Evaluation and validation of detailed and simplified models of the uncertainty of unified pHabsH2O measurements in aqueous solutions

Highlights • First detailed evaluation of the uncertainty of pHabsH2O measurements. • Bottom-up uncertainty evaluations proven valid for 95% confidence. • Monte Carlo Simulation of pHabsH2O measurement ladder with least-squares minimisation. • Described simplified and detailed bottom-up uncertainty evaluations are equivalent. • Measurements from 2 to 10 pHabsH2O with a 95% expanded uncertainty of 0.26–0.51.The use of the unified pH concept, pHabsH2O, applicable to aqueous and non-aqueous solutions, which allows interpreting and comparison of the acidity of different types of solutions, requires reliable and objective determination. The pHabsH2O can be determined by a single differential potentiometry measurement referenced to an aqueous reference buffer or by a ladder of differential potentiometric measurements that allows minimisation of inconsistencies of various determinations. This work describes and assesses bottom-up evaluations of the uncertainty of these measurements, where uncertainty components are combined by the Monte Carlo Method (MCM) or Taylor Series Approximation (TSM). The MCM allows a detailed simulation of the measurements, including an iterative process involving in minimising ladder deviations. On the other hand, the TSM requires the approximate determination of minimisation uncertainty. The uncertainty evaluation was successfully applied to measuring aqueous buffers with pH of 2.00, 4.00, 7.00, and 10.00, with a standard uncertainty of 0.01. The reference and estimated values from both approaches are metrologically compatible for a 95% confidence level even when a negligible contribution of liquid junction potential uncertainty is assumed. The MCM estimated pH values with an expanded uncertainty, for the 95% confidence level, between 0.26 and 0.51, depending on the pH value and ladder inconsistencies. The minimisation uncertainty is negligible or responsible for up to 87% of the measurement uncertainty. The TSM quantified measurement uncertainties on average only 0.05 units larger than the MCM estimated ones. Additional experimental tests should be performed to test these uncertainty models for analysis performed in other laboratories and on non-aqueous solutions

The perfluorinated alcohols (F5C6)(F3C)(2)COH and (F5C6)(F10C5)COH: synthesis, theoretical and acidity studies, spectroscopy and structures in the solid state and the gas phase

Trapp N, Scherer H, Hayes SA, et al. The perfluorinated alcohols (F5C6)(F3C)(2)COH and (F5C6)(F10C5)COH: synthesis, theoretical and acidity studies, spectroscopy and structures in the solid state and the gas phase. Physical Chemistry Chemical Physics. 2011;13(13):6184-6191.The syntheses of the perfluorinated alcohols (F5C6)(F3C)(2)COH (1) and (F5C6)(C5F10)COH (2) are described. Both compounds were prepared in reasonable yields (1: 65%, 2: 85%) by reacting the corresponding ketone with C6F5MgBr, followed by acidic work-up. The alcohols were characterized by NMR, vibrational spectroscopy, single-crystal X-ray diffraction, acidity measurements and gas-phase electron diffraction. A combination of appropriate 2D NMR experiments allowed the unambiguous assignment of all signals in the F-19 spin systems, of which that of 2 was especially complex. High acidity of the alcohols is indicated by acidity measurements as well as the calculated gas phase acidities. It is also supported by the crystal structure of 2, which exhibits only a single weak intermolecular hydrogen bridge with an O center dot center dot center dot O distance of 301 pm. This shows the low donor strength of the oxygen atom in the compound, which is partly compensated through formation of two intramolecular CF center dot center dot center dot H contacts of 220 and 232 pm length to the proton not involved in the hydrogen bridge. The pK(a) values in acetonitrile are 22.2 for 1 and 22.0 for 2; their calculated gas phase acidities are 1367 and 1343 kJ mol(-1) (MP2/TZVPP level)

Tris(benzophenoneimino)phosphane and related compounds

The dataset includes files supporting the article named "Tris(benzophenoneimino)phosphane and related compounds". Monocrystal XRD data, calculated .cosmo files, calculated pKaH values for related and correlation compounds, gas-phase geometry optimization and frequency calculation files, and NMR files are given

Experimental Basicities of Superbasic Phosphonium Ylides and Phosphazenes

Experimental basicities of some of the strongest superbases ever measured (phosphonium ylides) are reported, and by employing these compounds, the experimental self-consistent basicity scale of superbases in THF, reaching a p<i>K</i>_α (estimate of p<i>K</i>_a) of 35 and spanning more than 30 p<i>K</i>_a units, has been compiled. Basicities of 47 compounds (around half of which are newly synthesized) are included. The solution basicity of the well-known <i>t</i>-Bu-NP₄(dma)₉ phosphazene superbase is now rigorously linked to the scale. The compiled scale is a useful tool for further basicity studies in THF as well as in other solvents, in particular, in acetonitrile. A good correlation between basicities in THF and acetonitrile spanning 25 orders of magnitude gives access to experimentally supported very high (p<i>K</i>_a > 40) basicities in acetonitrile, which cannot be directly measured. Analysis of structure–basicity trends is presented

