Dimerization equilibria

HMO Predictions for Dimerization Equilibria of Substituted Ethylene Radical Anions ... 

The aggregation behaviour and tautomerism of three 0,0 -dihydroxy and one o-hydroxy-o -methoxy monoazo dyes have been studied by UV-visible spectroscopy [17]. Evidence of monomer-dimer equilibria was obtained for all four of these mordant dye structures. Intermolecular hydrogen bonding between the hydroxy groups and hydrophobic interaction between aryl nuclei contribute to the dimerisation effect. 

Other interesting problems solved by UB resonance include monomer-dimer equilibria in molecules of the type C12BNRP(CF3)2,<1968,24) the structure of (Me3PBH2NMe3)e (811B = + 8-7, ref. BF3-etherate)... 

Pr ), like their tin congeners (see above), were shown by H, C, N, Si and NMR spectra <901CA205> to be dimers while monomer-dimer equilibria at room temperature and above exist in the cases of l,3-di-R-4,4,5,5-tetramethyl-l,3,4,5,22Miazadisilaplumbolidines (R = Et, Pr ). 

Silver porphyrins are interesting because of the existence of disilver(I), paramagnetic silver(II), and diamagnetic silver(III) complexes which has been known for a long time [7, 8]. The latter are favorably prepared by electrochemical [299] or peroxodisulfate [300-302] oxidation of silver(II) porphyrins. The Ag(III) porphyrins were used for and 2H resonance experiments [299] on one hand and for the determination of monomer-dimer equilibria [301] or self-exchange rate constants [301] at water-soluble silver porphyrins. 

Smith, M. B., The monomer-dimer equilibria of liquid aluminum alkyls. I. Triethyl-aluminum, J. Phys. Chem. 71, 364 (1967). 

The minimal model comprises an uncatalyzed and unspecific direct formation of R and S (ko), a simple and unspecified description of the autocatalytic steps assuming monomers as catalytic species (k ), and the monomer-dimer equilibria (k2, M and (M M in which different rates of dimer formation for homochiral and heterochiral species is allowed. This model translates into the following set of differential equations ... 

AH°q -10.7 Real mol ) indicate a higher stability of the sulfur containing radical (86NJC345). A similar study concerning the dimerization of radicals 53 and 54 was carried out by Kawata and Niizuma (89BCJ2279). Owing to steric hindrance of the phenyl groups in the dimers, the dimerization equilibria were found to be endothermic. 

Trialkylaluminum compounds undergo monomer-dimer equilibria (equation 22) in noncoordinating solvents. Cryo-scopic molecular weight measurements indicate that trimethylaluminum is predominately a dimer in toluene solutions, and... 

Scheme li Charge-transfer and dimerization equilibria. A schematic description of a biselectro-phore and degenerate electron hopping... 

An alternate explanation assumes that variations in the position of possible monomer polymer equilibria with ionic radius cause the observed trend in volatility. There is longstanding evidence that acetyl-acetonate chelates of the lanthanides tend to polymerize in solution (2, 9), the molecular weight values (2) being anomalously high. Filipescu, Hurt, and McAvoy (8) have recently found other examples of what appear to be monomer-dimer equilibria. Considering the trend in volatility one can image that monomer polymer transformations could take... 

In the high-concentration cobalt systems, cobalt ions are reported to be present in several configurations, with the Co" dimer being the most active [71]. Some non-varivalent metal salts, such as ZrO(OAc)2, are reported to promote cobalt catalyzed oxidations. This may be the result of the influence of the promoter on coordination number and monomer-dimer equilibria [72, 73]. 

As shown in Example 24-5, deviations from Beer s law appear when the absorbing species undergoes association, dissociation, or reaction with the solvent to give products that absorb differently from the analyte. The extent of such departures can be predicted from the molar absorptivities of the absorbing species and the equilibrium constants for the equilibria involved. Unfortunately, since we are usually unaware that such processes are affecting the analyte, there is often no opportunity to correct the measurement. Typical equilibria that give rise to this effect include monomer-dimer equilibria, metal complexation equilibria when more than one complex is present, acid-base equilibria, and solvent-analyte association equilibria. 

Recently several dimeric derivatives of catechol have been prepared (1-3) and their monomer/dimer equilibria examined65. It was found that at a temperature of 59 °C 2 and 3 form predominantly monomeric compounds, whereas 1 exists predominantly as the dimer even at this temperature. This was rationalized by considering... 

These results may be interpreted in the following way (22) At room temperature the species exists mainly as dimer (or ion quadruplet) with rapid migration of MeaSi groups on the NMR time scale. As the sample is cooled to -20°C, trimethylsilyl migration becomes slow so that two peaks are observed for the dimer. The related compound LiN(SiMc3)2 shows similar monomer-dimer equilibria in solution in THF and hydrocarbons (72). Below —20°C, monomer (ion pair) is observable in equilibrium with dimer. Between -20°C and -60°C silyl migration is rapid in the monomer but slow in the dimer (compared to the proton spin relaxation time). Finally at -80°C anionic rearrangement for the monomer also becomes slow, so that two pairs of 2 1 peaks are observed. 

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