The effect of the solvent upon the rates and mechanisms of organometallic reactions : V. PMR spectra and structure of molecular complexes of methyltin halides in solutions

The complexation of methyltin halides in electron-donating solvents such as acetone, dioxane, dimethoxyethane, pyridine, dimethylformamide, dimethyl sulfoxide, hexamethylphosphoric triamide, and tetramethylethylenediamine has been studied by means of PMR spectroscopy. Equilibrium constants have been evaluated for Me₃SnHal·D (D = donor) complexes. The concentration and temperature dependences of J(¹¹⁹SnC¹H) in methyltin halides suggest some conclusions on the electron and spatial structures of complexes in solutions.     

The effect of solvent upon the rates and mechanisms or organometallic rections. : I. General aspects

The data reported to date on the effect of solvent upon the rates and mechanisms of organometallic reactions have been analysed and discussed and indicate that the effect is very complex. The analysis also shows that there is no overall explanation for the effect and that indeed such an explanation seems at present to be non-existent.It is suggested that the problem may be successfully approached through a study of the effect of the solvent upon the redistribution of organic groups between organometallic molecules. In these reactionis, not only the kinetics of the reactions in various solvents should be studied but also identification of the organometallic complexes is essential as well as the electronic structures and geometries of the complexes in the crystalline phase and in solution.     

The effect of the solvent upon the rates and machanisms of organometallic reactions: VII. A 19F NMR study of complexation of CF3 HgX (X = Cl,I, OCOCF3)

The concentration and temperature dependence of J(¹⁹⁹HgC¹⁹F) for solutions of CF₃HgX (X = Cl, I, OCOCF₃) in various solvents shows that in inert solvents these molecules exist mainly as non-solvent dimers (X = I or OCOCF₃) or as monomers (X = Cl). In strongly coordinated solvents $\text{1}\text{2}$ complexes are largely formed from CF₃HgX and the electron-donating solvent molecules. In pyridine solution an equilibrium exists between the $\text{1}\text{1}$ and $\text{1}\text{2}$ complexes. Complexes of the type CF₃HgX·D are T-shaped and have a higher relative content of s-electrons in the HgCF group compared with tetrahedral CF₃HgX·2D complexes.     

Dramatic effects of halogen substitution and solvent on the rates and mechanisms of nucleophilic substitution reactions of aziridines

In a previous study we reported that fluorine substitution at the carbon positions of aziridine results in profound enhancements of the rate of reaction with ammonia, a typical nucleophile, in the gas phase. In this study the investigation is extended to include chloro- and bromoaziridines. Because syntheses are largely performed in the condensed phase, the present computational investigation [(MP2(Full)/6-311++G(d,p)//MP2(Full)/6-31+G(d) level] was conducted with three typical solvents that cover a wide range of polarity: THF, CH3CN, and H2O. Nucleophiles can react with haloaziridines 1 by displacing a substituted amide ion by means of an SN2 mechanism (pathway a), producing 1,2-diaminohaloethanes (from the initially formed dipolar species 2). Alternatively, a rearrangement mechanism involving rate-determining departure of a halide ion (pathway b) to form an imidoyl halide, 3, is possible. Transition-state theory was used to compute relative reaction rates of these mechanistic possibilities and to assess the role of the halogen substituents and the reaction solvent. Gas-phase results provided the basis of mechanistic insights that were more apparent in the absence of intermolecular interactions. Fluoroaziridines were found to react at accelerated rates relative to aziridine exclusively by means of the a Menshutkin-type mechanism (SN2) in each solvent tested, while the reactions of the chloro- and bromoaziridines could be directed toward 2 in the highly nonpolar solvent, cyclohexane, or toward 3 in the more polar solvents. An assessment is made of the feasibility of using this chemistry of the haloazirdines in the synthetic laboratory.     

The complexation of 2,3-anthracenedicarboxylate by β- and γ-cyclodextrins in mixed solvent systems as studied by induced circular dichroism. The role of cosolvent hydrophobicity

The complexation of 2,3-anthracenedicarboxylate (ADC) by- and-cyclodextrins in water containing an organic solvent has been studied by induced circular dichroism. It has been shown that an increase of organic solvent ratio causes the degradation of the 1:1 ADC:-CD complex and the liberation of one guest molecule from the 2:1 ADC:-CD complex in water. The higher the hydrophobicity of the cosolvent, the weaker the complexation of ADC by-CD.     

The structure of the ionized form of 3-nitroaniline in HCl-DMF solutions studied by IR spectroscopy

The composition and structure of the ionized form of 3-nitroaniline in the HCl-DMF system were found by Multiple Attenuated Total Reflectance IR spectroscopy. The stage of ionization is the replacement of a DMF molecule in the HCl·DMF complex by an indicator molecule. The ionized form contains the strong quasi-symmetrical N…H…Cl hydrogen bond.     

The effect of slow heating rates on the reaction mechanisms of nano and micron composite thermite reactions

Thermal analyses were performed on Al+MoO₃ thermite reactions as a function of Al particle size (ranging from 50 to 20 μm) and heating rate (from 2.5 to 15 K min^–1 ). Results include ignition (onset) temperatures and heats of reaction. The nano-thermites initiate prior to reactant phase changes and at least 300°C below micron-thermites. The differences in ignition temperatures are suggestive of different ignition mechanisms. Nano-thermites display higher heats of reaction that are dependent on experimental conditions.     

The effect of heating rate upon the coupling of complex reactions. I. Independent and competitive reactions

Theoretical curves of the rate of conversion vs. temperature at constant heating rate for first-order reactions with activation energies of 80, 160, 240 and 320 kJ mole^?1 are compared over a range of heating rates from 10^?9 to 10⁵ K s^?1 for independent and competitive reactions. Independent reactions with different activation energies may be separated from one another by either increasing or decreasing the rate of heating. The spectrum of derivative peaks for two competing reactions at various heating rates has a dispersion effect in the region of change from low to high activation energy reactions. The practical range of heating rates in thermal analytical experiments and the application of these model cases to the understanding of the kinetics of complex systems at high and low temperatures are discussed.     

