Presence Of Dimers Research Articles

Computer simulation of sedimentation in the ultracentrifuge: VI. Monomer-tetramer systems in rapid chemical equilibrium

The velocity sedimentation of solutes consisting of a monomer in rapidly established equilibrium with a tetramer has been simulated using a realistic model and a digital computer. The results are compared with the no-diffusion treatments of the same systems and with the simulated experiments done previously for trimerizing solutes. In the absence of diffusion, most of the tetramerizing solutes examined would have produced bimodal sedimenting boundaries. When diffusion was taken into account, the tendency to exhibit bimodal gradient profiles was more obvious in monomer-tetramer systems than in comparable monomer-trimer systems. The characteristic boundary shapes predicted by the no-diffusion model were less seriously obscured by the presence of dimer in the monomer-tetramer case than in the monomer-trimer case. Diagnostically useful shoulders and subsidiary maxima persisted in the simulated schlieren patterns given by tetramerizing solutes with monomer molecular weights as low as 20,000. Hydrodynamic concentration dependence of the sedimentation coefficients of the various solute species did not greatly obscure the unusual structure of either the monomer-trimer or the monomer-tetramer gradient profiles.

Self-association of alcohols and phenols in nonpolar solvents

A number of alcohols (primary, secondary, and tertiary) and phenols, covering a pKa range from 9.4 to 19.0, gave on association a peak about 125 ± 5 cm−1 lower than the monomeric hydroxyl peak in the fundamental stretching region. Taken together with the extensive literature available on the association of alcohols and phenols in nonpolar solvents, this seems to indicate the presence of dimers.

Nuclear magnetic resonance studies in coordination chemistry : VII. Kinetic studies of exchange reaction involving Bicyclo[2.2.1]hepta-2.5-diene complexes of rhodium(I)

NMR studies of the coalescence of the signals of the inequivalent olefinic protosn of the monomer (Nor)RhCl(PPh 3) (Nor=bicyclo[2.2.1]hepta-2,5-diene) have indicated the occurance of monomer-monomer reactions. In the presence of free triphenylphosphine a fast ligand exchange reaction was observed with the monomer. When, however, besides the monomer the dimeric [(Nor)RhCl] 2 was present, below + 20° monomer-dimer reactions took place without exchange of PPh 3. Above +20° phosphine-exchange reactions occured between (Nor)RhCl(PPh 3) and (Nor)RhCl, the latter being formed by dissociation of the dimer. Addition of Cl− to solution of the monomer, and to mixtures of monomer and dimer, resulted in halide-exchange reactions, while addition of free norbornadiene had no observable effect. With the triphenylarsine system we could only study reactions of the monomer (Nor)RhCl(AsPh 3)in the presence of dimer. The arsine system is different from the phosphine one in that reactions were found of dissociated AsPh 3 with the arsine compound and the dimer, respectively. The result for these systems are compared with those for the 1.5-cyclooctradiene system of Rh 1 and Ir 1 and the π-allylpalladium systems.

Mass spectrometric studies at high temperatures. Part 24.—Thermodynamics of vaporization of SnF2and PbF2and the Dissociation energies of SnF and PbF

Vapour species in the SnF2, SnF2+ Sn, CaF2+ Sn and PbF2 systems have been studied mass spectrometrically. The presence of dimers and trimers as well as monomers of SnF2 was noted and the average SnF2—SnF2 bond energy for these polymers was 20.0±2 kcal mole–1. PbF2 does not dimerize but disproportionates into Pb + PbF4, and a value of 382±2 kcal mole–1 was measured for the heat of atomization of PbF4. The dissociation energies of PbF and SnF were 85±2 and 111.5±3 kcal mole–1 respectively.

