Spontaneous transfer mechanism

Higashimura and Nishi have recently studied the dimerisation of styrene and a-methylstyrene by acetyl perchlorate. An appropriate choice of temperature and solvent allowed the c timisation of linear dimers formation. These authors postulate a transition state for the formation of the linear dimers which resembled closely the spontaneous-transfer mechanism proposed thirteen years earlier by Gandini and Hesch in the pseudocationic polymerisation of styrene by perchloric acid. 

Decomposition of diphenoylperoxide [6109-04-2] (40) in the presence of a fluorescer such as perylene in methylene chloride at 24°C produces chemiluminescence matching the fluorescence spectmm of the fluorescer with perylene was reported to be 10 5% (135). The reaction follows pseudo-first-order kinetics with the observed rate constant increasing with fluorescer concentration according to = k [flr]. Thus the fluorescer acts as a catalyst for peroxide decomposition, with catalytic decomposition competing with spontaneous thermal decomposition. An electron-transfer mechanism has been proposed (135). 

For oxidation of G in duplex DNA, Steenken concluded that the proton on N-1 of G shifts spontaneously to N-3 of the cytosine in the normal Watson-Crick base pair to generate [C+(H)/G ]. Consistent with this proposal, calculations indicate that charge transfer in oxidized DNA is coupled with proton transfer from G to Experiments carried out in D2O also reveal a kinetic isotope effect for G oxidation, implicating a concerted proton-coupled electron transfer mechanism. However, density functional theory (DFT) calculations in the gas phase predict that the structure with a proton on G N-1 [C/HG ] is more stable than [C (H)/G ] by 1.4kcal/mol. " ... 

Alkoxyl radicals can be generated by a variety of methods including peroxide reduction, nitrite ester photolysis, hypohalite thermolysis, and fragmentation of epoxyalkyl radicals (for additional examples of alkoxyl radical generation, see Section 4.2.S.2). Hypohalites are excellent halogen atom donors to carbon-centered radicals, and a recent example of this type of cyclization from the work of Kraus is illustrated in Scheme 43.182 Oxidation of the hemiketal (57) presumably forms an intermediate hypoiodite, which spontaneously cyclizes to (58) by an atom transfer mechanism. Unfortunately, the direct application of the Barton method for the generation of alkoxyl radicals fails because the intermediate pyridine-thione carbonates are sensitive to hydrolytic reactions. However, in a very important recent development, Beckwith and Hay have shown that alkoxyl radicals are formed from N-alkoxypyridinethiones.183 Al-... 

Some authors 24 25) document that the transfer reaction mechanism is more complex because the rate of transfer is dependent on the monomer concentration. This phenomenon, though important from the mechanistic point of view, does not change the product of the transfer reaction metal-polymer bonds are formed even when a monomer is involved in the reaction path. Other commonly considered transfer reactions (with monomer, solvent, hydrogen, spontaneous transfer) do not result in the formation of metal-polymer bonds. 

Yamazaki and collaborators studied the homo- and co-dimerisation of styrenes by diphenylphosphate. These reactions were shown to proceed through a pseudocationic mechanism by the isolation of the active ester intermediate. With p-methoxystyrene oligomers were obtained, i.e. the ester was in this case more active towards propagation (very nucleophilic monomer) and less prone to spontaneous transfer. 

In the course of biological evolution, primitive enzymes which bind the appropriate transition states initially fairly poorly are slowly refined as a consequence of Natural Selection. Evolution by Natural Selection thus predicts that, wherever enzymes are found to catalyse equivalent reactions by very different mechanisms, as with aldose-ketose isomerisations by hydride transfer or proton transfer, the spontaneous, uncatalysed mechanisms will be found to proceed at comparable rates under ambient conditions. ... 

Table 4 presents the proton affinities PA and deprotonation enthalpies DPE of thiouracils calculated at the B3LYP/6-31+G(d,p) computational level. Inspecting this Table, we find that first, the thio substitution of uracil systematically increases its PAs by 3-6 kcal/mol and second, it decreases the DPEs of uracil by 6-13 kcal/mol. As far as the base pair A thioU is considered, it particularly implies a lowering, on the one hand, of the potential well at the Sio atom of thiouracil corresponding to the proton transfer from the N8 atom of adenine to the Sio of thiouracil and, on the other one, a raising of the potential well at the N3 atom of thiouracil involved in the proton transfer from the N3-H bond to the Ni atom of adenine. Therefore, summarizing, the thio substitution of uracil is in a favor to the double proton transfer mechanism of the occurrence of the spontaneous point mutations proposed by Lowdin [19]. Table 4 also includes the PAs of the T2 tautomer of uracil and thiouracils. A comparison with the corresponding PAs of the parental normal nucleobases shows that their tautomerization to the T2 form is accompanied by an increase of the proton affinity by 20-23 kcal/mol. 

Based on the known chemistry of flavin photolysis reactions, it appears unlikely that thymine dimer cleavage occurs via a direct energy transfer mechanism (160). One proposal suggests that in the model reaction with 1-deazariboflavin, the thymine dimer radical anion is formed via electron donation from the excited sensitizer (164). Alternatively, electron abstraction by the excited flavin could occur, resulting in the thymine dimer radical cation (159, 160), although it is unlikely that reduced flavin would act as an electron acceptor. A schematic for this mechanism is illustrated in Scheme 33, where the initial formation of a sensitizer-dimer complex is consistent with the observed saturation kinetics. The complex is activated by excitation of the ionized sensitizer (pH > 7), and electron donation to the dimer forms the dimer radical anion and the zwitterionic, neutral 1-deazariboflavin radical (162). Thymine dimer radical would spontane-... 

According to this mechanism, the catalyst adsorbs monomer on its surface pig. 9.7(a)] and initiation occurs when an adsorbed monomer is polarized by some speeies on the eatalyst surfaee converting it to an ion (or a radieal or an ion-radieal pair) bound to the surfaee pig. 9.7(b)]. Fhop-agation takes place along the surfaee pig. 9.7(e)] and the polymer ehain is eventually terminated and desorbed from the surface, being replaeed by fresh monomer. The ehain termination may be caused by transfer with monomer or spontaneous transfer or detaehment from the surface. The molecular regularity of the polymer formed depends on the surfaee layer line-up of the adsorbed monomer molecules. 

It is intuitively obvious that, unless there are special constraints (such as internal partitions), an equilibrium state must have thermal and mechanical equiUbrium. A temperature difference between two phases would cause a spontaneous transfer of heat from the warmer to the cooler phase a pressure difference would cause spontaneous flow of matter. 

It should be said that in spite of that chemistry of chain transfer to solvent and spontaneous transfer is different, the two mechanisms are identical both in mathematics and in results at ksp = k S. 

These negative values of AS may confirm the compactness of the intermediates and characterized to one- or two-electron transfer mechanisms of inner-sphere nature. Again, the positive values of AG obtained may confirm the non-spontaneity of the complexes formation in the rate-determining steps, as was suggested by the cited mechanisms. 

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