Determination of p K a values of fluorocompounds in water using 19 F NMR

Selectivity control towards CO versus H2 for photo-driven CO2 reduction with a novel Co(II) catalyst

Developing efficient catalysts for reducing carbon dioxide, a highly stable combustion waste product, is a relevant task to lower the atmospheric concentration of this greenhouse gas by upcycling. Selectivity towards CO2-reduction products is highly desirable, although it can be challenging to achieve since the metal-hydrides formation is sometimes favored and leads to H2 evolution. In this work, we designed a cobalt-based catalyst, and we present herein its physicochemical properties. Moreover, we tailored a fully earth-abundant photocatalytic system to achieve specifically CO2 reduction, optimizing efficiency and selectivity. By changing the conditions, we enhanced the turnover number (TON) of CO production from only 0.5 to more than 60 and the selectivity from 6% to 97% after four hours of irradiation at 420 nm. Further efficiency enhancement was achieved by adding 1,1,1,3,3,3-hexafluoropropan-2-ol, producing CO with a TON up to 230, although at the expense of selectivity (54%).

Accurate Computation of Aqueous pKas of Biologically Relevant Organic Acids: Overcoming the Challenges Posed by Multiple Conformers, Tautomeric Equilibria, and Disparate Functional Groups with the Fully Black-Box pK-Yay Method

The use of static electronic structure calculations to compute solution-phase pKas offers a great advantage in that a macroscopic bulk property could be computed via microscopic computations involving very few molecules. There are various sources of errors in the quantum chemical calculations though. Overcoming these errors to accurately compute pKas of a plethora of acids is an active area of research in physical chemistry pursued by both computational as well as experimental chemists. We recently developed the pK-Yay method in our attempt to accurately compute aqueous pKas of strong and weak acids. The method is fully black-box, computationally inexpensive, and is very easy for even a nonexpert to use. However, the method was thus far tested on very few molecules (only 16 in all). Herein, in order to assess the future applicability of pK-Yay, we study the effect of multiple conformers, the presence of tautomers under equilibrium, and the impact of a wide variety of functional groups (derivatives of acetic acid with substituents at various positions, dicarboxylic acids, aromatic carboxylic acids, amines and amides, phenols and thiols, and fluorine bearing organic acids). Starting with more than 1000 conformers and tautomers, this study establishes that overall errors of ∼ 1.0 pKa units are routinely obtained for a majority of the molecules. Larger errors are noted in cases where multiple charges, intramolecular hydrogen bonding, and several ionizable functional groups are simultaneously present. An important conclusion to emerge from this work is that, the computed pKas are insensitive (difference <0.5) to whether we consider multiple conformers/tautomers or only choose the most stable conformer/tautomer. Further, pK-Yay captures the stereoelectronic effects arising due to differing axial vs equatorial pattern, and is useful to predict the dominant acid-base equilibrium in a system featuring several equilibria. Overall, pK-Yay may be employed in several chemical applications featuring organic molecules and biomonomers.

A pH-responsive sensor based on intramolecular internal standard for reproducible detection of strong acids and bases via 19F NMR spectroscopy

Numerous methods, including pH meters and optical sensors, have been developed for the detection of pH, which is an important indicator in various fields. However, those methods are susceptible to errors in strongly acidic and basic ranges and inaccurate pH measurement due to sample turbidity, hindering their application such as photographic industries and wastewater treatment facilities. Eco-friendly and non-invasive ¹⁹F NMR spectroscopy is a promising technique for measurement of strong acids and bases owing to its high sensitivity and little interference; nevertheless, inconsistencies in reproducibility impede its widespread adoption. Herein, we developed a¹⁹F NMR-based pH sensor by introducing an intramolecular internal standard strategy into a pH-responsive fluorinated material. Based on the acceptable deviation (Δδ_F = 17-19 ppb) in the evaluation of the internal standard signal, this pH-sensing platform enabled reproducible pH measurements in strongly acidic and basic environments. Moreover, its ¹⁹F NMR response showed reversibility and high stability to potential interfering factors, and the low absolute difference (0.026-0.086 in pH) for real samples such as diet Coke suggests its potential suitability for various acidic beverages.

Quantifying Acidity in Heterogeneous Systems: Biphasic pKa Values

Acidities of lipophilic compounds, such as various ligands or catalysts, in systems consisting of an aqueous phase at equilibrium with a water-immiscible phase (lipid bilayers, phase transfer catalysis, sensor membranes, to name just few) are typically approximated by the aqueous pK_a values. Our research shows that such approximations can lead to seriously biased estimations of the acidities as the bulk of solvated H⁺ ions reside in the aqueous phase, while the lipophilic species─both neutral acid and anion─predominantly reside in the organic phase. Therefore, the use of aqueous pK_a in such situations is not justified. In this work, we provide a more accurate description of the acidities of acids in such systems by applying the biphasic pK_a concept. Biphasic pK_a values (pK_a^ow values) of 35 acids of various structures and chemical properties were determined in a 1-octanol:water system. We provide detailed descriptions of the UV-vis and NMR measurement methods. The directly obtained (apparent) pK_a^ow values depend on concentration. Concentration-independent values were obtained by extrapolating the apparent values to zero concentration using a Debye-Hückel model.