Nuclear magnetic resonance studies in coordination chemistry. Part II: Kinetic studies of reactions occurring in the system [(π‐C4H7)PdCl]2‐triphenylarsine

AbstractAn NMR study has been made of the reactions occurring in systems which contain either the monomeric complex (π‐C4H7)PdCl(Ph3As) and the dimer [(π‐C4H7)PdCl]2, or only monomer and free triphenylarsine (in CDCl3). In the former case reactions were observed of Ph3As ‐ formed by the dissociation of the monomer (reaction (a)) ‐ with the dimer (b) and the monomer (c), respectively: In the latter case, where only monomer and Ph3As are present, two reactions were studied. The first, which was observed between ‐70° and ‐40° with a very small excess of Ph3As, and which involves the exchange of Ph3As (reaction (c)), is first‐order in the arsine ligand. This reaction and the reactions in the presence of dimer were studied by measuring the rates at which the syn‐protons (or the anti‐protons) of the allyl group become magnetically equivalent. The second reaction, in which also exchange of Ph3As takes place, was measured between +10° and +80° with a higher excess of free Ph3As. This reaction is also first‐order in Ph3As, but involves the formation of either a s̀‐allyl intermediate or an activated state resembling the s̀‐allyl form. The activation energies and frequency factors of the above reactions are reported.At still higher excess of Ph3As (Ph3As/Pd ∼ 2‐3) an equilibrium (π‐C4H7)PdCl(Ph3As) + Ph3As ⇆ [(π‐C4H7)Pd(Ph3As)2]+Cl− becomes notticeable. This equilibrium moves to the right with increasing Ph3As/Pd ratio and decreasing temperature. The amount of ionic species, however, is so small that the kinetics of the above reactions are but little influenced by it.

Aggregation studies on long chain alkyl-ammonium salts in organic diluents—I The chloride, nitrate salts in benzene

The aggregation equilibria of tridodecyl-ammonium chloride and tridodecyl-ammonium nitrate in benzene have been investigated by studying the vapour-pressure lowering of the solutions with matched termistors at 25°C. For the chloride salt, measurements have been also carried out at 37°C. The existence of a monomer-dimer equilibrium for the chloride salts fits the experimental data. The equilibrium constants K 2 = 30 ± 3 at 37°C, K 2 = 50 ± 5 at 25°C have been calculated. The presence of dimer and trimer is necessary to explain the experimental data of the nitrate salt. K 2 = 65 ± 7 and K 3 = 39 ± 4 at 25°C have been calculated.

Unsaturation of Polyisobutylenes and Butyl Rubber. Reactions with Nitrosobenzene, Thiocyanogen, Iodine Monochloride, and Ozone

Abstract Table III summarizes unsaturation values obtained for the various polymers. The ranges indicated are, in most cases, extreme values of determinations made with two or more different samples of a given polymer type. Taking into consideration the small amounts of unsaturation involved, the values based on end-group determinations are in reasonably good agreement with one another and with the values based on the viscosity of the degraded polymer. Higher values are observed for the nitrosobenzene determinations, but these are considered to be less reliable because of the ease with which oxidation phenomena can occur with this reagent. The thiocyanogen values are the lowest of all; on the basis of values obtained by the other methods as well as of experimental evidence not included in this paper, these values are believed to result from incomplete reaction. As a class the iodine monochloride methods give considerably higher values than those obtained with the other reagents. This may be attributed to the substitution reaction that readily occurs with that reagent. The effect is perhaps most marked in the data obtained for polyisobutylene sample 2 with 0.1 N Wijs reagent; the upper value (0.4) is far greater than can be explained on the basis of the knowa molecular weight of this polymer and the corresponding terminal double bond contribution. Furthermore, since this polymer sample was fractionated in such a way as to remove all species of viscosity-average molecular weight less than about 50,000, the high value cannot be ascribed to the presence of dimers, trimers, or other small molecules formed during polymerization. In view of the data and the above considerations, it is believed that the unsaturation values based on ozone degradation are correct within the limits of experimental error, and that this reagent makes possible a relatively good estimation of the number of double bonds in polymers such as have been described. A modified procedure more suitable for rapid evaluations is the object of a current investigation.