Reversible dimerization

Hydrolysis of ferric solutions is readily induced by addition of base. Initially, at rather acid pH, the purple ferric aqua-ion Fe(H20)63+, upon addition of base undergoes the first deprotonation step (equation 5), which is followed by reversible dimerization (equation 6), giving a yellow solution of mono- and dinuclear species ... 

The future prospects for the capsule project emerge from these considerations. Further increasing the size of the capsule and building chemical functionalities into the inner cavity would allow a closer emulation the functions of enzymes, especially those that require cofactors in order to catalyze chemical transformations. Another important aspect is to design capsules that can combine stereospecificity and catalysis - that is accelerate stereoselective transformations. Capsules that reversibly dimerize in water would probably contribute a lot more to our understanding of non-covalent forces and solvent effects in this most biorelevant medium. So far, water solubility and assembly have not been achieved with hydrogen-bonded capsules. 

A new class of conjugated hydrocarbons is that of the fullerenes [11], which represent an allotropic modification of graphite. Their electrochemistry has been studied in great detail during the last decade [126]. The basic entity within this series is the Ceo molecule (23). Because of its high electron affinity, it can be reduced up to its hexaanion (Fig. 4) [14,127]. Solid-state measurements indicate that the radical anion of Ceo reversibly dimerizes. NMR measurements confirm a u-bond formation between two radical anion moieties [128,129]. 

Konarev DV, Kovalevsky AY, Otsuka A, Saito G, Lyubovskaya RN (2005) Neutral and ionic complexes of Cgo with metal dibenzyldithiocarbamates. Reversible dimerization of Ceo" in ionic multicomponent complex [Cr (C6H6)2" ] (C6o ) 0.5[Pd(dbdtc)2]. Inorg Chem 44 9547-9553... 

Nitrogen dioxide contains an unpaired electron. It reversibly dimerizes to form dinitrogen tetroxide, which can exist in several isomeric forms shown in decreasing order of stability (Jones, 1973). 

Reversible dimerization reactions have been described for a number of aromatic radical ions. In aromatic oligomers and polymers with a more extensive delocalization of the charge and the spin such dimerizations are disfavoured. This was shown in a study of several aromatic molecules with extended it systems at different redox stages179. An extensive overview on fluorinated aromatic radical anions has been presented180. 

All three 2-oxaheterocyclopentanes will reversibly dimerize, the tin compound through association and not ring opening. Aldehydes and ketones will also readily insert the M—O bond of all three heterocycles (73RTC321). 

The formation of relatively stable semiquinone radicals by electrolytic reduction of quinones has been established by a variety of methods. Some semiquinone radicals undergo reversible dimerization reactions to form peroxides. 

The action of venom exonuclease is also blocked by thymine dimers produced as a result of UV irradiation of DNA at 280 nm. Setlow et al. (66) subjected irradiated DNA to exhaustive digestion by venom exonuclease. They isolated and identified the products of the reaction, which were composed of large amounts of all four 5 -mononucleotides and of small amounts of trinucleotides of the type d-pNpTpT where N was any of four common nucleosides and TpT was the irradiation-induced dimer of thymidine. These trinucleotides were totally resistant to further digestion with venom exonuclease but became partially susceptible after UV irradiation at 240 nm, known to reverse dimerization. The authors picture the action of venom exonuclease as proceeding linearly from the internal bond one base beyond the dimer. From there on conventional hydrolysis is resumed until the next block. This experiment touches upon one of the most pressing problems connected with venom exonuclease. Is the endonucleolytic activity an intrinsic property of the enzyme ... 

The cation radicals of terthiophene 18 reversibly dimerize even at low concentration (92JA2728). As a consequence, 71-dimers and 7r-stacks deserve attention as entities responsible for the properties of oxidized polythiophene and other conducting polymers. 

In a process related to RAFT, BASF workers have shown that 1,1-diphenylethylene will control the molecular weight of PMMA and polystyrene, and permit block polymer synthesis [92]. They propose that radical chain ends add to the diphenylethylene to form a stable diphenylalkyl radical that does not add more monomer but can reverse to diphenylethylene and the same radical chain end for addition of more monomer. The diphenylalkyl radical cap has the additional possibility of forming a reversible dimer (Scheme 35). 