Efficient and reversible absorption of low pressure NH3 by functional type V deep eutectic solvents based on phenol and hydroxypyridine

Highly efficient and reversible absorption of low pressure ammonia by phenol-hydroxypyridine deep eutectic solvents.

Acid-Base and Anion Binding Properties of Tetrafluorinated 1,3-Benzodiazole, 1,2,3-Benzotriazole and 2,1,3-Benzoselenadiazole

The influence of fluorination on the acid-base properties and the capacity of structurally related 6-5 bicyclic compounds - 1,3-benzodiazole 1, 1,2,3-benzotriazole 2 and 2,1,3-benzoselenadiazole 3 to σ-hole interactions, i. e. hydrogen (1 and 2) and chalcogen (3) bondings, is studied experimentally and computationally. The tetrafluorination increases the Brønsted acidity of the diazole and triazole scaffolds and the Lewis acidity of selenadiazole scaffold decreases the basicity. Increased Brønsted acidity facilitates anion binding via the formation of hydrogen bonds; particularly, tetrafluorinated derivative of 1 (compound 4) binds Cl^- . Increased Lewis acidity of tetrafluorinated derivative of 3 (compound 10), however, is not enough for binding with Cl^- and F^- via chalcogen bonds in contrast to previously studied Te analog of 10. It is suggested that the maximum positive values of molecular electrostatic potential at the σ-holes, V_S,max , can be a reasonable metric for design and synthesis of new anion receptors with selenadiazole-diazole/triazole hybrids as a special target. Related chlorinated compounds are also discussed.

Perfluoro-tert-butanol: a cornerstone for high performance fluorine-19 magnetic resonance imaging

As a versatile quantification and tracking technology, 19F magnetic resonance imaging (19F MRI) provides quantitative "hot-spot" images without ionizing radiation, tissue depth limit, and background interference. However, the lack of suitable imaging agents severely hampers its clinical application. First, because the 19F signals are solely originated from imaging agents, the relatively low sensitivity of MRI technology requires high local 19F concentrations to generate images, which are often beyond the reach of many 19F MRI agents. Second, the peculiar physicochemical properties of many fluorinated compounds usually lead to low 19F signal intensity, tedious formulation, severe organ retention, etc. Therefore, the development of 19F MRI agents with high sensitivity and with suitable physicochemical and biological properties is of great importance. To this end, perfluoro-tert-butanol (PFTB), containing nine equivalent 19F and a modifiable hydroxyl group, has outperformed most perfluorocarbons as a valuable building block for high performance 19F MRI agents. Herein, we summarize the development and application of PFTB-based 19F MRI agents and analyze the strategies to improve their sensitivity and physicochemical and biological properties. In the context of PFC-based 19F MRI agents, we also discuss the challenges and prospects of PFTB-based 19F MRI agents.

Determination of acid dissociation constants of Alizarin Red S, Methyl Orange, Bromothymol Blue and Bromophenol Blue using a digital camera

Acid dissociation constants (pK_a) are important parameters for the characterization of organic and inorganic compounds. They play a crucial role in different physical, chemical, and biological studies. Herein, we introduce a new approach for the determination of acid dissociation constant based on digital image analysis using a low-cost, precise, accurate, sensitive, and portable home-made, camera-based platform. Digital images of Alizarin Red S, Bromophenol Blue, Bromothymol Blue, and Methyl Orange solutions were captured at various pH values. The captured images were analysed to obtain the RGB (Red, Green, and Blue) colour intensities that are used to calculate the RGB colour absorbances. The pK_a values were calculated from the RGB colour absorbance–pH relationship using graphical and mathematical methods, and with the aid of DATAN software. For the four studied dyes, the results obtained from digital image analysis were in excellent agreement with the data of sophisticated spectrophotometers and the previously reported literature data.

Acid dissociation constants (pK_a) are important parameters for the characterization of organic and inorganic compounds.

Retention of acidic and basic analytes in reversed phase column using fluorinated and novel eluent additives for liquid chromatography-tandem mass spectrometry

This research focuses on retention mechanisms in a LC column with C18 stationary phase when novel eluent additives (HFIP, HFTB and TFE as well as NFTB and perfluoropinacol) are used. The retention factors between novel eluent additives and conventional ones like ammonium acetate and ammonium bicarbonate at different eluent pH values were compared. A simple set of drug-like molecules, widely spread over different logP values, containing protonated and deprotonated acids and bases was selected for this investigation. HFIP, HFTB, NFTB and PP demonstrated strong influence on basic polar analytes in basic medium. These additives drastically increased retention. A decrease in retention was observed for acidic analytes when novel eluent additives were used. Additionally, for the first time, the absolute pH (pH_abs) scale was used for expressing the mobile phase pH.