Methanol solution stability study

Watts and Bunton examined the effect of Cr(CO)3 coordination on pAR for the tropylium ion.194 Because of a competing reaction at high pH in water, the study was conducted in methanolic solutions in which pAR is decreased by up to 5 log units.195 This allowed determination of pAR = 6.6, in methanol which translates to perhaps 9.5 in water. For the uncoordinated tropylium ion pAR is reported as 2.15 in methanol compared with 4.7 in water.195 The moderate stabilization of the tropylium ion by coordination of Cr(CO)3 is pertinent to Schleyer s conclusion that coordination of Cr(CO)3 does not impair the aromaticity of benzene.181... 

Also Fryar and Kaufman8 studied the solvent effect on the stability of barium dinonylnaphthalene sulfonate in toluene, toluene/methanol, and methanol solutions by ultracentrifugation and viscometry. The aggregation number of the micelles reduced from about 10 in toluene to about 4 when the mole fraction of free methanol in the solvent mixture was approximately 0.03. In pure methanol BaDNNS micelles did not exist. 

The data for bromo complexes were obtained in aqueous methanolic solutions and outer-sphere bromo complexes with K = 1.3-1.9 were obtained for Pr, Nd, Sm, which are larger than the values of Ho (0.97) and Er (0.70). Chloro and bromo complex formation in dimethyl formamide studied by titration calorimetry [122] showed the evidence for MC12+, MCC. MCI3, and MCI4 species in solutions. In the case of bromide, monobromo and dibromo inner sphere complexes have been detected. The stepwise formation constants could not be determined for iodo complexes due to the small value of enthalpies of reaction. The stability constants data obtained in DMF are given in Table 4.8. 

Kinetic studies [81] of tetraazaporphine acetylacetonate mixed complexes of Dy(III), Eu(III) and Nd(III) in aqueous and methanolic solutions show the stability to be in the order DyTAP(acac) > EuTAP(acac) > NdTAP(acac). These complexes undergo decomposition on protonation. 

Stable in methanol than in acetonitrile however, in both solvents, the sample solutions are stable in the refrigerator (4°C) for up to 50 hours. Solution light stability studies (dark and light chamber) in both solvents at ambient temperature were also performed in order to further determine whether the formation of the degradation products is photocatalyzed or generated at ambient temperature ( 25°C). 

Handling, Storage, and Precautions R-SpirOP is obtained as a white solid that is stable in air but decomposes gradually in alcoholic solution. To help prevent oxidation, storage under a nitrogen atmosphere is recommended. The stability of R-SpirOP has been tested in methanol solution through a p NMR study. Observable decomposition occurred in 2 h and complete decomposition in 24 h. 

A series of pyridoxal aroylhydrazones 17 has been synthesized and studied by UV, IR, and JH NMR spectroscopy. In absolute methanol, these hydrazones exist in the neutral form 17a, however, gradual addition of water to the methanol solution results in the appearance of new bands in the UV spectra, indicating the formation of zwitterionic tautomer 17b. In aqueous solution, the equilibrium is greatly shifted toward 17b rather than earlier proposed ketoenamine 17c. The dipolar species 17b are stabilized by intermolecular hydrogen bonding with water molecules (92JCS(P2)213). 

Hiller and coworkers reported an NMR and LC-MS study on the structure and stability of l-iodosyl-4-methoxybenzene and 1-iodosyl-4-nitrobenzene in methanol solution [195]. Interestingly, LC-MS analyzes provided evidence that unlike the parent iodosylbenzene, which has a polymeric structure (Section 2.1.4), the 4-substituted iodosylarenes exist in the monomeric form. Both iodosylarenes are soluble in methanol and provide acceptable and NMR spectra however, gradual oxidation of the solvent was observed after several hours. Unlike iodosylbenzene, the two compounds did not react with methanol to give the dimethoxy derivative ArI(OMe)2 [195]. 

EPR spectroscopy was employed by Rockenbauer et al [Ro 72] to study the equilibrium stability constants of low-spin cobalt(II) mixed complexes. They determined the equilibrium constants of formation of the mixed complexes of the bis(dimethylglyoximato)cobalt(II) parent complex with pyridine ligands, in methanol. It was shown that in a methanolic solution of the parent complex, two methanol molecules are coordinated along the z axis, and these methanol molecules can be replaced stepwise with pyridine. 

In a liquid state high-resolution NMR study of adsorption on colloidal palladium, [33] it was found that under 3 atm. of 99% CO, the resonance which would correspond to carbon monoxide adsorbed on the 7.0 nm crystalline palladium colloid stabilized in methanol solution with PVP could not be directly... 

Carson [99] investigated the behavior of different aliphatic amines towards d-fructose, and encountered problems of undesired disubstitution, epimerization, as well as lack of stability of the 2-amino-2-deoxyglucose derivatives during his studies. He performed the reaction in a two-step sequence by initial formation of the ketosylamine, which was isolated as a crystalline intermediate, followed by the rearrangement reaction in methanolic solutions containing glacial acetic acid. These conditions allowed access to increased yields that could be further improved employing benzylamine as the nucleophilic component. 

In buffered solution, the DPPH radical can show variations in its stability. Al-Dabbas et al. (2007) found that in methanol solution containing acetate buffer (pH 5.0), the absorbance of the DPPH radical was not reduced in a wide range of concentrations examined (0.01-0.2 mmol L ), while in phosphate buffer (pH 7.0), a reduction of the DPPH radical absorbance was observed at concentrations above 0.05 mmol L. In other studies, Ozcelik et al. (2003) evaluated the variation in stability of the DPPH radical after 120 min. The absorbance of DPPH radical in potassium biphthalate buffer (pH 4.0) decreased by 25% in methanol solution and by -45% in acetone solution. DPPH radical in sodium bicarbonate buffer (pH 7) was stable in an acetone system (less than 10% reduction), but an -30% decrease occurred in the absorbance in a methanol system. DPPH radical in potassium carbonate-potassium borate-potassium hydroxide buffer (pH 10) was stable in a methanol system (less than 10% reduction), but a decrease of -70% occurred in the absorbance in an acetone system. Thus, the stability of DPPH in pH buffer solution mainly depends on the types of buffer and solvent used. 

The interaction between bromide and lanthanide ions was also studied, but experimental data are available for aqueous methanol solutions only (Kozachenko et al. 1973). Using a spectrophotometric method, the formation of rather weak outer-sphere bromo complexes was evidenced, and their stability constants for Pr, Nd, Sm, and Ho were determined in water and in 50% and 90% methanol (table 3). For solutions in 50% methanol, the stability of the outer-sphere bromo complexes is larger for Pr, Nd, and Sm Ki = 1.3—1.9) compared to Ho (0.97) and Er (0.70). Kozachenko et al. (1973) explained this behaviour as reflecting a higher stability for the solvates of the heavier lanthanide ions. A similar trend was observed in the stability constants of the chloro complexes in absolute methanol vide supra). Finally, the stability of the bromo complexes of the lanthanides increases as the dielectric constant of the medium is reduced. 

The coordination of several ions to La(III) in methanol has been studied by La NMR (Bunzii et al. 1987). The chemical shift of the La-NMR resonance is indeed governed by the composition of the iimer coordination sphere. The chemical shifts measured for several LaX methanolic solutions are shown in fig. 9. The large variation of these shifts versus the concentration of the salts is indicative of the presence of a significant inner-sphere interaction between La(III) and all the anions studied to form complexes of the type [LaX (solv)x] "". The absolute stability constants (table 8) for the anion complexation can be obtained from the following equations, where x is the mole fraction of LaX ", and where the solvation has been neglected ... 

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