Basicity of Very Weak Bases in 1,2‐Dichloroethane

The basicity scale of very weak bases has been set up in 1,2‐dichloroethane to give, for the first time, reliable quantitative insights into the basic properties of weak bases in a low‐polarity solvent. The scale contains 30 compounds, including anilines; phosphanes, and carbonyl bases, such as esters and amides, linked by 53 relative basicity measurements. The scale spans more than 12 pK_ip units, expanding to as low pK_ip values as possible with our current experimental methodology.     

Influence of Water Content on Basicities in Acetonitrile

The influence of water content at impurity level (5–10,000 ppm) in acetonitrile, on the changes in relative basicity differences (ΔpK _a values) of 13 pairs of bases, was studied both experimentally and computationally (COSMO-RS). The ΔpK _a values involving smaller bases with localized charge in the cationic form were found to be more affected. A computational parameter, weighted average negative sigma (WANS), was proposed to quantify the charge delocalization in cations and succeeded in describing the observed changes of ΔpK _a. The results validate the previously published basicity scale in acetonitrile with respect to solvent dryness and give guidelines for better experimental planning.     

Influence of water content on the acidities in acetonitrile. Quantifying charge delocalization in anions

The effect of traces of water on the relative strengths of acids (ΔpK(a) values) in acetonitrile was quantitatively evaluated experimentally and computationally (COSMO-RS). Water affects first of all the anions by selective solvation. Expectedly, the more localized is the charge in acid anions the higher is the effect of water. The energetic effect of increasing water content from 0 to ca. 10,000 ppm on solvation enthalpies of anions ranged from 0.2-0.4 kcal mol?1 (anions with delocalized charges) to 15 kcal mol?1 in the case of the highly charge-localized acetate ion. In the case of ΔpK(a) values the change ranges from 0.01 to ca. 1.7 pK(a) units (acid pair involving acetic acid). The COSMO-RS method was found to satisfactorily describe the trends in ΔpK(a) values. To quantify the extent of charge localization/delocalization in anions a parameter, weighted average positive σ (WAPS), was introduced, which can be conveniently computed using the COSMO approach. WAPS characterizes the distribution of charge density across the molecular surface and was found to correlate well with the extent of water influence on the dissociation of the respective acid.     

Basicity Limits of Neutral Organic Superbases

The potential limits of superbasicity achievable with different families of neutral bases by expanding the molecular framework are explored using DFT computations. A number of different core structures of non‐ionic organosuperbases are considered (such as phosphazenes, guanidinophosphazenes, guanidino phosphorus ylides). A simple model for describing the dependence of basicity on the extent of the molecular framework is proposed, validated, and used for quantitatively predicting the ultimate basicities of different compound families and the rates of substituent effect saturation. Some of the considered bases (guanidino phosphorus carbenes) are expected to reach gas‐phase basicity around 370 kcal mol^?1, thus being the most basic neutral bases ever reported. Also, the classical substituted alkylphosphazenes were predicted to reach pK_a values of around 50 in acetonitrile, which is significantly higher than previously expected.     

Adduct Formation in ESI/MS by Mobile Phase Additives

Adduct formation is a common ionization method in electrospray ionization mass spectrometry (ESI/MS). However, this process is poorly understood and complicated to control. We demonstrate possibilities to control adduct formation via mobile phase additives in ESI positive mode for 17 oxygen and nitrogen bases. Mobile phase additives were found to be a very effective measure for manipulating the formation efficiencies of adducts. An appropriate choice of additive may increase sensitivity by up to three orders of magnitude. In general, sodium adduct [M + Na]⁺ and protonated molecule [M + H]⁺ formation efficiencies were found to be in good correlation; however, the former were significantly more influenced by mobile phase properties. Although the highest formation efficiencies for both species were observed in water/acetonitrile mixtures not containing additives, the repeatability of the formation efficiencies was found to be improved by additives. It is concluded that mobile phase additives are powerful, yet not limiting factors, for altering adduct formation.

