Dimerization Association

Several industrially important materials form dimers (or trimers, or tetramers.) reversibly, in both gas and liquid phases. Important examples are sulfur [12] 

FIGURE 13.8 Gas-liquid equilibrium with chemical equilibrium in both phases. 

FIGURE 13.9 Structure of dimmers formed hy carboxyhc acids. For HAc, R is the methyl group, CH3. 

FIGURE 13.10 Experimental compressibility factors of HAc as a function of pressure at very low pressures [14]. 

Example 13.13 Estimate the value of the compressibility factor z for HAc at 25 °C and 11.38 torr from the measured value of the specific volume (0.6525 L/0.04346 g of HAc) [14] with the assumption that the HAc is all in the form of monomer with molecular weight 60.05 g/mol. 

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It has been suggested that benzylic radicals may form a dimeric association complex which may easily collapse to the combination product but be geometrically unfavorable for disproportionation.1,8-179 Even if this applies for the aralkyl radicals, it cannot account for the behavior of systems with other / -substituents. 

However, the mechanisms by which the initiation and propagation reactions occur are far more complex. Dimeric association of polystyryllithium is reported by Morton, al. ( ) and it is generally accepted that the reactions are first order with respect to monomer concentration. Unfortunately, the existence of associated complexes of initiator and polystyryllithium as well as possible cross association between the two species have negated the determination of the exact polymerization mechanisms (, 10, 11, 12, 13). It is this high degree of complexity which necessitates the use of empirical rate equations. One such empirical rate expression for the auto-catalytic initiation reaction for the anionic polymerization of styrene in benzene solvent as reported by Tanlak (14) is given by ... 

Miyake, R., Murakami, K., Owens, J.T., Greiner, D.P., Ozoline, O.N., Ishihama, A., and Meares, C.F. (1998) Dimeric association of Escherichia coli RNA polymerase alpha subunits, studied by cleavage of single-cysteine alpha subunits conjugated to iron-(S)-l-[p-(bromoacetamido)benzyl]ethylenediaminetet raacetate. Biochemistry 37(5), 1344-1349. 

Fig. 88. The dimer association of uteroglobin, with one subunit shown shaded and one open.

A study of the state of association of the functionalized organolithium compounds 204a-d was carried out by multinuclear ( H, Li, Li, C, N and P) NMR spectroscopy, using Li- and N-enriched species. Spectral evidence, supported in part by XRD crystallographic evidence, points to compounds 204a-c being dimerically associated in etheric solutions in three different forms (205-207). The interconversion among these three... 

Furthermore, the proposed dimeric association of the polymer lithium species was actually confirmed (l) by viscosity measurements on the polymerized solutions, using the well-known relation (12) which applies to melts or concentrated solutions... 

One additional item of experimental evidence for the dimeric association of polyisoprenyl lithium was provided by a light scattering study (21), in n-hexane at 25°C., where it was found that the molecular weight of the terminated polymer was very close to one-half that of the active polymer. All of these data seem to leave no doubt that the active chain ends in the organo-lithium polymerization of styrene, isoprene and butadiene, in non-polar solvents, are associated as pairs, at least at chain-end concentrations of 10 2 M or less. This conclusion has also been supported by data obtained in four other laboratories (22,... 

It must be concluded, therefore, that the kinetic scheme proposed in Equations (2) and (3) cannot be valid for these systems. Although the seeming correspondence between the half-order kinetics and the dimeric association in the case of styrene might validate the above kinetic scheme, even this hypothesis has been recently contradicted (18). Thus Fetters and Young... 

Several studies have appeared (12,13,14) in which the propagation reactions involving styryllithium were examined in mixed solvent systems comprising benzene or toluene and ethers. The kinetics were examined under conditions where the ether concentration was held constant and the active center concentration varied. In most cases, the kinetic orders of the reactions were identical to those observed in the absence of the ether. Thus, in part, the conclusion was reached (13,14) that the ethers did not alter the dimeric association state of polystyryllithium. The ethers used were tetrahydrofuran, diphenyl ether, anisole, and the ortho and para isomers of ethylanisole. 

The state of association of poly(dienyl)lithium compounds in hydrocarbon solutions is a matter of current controversy (15-18). Aggregation states of two (.16 and four (.15) have been reported based on light-scattering and concentrated solution viscosity measurements. The most recent concentrated solution viscosity studies ( 1 6 J 7.) > which include results of various endcapping and linking techniques, provide convincing evidence for predominantly dimeric association of poly(isoprenyl)lithium in hydrocarbon solution. The effect of tetrahydrofuran on the degree of association of poly(isoprenyl)lithium has also been examined by concentrated solution viscosity measurements (13). These results indicate that the equilibrium constant for the process shown in eq 3 [PILi = poly(isoprenyl)lithium] exhibits an equilibrium... 

Figure 2.2 Dimeric association of gold complexes with TPA (a) and protonated TPA (b).

Figure 2.5 Dimeric association of complexes with N-donor ligands.

Figure 2.42 Dimer associations of dichalcogenolate complexes (a and b). Trinuclear derivative (c).

Figure4.14 (a) The structure of ThA lQCIsj acetone) 36 [62], and (b) the dimer association through hydrogen bonding in the crystal of 12 2( 15)4( )(4,4 - ) 39 [63].

Figure 4.24 Dimer association in the crystal of (a) (Ph3As) AuGeCI3 82 [76], and ( ) [(2- 6 4)3 ] 13 83 [77].

The crystal structure of the complex LiBr-pmdeta [pmdeta = permethylated (11)] reveals a dimeric association in the solid state each Li+ cation is five-coordinated by the three nitrogen donors and by two bridging bromide anions. The bridging —Li2Br2— unit is non-symmetric,... 

DNA polymerase III can polymerize DNA, but with a much lower processivity than one would expect for the organized replication of an entire chromosome. The necessary increase in processivity is provided by the addition of the J8 subunits, four of which complete the DNA polymerase III holoenzyme. The J3 subunits associate in pairs to form donut-shaped structures that encircle the DNA and act like clamps (Fig. 25-10b). Each dimer associates with a core subassembly of polymerase III (one dimeric clamp per core subassembly) and slides along the DNA as replication proceeds. The J8 sliding clamp prevents the dissociation of DNA polymerase III from DNA, dramatically increasing processivity—to greater than 500,000 (Table 25-1). 

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