Predict the form of the rate law for the reaction 2A + B - products, if the first step is the reversible dimerization of A (2A A2), followed by reaction of A2 with B in a bimolecular step. Assume that the equilibrium concentration of A2 is very small compared with [A],... 

Although the constant K includes a contribution from the reversible dimerization of the pyranyl radical in Eq. (41), the trend generally shows an increase with acceptor strength ( %) of the pyrylium cation and donor strength ( °,) of the oiganometalhc anion. 

A pH dependent reversible dimerization was observed for complexes containing the [Ru(N2H2S2)] fragment. The [Ru(N2H2S2)] fragments are isoelec -tronic with [Ru(S4)J fragments, but have NH donors that can be deprotonated. As in the case shown in Eq. 13, they can also exhibit a different coordination geometry with cis- instead of fraws-thiolate donors. 

Soy globulins also have a tendency to undergo a reversible dimerization with changes in ionic strength, the 7S globulin converting to a 9S dimer at low ionic strength (2). 

The reversible photodimerization of polycyclic aromatic hydrocarbons offers a number of advantages as a general technique for refractive-index imaging. We mentioned earlier the well-known reversible dimerization of anthracene and its relatives,... 

A reversible dimer-monomer transition has been reported independent of FAD dissociation 174, 179-182). This transition seems to be potentiated by the exhaustive removal of phosphate ion or by freezing of dilute... 

More re( ent work by Hawkins and coworkers [R. E. Fuguitt and J. E. Hawkins, J. Am. Chem. Soc., 69, 319 (1947) 67, 242 (1945) H. G. Hunt and J. E. Hawkins, ihid.y 72, 5618 (1950)] on the licjuid-phase isomerization. shows that there are three simultaneous reactions of first order occurring, the racemization to /3-pinenc with log A = 14.0, E = 44.1 Kcal/mole, and the two listed above. In addition, the allocimcne undergoes reversible dimerization and irreversible first-order decomjwsition to pyrenones. The values of Smith in parentheses cannot therefore be taken too seriously. 

The reversible dimerization of C2F4 proceeds in the gas phase between 300 and 450°C to form cyclo-C4F8. The forward reaction is second-order in C2F4 and the reverse is first-order in C4F8 ... 

Accordingly, we suggest that the photolysis is preceded by a reversible dimerization of the hydride complex that subsequently imdergoes a reductive elimination without the formation of radicals ... 

The dimerization of anthracene " has been studied extensively [243, 244]. With strongly electron-withdrawing substituents at position 9 the radical anions undergo reversible dimerization in aprotic solvents such as DMF, MeCN, propylene carbonate, DMSO etc. followed by rate determining a bond formation to furnish the stable dimer dianion [245]. In DMF k m were found to decrease in the... 

Reductive dimerization of benzene [246], cyano biphenyl ether [247], pyridine [248], and acridine [249] derivatives has also been investigated. Radical anions of diesters of pyridine and benzene undergo rapid reversible dimerization (A dim = 10 -10 M" s" ) [250]. 

Phenoselenazine cation-radical 250 undergoes reversible dimerization in aqueous sulfuric acid. The equilibrium constant and thermodynamic parameters for the dimerization of 250 and its benzo[c] derivative have been determined by Matsunaga and Tanaka. ... 

Let us now consider a reversible electron transfer reaction followed by a reversible chemical reaction. Typical examples, including the reaction of B with a reagent X (ii), here with p equivalents, and the reversible dimerization (iii), are shown in Scheme 4. 

Reversible dimerizations are observed less frequently, since often the reactions are not fast enough to be treated as thermodynamic equilibria. Examples are the dimerization of the radical cations of thianthrenes [110] and thiophene derivatives [111]. 

Figure 16. DCV working curves for a reversible dimerization followed by an irreversible reaction for (a) KiiCX = 1 and (b) Ki,CX = 10 [Ki,Ca = K QA) in the figures]. The labels 1, 0.1," and 0.01" at the working curves refer to the values of kvCx/k n. The rate constant kj- in the figure corresponds to ku in Scheme 6 and the constant a is equal to vnF/(RT). (From Ref. 117.)...

Logarithmic analyses can be carried out on the neopolarogram in much the same way as with classical polarography (see Sec. V). Examples that illustrate the application of the convolution technique include reversible dimerization of radical cations [114], the study of dissociative electron transfer reactions [175,176], and investigations of the possible potential dependence of the transfer coefficient a [174,177]. 

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