Experimental Basicities of Phosphazene, Guanidinophosphazene, and Proton Sponge Superbases in the Gas Phase and Solution

Experimental gas-phase superbasicity scale spanning 20 orders of magnitude and ranging from bicyclic guanidine 7-methyl-1,5,7-triazabicyclo[4.4.0]­dec-5-ene to triguanidinophosphazenes and P₃ phosphazenes is presented together with solution basicity data in acetonitrile and tetrahydrofuran. The most basic compound in the scaletriguanidinophosphazene Et–NP­[NC­(NMe₂)₂]₃has the highest experimental gas-phase basicity of an organic base ever reported: 273.9 kcal mol^–1. The scale includes besides the higher homologues of classical superbasic phosphazenes and several guanidino-substituted phosphazenes also a number of recently introduced bisphosphazene and bis-guanidino proton sponges. This advancement was made possible by a newly designed Fourier transform ion cyclotron resonance (ICR) mass spectrometry setup with the unique ability to generate and control in the ICR cell sufficient vapor pressures of two delicate compounds having low volatility, which enables determining their basicity difference. The obtained experimental gas-phase and solution basicity data are analyzed in terms of structural and solvent effects and compared with data from theoretical calculations

¹⁵N NMR Spectroscopy, X‑ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases

Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}­C₅H₄N (<b>I</b>) and 2,6-{hpp}₂C₅H₃N (<b>II</b>) (hppH = 1,3,4,6,7,8-hexahydro-2<i>H</i>-pyrimido­[1,2-<i>a</i>]­pyrimidine) were protonated using [HNEt₃]­[BPh₄] to afford [<b>I</b>-H]­[BPh₄] (<b>1a</b>), [<b>II</b>-H]­[BPh₄] (<b>2</b>), and [<b>II</b>-H₂]­[BPh₄]₂ (<b>3</b>). Solution-state ¹H and ¹⁵N NMR spectroscopy shows a symmetrical cation in <b>2</b>, indicating a facile proton-exchange process in solution. Solid-state ¹⁵N NMR data differentiates between the two groups, indicating a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for <b>1a</b> by single-crystal neutron diffraction. The crystal structure of <b>1a</b> shows association of two [<b>I</b>-H]⁺ cations within a cage of [BPh₄]⁻ anions. Computational analysis performed in the gas phase and in MeCN solution shows that the free energy barrier to transfer a proton between imino centers in [<b>II</b>-H]⁺ is 1 order of magnitude lower in MeCN than in the gas phase. The results provide evidence that linking hpp groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished p<i>K</i>_a(MeCN) values. Spectrophotometrically measured p<i>K</i>_a values are in excellent agreement with calculated values and confirm that <b>I</b> and <b>II</b> are superbases in solution

Symmetric Potentiometric Cells for the Measurement of Unified pH Values

A unified pH scale of absolute values (pHabs scale) enables the comparison of acidities in different solvents. To date, very few different experimental setups have been used for the measurement of values on this scale. The article describes the design and performance of the different symmetric cells used for unified pH measurement by several institutions. Well-established and reliable standard aqueous buffer solutions are the first step of method validation necessary to achieve a robust metrological level for more complex media. The pH of aqueous standard buffers was measured by differential potentiometry, where the potential between two glass electrodes is measured directly. All the tested electrochemical cells prove to be suitable for unified pH measurements. This validation highlights that the method is, to a large extent, independent of the used equipment, including the cell geometry. The inherent symmetry of the cell design helps to reduce the experimental workload and improve the accuracy of obtained results

¹⁵N NMR Spectroscopy, X‑ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases

Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}­C₅H₄N (<b>I</b>) and 2,6-{hpp}₂C₅H₃N (<b>II</b>) (hppH = 1,3,4,6,7,8-hexahydro-2<i>H</i>-pyrimido­[1,2-<i>a</i>]­pyrimidine) were protonated using [HNEt₃]­[BPh₄] to afford [<b>I</b>-H]­[BPh₄] (<b>1a</b>), [<b>II</b>-H]­[BPh₄] (<b>2</b>), and [<b>II</b>-H₂]­[BPh₄]₂ (<b>3</b>). Solution-state ¹H and ¹⁵N NMR spectroscopy shows a symmetrical cation in <b>2</b>, indicating a facile proton-exchange process in solution. Solid-state ¹⁵N NMR data differentiates between the two groups, indicating a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for <b>1a</b> by single-crystal neutron diffraction. The crystal structure of <b>1a</b> shows association of two [<b>I</b>-H]⁺ cations within a cage of [BPh₄]⁻ anions. Computational analysis performed in the gas phase and in MeCN solution shows that the free energy barrier to transfer a proton between imino centers in [<b>II</b>-H]⁺ is 1 order of magnitude lower in MeCN than in the gas phase. The results provide evidence that linking hpp groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished p<i>K</i>_a(MeCN) values. Spectrophotometrically measured p<i>K</i>_a values are in excellent agreement with calculated values and confirm that <b>I</b> and <b>II</b> are superbases in solution