Aggregation of methylene blue in frozen aqueous solutions studied by absorption spectroscopy

The paper presents a qualitative as well as quantitative spectroscopic study of methylene blue (MB) aggregation that occurs upon freezing the aqueous solutions over a wide concentration range. The Gaussian curve analysis and the multivariate curve resolution-alternating least squares method were used to determine the number and concentration of chemical species responsible for the overlaying absorption visible spectra measured. The results show the extent of aggregation for the concentrations above 10(-7) mol L(-1), being dependent on the freezing rate and the initial concentration. While the local concentration of MB at the grain boundaries of polycrystalline ice increased by approximately 3 orders of magnitude upon fast freezing at 77 K compared to the liquid phase, the concentration raised at least by 6 orders of magnitude upon slow freezing at 243 K. Since enhancement of the local concentration of solutes plays an important role in (photo)chemical transformations in solid aqueous media, this work helps to understand how the initial conditions control the course of the process. The results are relevant in other interdisciplinary fields, such as environmental chemistry, cosmochemistry, or geochemistry.     

Porphyrins. 12. Investigation of the protonation of meso-substituted prophyrins by PMR spectroscopy

The dependence of the chemical shifts of the protons in the PMR spectra of mesodimethylaminomethyl derivatives of etioporphyrin and octaethylporphyrin, as well as the Schiff bases of meso-formylporphyrins, on the concentration of trifluoroacetic acid was studied. The order of protonation of the nitrogen atoms of these compounds was determined. The effect of various solvents and acids on the ratio of the equilibria of proton transfer along the hydrogen bond and dissociation of the H complexes with the formation of solvated ions was examined. The PMR spectra of various salts of the Schiff bases of meso-formylporphyrins were studied. See [1] for communication No. 11. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 769–776, June, 1980.     

The complexation of uranium(VI) and atmospherically derived CO2 at the ferrihydrite-water interface probed by time-resolved vibrational spectroscopy

The sorption reactions of uranium(VI) at the ferrihydrite(Fh)-water interface were investigated in the absence and presence of atmospherically derived CO(2) by time-resolved in situ vibrational spectroscopy. The spectra clearly show that a single uranyl surface species, most probably a mononuclear bidentate surface complex, is formed irrespective of the presence of atmospherically derived CO(2). The character of the carbonate surface species correlates with the presence of the actinyl ions and changes from a monodentate to a bidentate binding upon sorption of U(VI). From the in situ sorption experiments under mildly acid conditions, the formation of a ternary surface complex is derived where the carbonate ligands coordinate bidentately to the uranyl moiety (≡UO(2)(O(2)CO)(x)). Furthermore, the release reaction of the carbonate ligands from the ternary surface complex is found to be considerably retarded compared to those from the pristine surface suggesting a tighter bonding of the carbonate ions in the ternary complex. Simultaneous sorption of U(VI) and atmospherically derived carbonate onto pristine Fh shows formation of binary monodentate carbonate surface complexes prior to the formation of the ternary complexes.     

The effect of complexation on the rates of electron transfer of iron porphyrins

A study has been made of the heterogeneous rate constants of iron porphyrins with pyridines and other nitrogenous base ligands. A direct relationship between the pK_a of the base and the rate constant has been found with a few exceptions. The measurements were by cyclic voltammetry. The larger the pK_a the stronger the complex between iron porphyrins and axial ligands. With strong ligands no ligand exchange or spin state change is expected. Both of these actions show the rate in agreement with the rates found in this work.     

The solvent trapping or co-adsorbing effect during the self-assembly monolayer studied by surface-enhanced Raman scattering

The possibility of solvent molecules being trapped within the monolayer interior during the self-assembly of 2,5-dimercapto-1,3,4-thiadiazole (DMTD) molecules from alcoholic solution, or co-adsorbing together with the solute molecules onto the silver surface was verified by means of surface-enhanced Raman scattering (SERS). Variations of the relative intensity of the solvent bands to the concentrations of the solution were investigated. To a certain range of the concentration, the smaller the concentration is, the larger the relative intensity of the solvent bands is. A new method for obtaining the SERS spectra of the organic compounds that are insoluble in water was developed in our laboratory, which also provides further evidence for the solvent trapping or co-adsorbing effect.     

Pressure induced phase transitions of TTAB-micellar solutions studied by SANS and Raman spectroscopy

Using small angle neutron scattering in combination with optical Raman spectroscopy, phase transitions in the micellar system tetradecyl-trimethylammonium bromide (TTAB) have been investigated in a wide range of temperatures, pressures and surfactant concentrations. Allowing for the sluggish kinetics of crystallisation, the phase diagram in the p-T-c space could be determined that is dominated by a pronounced two-phase region of coexisting liquid and solid phases at elevated pressures.     

The effect of absorbancy upon Raman intensities in solutions

Reasonable agreement is obtained in the relationship between intensities of Raman lines as measured experimentally and as calculated from an equation derived on the basis of absorbancy laws. A direct relationship between Raman intensity and concentration exists with excitation outside an absorption band, and an inverse relationship over some or all concentration ranges is found with excitation within an absorption band.     

Equilibrium in water-peroxide solutions of cesium tellurate studied by 125Te NMR spectroscopy

Water-peroxide solutions of cesium tellurate have been studied by ¹²⁵Te NMR. The system contains different Te(VI) anionic species in equilibrium with one another, including those with coordinated peroxo and hydroperoxo groups.