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Vegetational variation of phenolic compounds in Epilobium angustifolium

Epilobium angustifolium L. herbs are used in ethnomedicine to treat benign prostate hyperplasia. The aim of the study was to investigate the phenolic contents in distinct E. angustifolium organs during the whole vegetational period from May to October. The plants were obtained from a remote habitat in Estonia and spectrophotometrically analysed for the total polyphenol, tannin, and flavonoid contents. The total polyphenol content was the highest in roots (85?mg?g(-1)) and stems (67?mg?g(-1)) in July. The total flavonoid content was the highest in leaves (2.36?mg?g(-1)) and flowers (2.09?mg?g(-1)) and it remained relatively stable during the summer months. The highest tannin content was found in small growing plants in May; in older ones it declined, whereas the absolute yield per plant was greater. In sum, the aerial organs without stems collected in July-August are the best choice to get E. angustifolium plant material with stable high phenolic content.     

Sodium adduct formation efficiency in ESI source

Formation of sodium adducts in electrospray (ESI) has been known for long time, but has not been used extensively in practice, and several important aspects of Na⁺ adduct formation in ESI source have been almost unexplored: the ionization efficiency of different molecules via Na⁺ adduct formation, its dependence on molecular structure and Na⁺ ion concentration in solution, fragmentation behaviour of the adducts as well as the ruggedness (a prerequisite for wider practical use) of ionization via Na⁺ adduct formation. In this work, we have developed a parameter describing sodium adducts formation efficiency (SAFE) of neutral molecules and have built a SAFE scale that ranges for over four orders of magnitude and contains 19 compounds. In general, oxygen bases have higher efficiency of Na⁺ adducts formation than nitrogen bases because of the higher partial negative charge on oxygen atoms and competition from protonation in the case of nitrogen bases. Chelating ability strongly increases the Na⁺ adduct formation efficiency. We show that not only protonation but also Na⁺ adduct formation is a quantitative and reproducible process if relative measurements are performed. Copyright © 2013 John Wiley & Sons, Ltd.     

The Evolution of Electrospray Generated Droplets is Not Affected by Ionization Mode

Ionization efficiency and mechanism in ESI is strongly affected by the properties of mobile phase. The use of mobile-phase properties to accurately describe droplets in ESI source is convenient but may be inadequate as the composition of the droplets is changing in the plume due to electrochemical reactions occurring in the needle tip as well as continuous drying and fission of droplets. Presently, there is paucity of research on the effect of the polarity of the ESI mode on mobile phase composition in the droplets. In this paper, the change in the organic solvent content, pH, and droplet size are studied in the ESI plume in both ESI+ and ESI– ionization mode. We introduce a rigorous way – the absolute pH (pH_abs ^H ₂ ^O) – to describe pH change in the plume that takes into account organic solvent content in the mobile phase. pH_abs ^H ₂ ^O enables comparing acidities of ESI droplets with different organic solvent contents. The results are surprisingly similar for both ionization modes, indicating that the dynamics of the change of mobile-phase properties is independent from the ESI mode used. This allows us to conclude that the evolution of ESI droplets first of all proceeds via the evaporation of the organic modifier and to a lesser extent via fission of smaller droplets from parent droplets. Secondly, our study shows that qualitative findings related to the ESI process obtained on the ESI+ mode can almost directly be applied also in the ESI– mode.

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Structure–Reactivity Relationship in the Frustrated Lewis Pair (FLP)‐Catalyzed Hydrogenation of Imines

The autoinduced, frustrated Lewis pair (FLP)‐catalyzed hydrogenation of 16‐benzene‐ring substituted N‐benzylidene‐tert‐butylamines with B(2,6‐F₂C₆H₃)₃ and molecular hydrogen was investigated by kinetic analysis. The pK_a values for imines and for the corresponding amines were determined by quantum‐mechanical methods and provided a direct proportional relationship. The correlation of the two rate constants k₁ (simple catalytic cycle) and k₂ (autoinduced catalytic cycle) with pK_a difference between imine and amine pairs (ΔpK_a) or Hammett's σ parameter served as useful parameters to establish a structure–reactivity relationship for the FLP‐catalyzed hydrogenation of imines.