Gas-Phase Lithium Cation Basicity: Revisiting the High Basicity Range by Experiment and Theory

According to high level calculations, the upper part of the previously published FT-ICR lithium cation basicity (LiCB at 373 K) scale appeared to be biased by a systematic downward shift. The purpose of this work was to determine the source of this systematic difference. New experimental LiCB values at 373 K have been measured for 31 ligands by proton-transfer equilibrium techniques, ranging from tetrahydrofuran (137.2 kJ mol^?1) to 1,2-dimethoxyethane (202.7 kJ mol^?1). The relative basicities (ΔLiCB) were included in a single self-consistent ladder anchored to the absolute LiCB value of pyridine (146.7 kJ mol^?1). This new LiCB scale exhibits a good agreement with theoretical values obtained at G2(MP2) level. By means of kinetic modeling, it was also shown that equilibrium measurements can be performed in spite of the formation of Li⁺ bound dimers. The key feature for achieving accurate equilibrium measurements is the ion trapping time. The potential causes of discrepancies between the new data and previous experimental measurements were analyzed. It was concluded that the disagreement essentially finds its origin in the estimation of temperature and the calibration of Cook’s kinetic method. Graphical Abstract

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Effect of frequency on sonochemical reactions. I: Oxidation of iodide

The effect of continuous sonication on the kinetics of iodine formation in aqueous iodide solutions was studied in the presence of air and argon at two frequencies, 20 and 900 kHz. Most workers in this area assume that frequency has a negligible effect on sonochemical reactions. The present results indicate, however, that there is a significant effect in the above solutions. The rate of sonochemical oxidation of iodide in aqueous solution is 3.1 times greater in presence of air than argon at 900 kHz, in contrast to the situation at 20 kHz, where the ratio is 0.9 A 900 kHz apparatus was specially designed to make it possible to measure the absolute acoustic power delivered into the solution. The rate of oxidation per unit power in this new 900 kHz apparatus can be more than 30 times greater than that at 20 kHz for the oxidation of iodide in the presence of air.     

D'Alembert's letter to Euler of 3 march 1766

Just before Leonhard Euler resigned his post at the Berlin Academy to return to St. Petersburg, where he spent the rest of his life, he received a letter from d'Alembert urging him to remain in Berlin. This letter is here published for the first time with commentary on the historical context.     

Bromine formation in solid NaBr/KNO<Subscript>3</Subscript> mixture and assay of this reaction via bromination of activated aromatics

Bromine formation in the mixture of solid NaBr and KNO₃ was observed and the process was studied in different acidified organic solvent–water mixtures by monitoring the bromination of acetanilide and other compounds, containing activated aromatic substituents. This assay is based on fast bromination reaction of these aromatic compounds, as differently from the assay of Br₂, the brominated aromatics can be easily determined by conventional gas chromatography–mass spectrometry (GC–MS) methods. It was found that bromine was generated autocatalytically on the surface of salt crystals and the reaction was characterized by a lag period, the duration of which depended on reaction conditions, and importantly on the type of the organic solvent in the reaction mixture. As the bromine formation could be easily controlled by reaction conditions, it was suggested that the studied reaction might have practical applications as an environmentally friendly and economically feasible bromination method. It was also shown that the bromination of aromatics followed the mechanism of classical electrophilic aromatic substitution reaction.     

The effect of frequency on sonochemical reactions III: dissociation of carbon disulfide

Investigations were made of the effects of frequency, temperature, intensity and gases on the rate of sonochemical dissociation of carbon disulfide. Application of 900 kHz ultrasound did not produce any noticeable change. When carbon disulfide was irradiated with 20 kHz, the liquid formed a heterogeneous mixture of black particles in a yellow solution. The rate of dissociation decreased with increasing temperature, in agreement with most sonochemical reactions. The rate also decreased with decreasing area of the horn tip, keeping total power constant. This dependence on the horn tip area, as well as that on the frequency, is in opposition to the dependence for the formation of iodine from the sonication of aqueous potassium iodide solution [See Part II, Ultrasonics Sonochemistry 3 (1996) 19]. The X-ray spectrum of the black particles and the yellow residue obtained after evaporation showed the presence of amorphous carbon and monoclinic sulfur. The rate of sono-dissociation of carbon disulfide in the presence of different gases is in the order He > H2 > Air > Ar > O2 > CO2.     

Sonication of aqueous solutions of chlorobenzene

Sonication at two frequencies (20 and 900 kHz) was carried out on dilute (220 ppm) aqueous solutions of chlorobenzene. The formation of chloride ions was followed using ion chromatography. The solutions became more colored with time; the absorbance maximum was around 270 nm. Some of the compounds remaining in the solution could be identified; they were chlorinated phenols, chloronaphthalene, mono and dichlorobiphenyls, etc. At the same acoustic power, the rate of chloride formation with 20 kHz ultrasound was greater when a probe with a larger tip area was used, but significantly less than the rate with 900 kHz. The use of ultrasound for conversion of chlorine in organic compounds in water to chloride can thus be performed more efficiently using a higher frequency and with a lower intensity (power per area). There is, however, a possibility that the toxicity of the aqueous solution is increased by such treatment.