Cryoscopic molecular weight

Annular nitrogen atoms can form hydrogen bonds, and if the azole contains an NH group, association occurs. Imidazole (84) shows a cryoscopic molecular weight in benzene 20 times that expected. Its boiling point is 256 °C, which is higher than that of 1-methyl-imidazole (198 °C). 

The polysaccharide was acetylated with acetic anhydride in the presence of pyridine. Purification of the acetate consisted in precipitation of a 10% boiling benzene solution with petroleum ether. One hundred and fifty such precipitations were necessary before a constant-rotating product resulted [a]D20 = — 20.1° (c = 1.0, chloroform). Cryoscopic molecular weight determinations in benzene solution gave an average value of 3918. 

There is an equilibrium between the dimer and monomer, and molecular orbital study suggests that the heterochiral dimer is more stable than the homochiral isomer. The existence and behavior of the dimeric species were well confirmed by experiments such as cryoscopic molecular weight and NMR measurement. In the NMR study of a DAIB-catalyzed dialkylzinc addition reaction, noticeable changes were observed in the spectrum of the homochiral dimer on the addition of benzaldehyde, while the spectrum of the heterochiral complex remained the same. This may imply that the heterochiral complex is very stable and does not react, and the homochiral dimer leads to the reaction product. 

In support of the association theory, colloid chemists cited non-reproduceable cryoscopic molecular weight determinations (which were eventually shown to be caused by errors in technique) and claimed that the ordinary laws of chemistry were not applicable to matter in the colloid state. The latter claim was based, not completely without merit, on the ascerta-tion that the colloid particles are large aggregates of molecules, and thus not accessible to chemical reactants. After all many natural colloids were shown to form double electrical layers and adsorb ions, thus they were "autoregulative" by action of their "surface field" (29). Furthermore, colloidal solutions were known to have abnormally high solution viscosities and abnormally low osmotic pressures. 

Proof for the structure of the triphosphate anion comes from its composition and the cryoscopic molecular weight in molten Glauber s salt [values found, 372 (19) and 364 (317) calculated, 367.9]. Assuming that phosphorus has a coordination number of four with respect to oxygen, the anion structure can only be represented by the formula... 

Not all of the compounds listed in Table 13 have been isolated in the solid state. The existence of some of the ester adducts has been inferred from electric polarization and cryoscopic molecular weight measurements on 1 1 and 1 2 stoichiometric mixtures of the metal tetrahalide and the ester in benzene solution. The concentration dependence of the polarizations and molecular weights294-297,304 indicates that the 1 1 complexes tend to dimerize in the more concentrated solutions and the 1 2 complexes tend to dissociate (equation 41) in... 

Deters, and Huang (129) describe the formation of graft copolymers of cellulose triacetate and vinyl chloride in vibratory mill treatments. Through hydrolytic degradation of the triacetate backbone, they isolated the polyvinyl chloride side chains and characterized them by infrared spectroscopy and cryoscopic molecular weight determination. The length of the side chains has been found to be between 15 and 30 vinyl chloride units. 

Although the amide derivatives of the alkali metals are normally only sparingly soluble in hydrocarbon solvents,91 the very bulky bistrimethylsilylamide derivatives are soluble in benzene. Cryoscopic molecular weight measurements indicate a dimeric nature for MN(SiMe3)2 (M = Li, Na and K).92,93 Both the lithium and sodium derivatives can be distilled under reduced pressure. 

The alkylmetal alkoxides and phenoxides of the alkaline earth metals and zinc are much more soluble and volatile and have been extensively studied. Cryoscopic molecular weight measurements on compounds RZn(OR ) and RZn(OAr) in benzene indicate tetrameric species in splution.95"98 H NMR studies of a number of such compounds have been carried out.98... 

The poly(metal)silazanes 62-77 are isolated usually as colorless crystals. They are unstable with respect to water and oxygen and are soluble in nonpolar solvents (with the exception of compound 77), The formulas drawn for 64-67 are merely formal and do not imply that these substances have a considerable ionic character. They are soluble in benzene and are monomo-lecular as found by cryoscopic molecular weight determinations. The H NMR spectra of 64 and 65 (64) demonstrate that the metal atoms are coordinated by methyl groups, the most reasonable structure being depicted in the formulas 64 and 65. The trigonal-bipyramidal arrangement of the N2SiAlM... 

In 1940, Murakami published a study21 of the constitution of verbascose. To prepare this sugar, Murakami extracted the finely ground roots with 95% alcohol, clarified the extract with lead acetate, and precipitated the verbascose with barium hydroxide and alcohol. After processing the barium complex in the usual way, and precipitating the saccharide several times from an aqueous solution by alcohol, Murakami obtained verbascose in 0.5-mm. spherocrystals composed of fine needles these had m.p. 253°, [a]20D + 170.2° (in water). The elementary analysis and the cryoscopic molecular-weight values were appropriate for a pentasaccharide. 

The IR spectra indicated the presence of disubstituted hexacarbonyl-metal derivatives with two different ligands in trans positions (trans-(CO MR R2). Moreover, the cryoscopic molecular weight determination proved that the complexes must be monomers. 

Cryoscopic molecular weight determinations are not fully conclusive because of ligand dissociation in solution. A dimeric structure with two bridging C02... 

The polyformal (I) was characterized by wet chemical and infrared analyses, and by cryoscopic molecular weight determinations. For the latter, the preferred method was the freezing point depression of an ethylene bromide solution. The other polyacetals were characterized primarily by infrared analysis and molecular weight determinations. 

The formation of the first monomeric heavy group 2 element alkoxides has been postulated based on oUgoether ligands (14), the molecularity of which is supported by cryoscopic molecular weight data. This contrasts with the initial publications of Ba(OR)2 preparations. When a saturated hydrocarbon is the parent alcohol, apparently only involatile and insoluble alkoxides are formed. Quite recently, a benzene soluble bis(aryloxide), Ba(bht)2(THF)2, has been prepared however, no volatility was reported. ... 

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