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The equilibrium acidity scale (pKa scale) in acetonitrile has been supplemented by numerous new compounds and new ΔpKa measurements. It now contains altogether 231 acids – over twice more than published previously – linked by 569 ΔpKa measurements and spans between the pKa values of hydrogen iodide (2.8) and indole (32.57), covering close to 30 orders of magnitude. Measurement results acquired over the last 15 years were added to the scale and new least‐squares treatment was carried out. The treatment yielded revised pKa values for the compounds published previously, with the root mean square difference between revised and previous values 0.04, demonstrating very good stability of the scale. Correlation equations were developed for estimating pKa values for the studied types of compounds in water, DMSO, DMF, and 1,2‐dichloroethane on the basis of pKa values in acetonitrile. These equations enable predicting pKa values with an average error around or less than 1 pKa unit, which is a sufficient accuracy for many applications. The scale is expected to be a useful tool for the widest possible research areas in organic chemistry, electrochemical power sources, catalysis, etc.

Rationale The first comprehensive quantitative scale of the efficiency of electrospray ionization (ESI) in the positive mode by monoprotonation, containing 62 compounds, was published in 2010. Several trends were found between the compound structure and ionization efficiency (IE) but, possibly because of the limited diversity of the compounds, some questions remained. This work undertakes to align the new data with the originally published IE scale and carry out statistical analysis of the resulting more extensive and diverse data set to derive more grounded relationships and offer a possibility of predicting logIE values. Methods Recently, several new IE studies with numerous compounds have been conducted. In several of them, more detailed investigations of the influence of compound structure, solvent properties, or instrument settings have been conducted. IE data from these studies and results from this work were combined, and the multilinear regression method was applied to relate IE to various compound parameters. Results The most comprehensive IE scale available, containing 334 compounds of highly diverse chemical nature and spanning 6 orders of magnitude of IE, has been compiled. Several useful trends were revealed. Conclusions The ESI ionization efficiency of a compound by protonation is mainly affected by three factors: basicity (expressed by pKaH in water), molecular size (expressed by molar volume or surface area), and hydrophobicity of the ion (expressed by charge delocalization in the ion or its partition coefficient between a water–acetonitrile mixture and hexane). The presented models can be used for tentative prediction of logIE of new compounds (under the used conditions) from parameters that can be computed using commercially available software. The root mean square error of prediction is in the range of 0.7–0.8 log units.

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This work explores the 7-amino-4-(trifluoromethyl)coumarin backbone for designing lipophilic fluorescent pH indicators. Four novel fluorescent phosphazene bases with an advantageous combination of properties-high lipophilicity of both neutral and charged forms, lack of localized charges in the cations, and prominent spectral changes upon protonation/deprotonation-were synthesized using the Staudinger reaction. The molecular structures of the synthesized compounds were confirmed using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FT-IR), nuclear magnetic resonance (NMR), and Fourier-transform ion-cyclotron resonance (FT-ICR)high-resolution mass spectrometry. The compounds were further characterized by ultraviolet-visible (UV-Vis) absorption and fluorescence emission spectra, lipophilicities (logP values) and pK a values in water and acetonitrile, as well as "biphasic" pK a values in the water/1-octanol solvent system. The bases could potentially be useful as working agents in optical pH sensors and acid-base indicators for lipophilic membranes.