodynamic sink. 22... 

The isomerization barrier of 15.0-20.0 kcal mol-1 (AG ) can be considered to be large enough to allow isolation and characterization of bis(q3-<2 /),A- nms-dodecatrienediyl-Nin stereoisomers of 7b41 as reactive intermediates in the stoichiometric cyclotrimerization process. Furthermore, the trans orientation of the two allylic groups gives rise to an insurmountable barrier for reductive elimination for these cases, which prevents these species from readily leaving the thermodynamic sink via a facile reductive elimination. The isolated intermediates clearly constitute dead-end... 

The c,c,t-CDT and c,t,t-CDT production paths are shown to be not assisted by incoming butadiene, while the square-planar transition state involved along the all-t-CDT path is significantly stabilized by an axial coordination of butadiene. Hence, the all-t-CDT route becomes the most facile of the three CDT production paths with a free-energy barrier for reductive elimination of 23 kcal mol-1, that perfectly corresponds with experimental estimates.44 Accordingly, the production of C12-cyclo-oligomers requires moderate reaction conditions,9 although 7b represents a thermodynamic sink within the catalytic cycle. 

Kinetics (oligomer formation and aging) Thermodynamic Sinks (non-stoichiometric molecules)... 

The thermodynamic stability of the macrocyclic complex provides one of the driving forces for cyclisation in template reactions. In a way, co-ordination of the macrocycle to the metal ion provides a thermodynamic sink into which the reaction product can fall. This is clearly of importance when we consider the reactions such as the formation of metal... 

Flash vacuum thermolysis (FVT) of 9-azidophenanthrene, 6-(5-tetrazolyl)phenan-thridine, and [l,2,3]triazolo[l,5-/]phenanthridine give 9-cyanofluorene as the principal product and 4-cyanofluorene as a minor product.74 A nitrene intermediate was detected by ESR in the case of 9-azidophenanthrene a carbene intermediate was detected from the other two starting materials. DFT calculations were applied to examine other potential reaction products and it was concluded that although the observed products are not the kinetically favoured products, they are the thermodynamic sink and cannot be avoided under FVT conditions. 

A prototype example is the famous McMurry coupling of carbonyl compounds to alkenes (Scheme 1) [4]. The very high stability of the accumulating titanium oxides constitutes the thermodynamic sink which drives the conversion but demands the use of stoichiometric or excess amounts of the low-valent titanium reagent [Ti]. Only recently has it been possible to elaborate a procedure that for the first time enables us to perform intramolecular carbonyl coupling reactions catalyzed by titanium species [5]. 

The observed product is the thermodynamic sink. Torquoselection is preserved according to this mechanism, and fully accounts for the experiment. 

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