Jaan Saame

Equilibrium acidities of superacids.

In this paper, we report the most comprehensive equilibrium superacidity scale that is available to date. Contrary to most of the past works, this scale is set up in a medium of constant composition and the obtained acidity values characterize the acidities of molecules rather than acidities of media. The current scale is thus complementary to the well-known H(0) scale in the information that it provides. The solvent used is 1,2-dichloroethane (DCE). DCE has very weak basic properties (but sufficiently high polarity) and is an appropriate solvent for measuring acidities of very strong acids of diverse chemical nature. DCE acidities of well-known superacids (CF(3)SO(2)OH, (CF(3)SO(2))(2)NH, cyanocarbon acids, etc.) as well as common mineral acids (H(2)SO(4), HI, HBr, etc.) are reported. Acidities of altogether 62 acids have been determined from 176 interlinked relative acidity measurements. The scale spans 15 orders of magnitude (from picric acid to 1,1,2,3,3-pentacyanopropene) and is expected to be a useful tool in design, use, and further acidity measurements of superacidic molecules.

Revision of the Gas-Phase Acidity Scale below 300 kcal mol-1

The gas-phase acidity (GA) scale from (CF(3)CO)(2)NH to (C(2)F(5)SO(2))(2)NH--about a 24 kcal mol(-1) range of gas-phase acidities--was reexamined using the Fourier transform ion cyclotron resonance equilibrium measurement approach. Some additions and modifications to the standard methodology of GA measurements were introduced (estimation of partial pressures from mass spectra of the compounds, instead of the pressure gauge readings and use of long reaction times) to achieve higher reliability. Gas-phase acidities of 18 compounds were determined for the first time. The results reveal a contraction of the previously published values in this part of the scale. In particular, the GA values of (CF(3)SO(2))(2)NH and (C(2)F(5)SO(2))(2)NH (important components of lithium ion battery electrolytes and ionic liquids) were revised toward stronger acidities from 291.8 kcal mol(-1) to 286.5 kcal mol(-1) and from 289.4 kcal mol(-1) to 283.7 kcal mol(-1) (i.e., by 5.3 and 5.7 kcal mol(-1)), respectively. Experimental and computational evidence is presented in support of the current results.

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.

Molecular structure and acid/base properties of 1,2-dihydro-1,3,5-triazine derivatives

It is shown that guanidine and its N,N-dimethyl-derivative react with substituted carbodiimides, affording hitherto unknown 1,2-dihydro-1,2,3-triazine derivatives. The structures of three novel compounds of this type and their perchlorate salts were elucidated by spectroscopic (IR, 1H and 13C NMR and 15N solid-state NMR) and X-ray diffraction methods. The acid/base properties were also determined experimentally and by using DFT calculations with the B3LYP functional. The most basic compound was found to be dihydrotriazine 3, the basicity of which with the pKa value of 23.3 is of the same order of magnitude as that of tetramethylguanidine. Acidity measurements revealed that all the compounds studied are very weak acids with pKa values in the range of 25.8–30.8 pKa units in acetonitrile.

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 (I) and 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2), and [II-H2][BPh4]2 (3). Solution-state (1)H and (15)N NMR spectroscopy shows a symmetrical cation in 2, indicating a facile proton-exchange process in solution. Solid-state (15)N 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 1a by single-crystal neutron diffraction. The crystal structure of 1a shows association of two [I-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 [II-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 pKa(MeCN) values. Spectrophotometrically measured pKa values are in excellent agreement with calculated values and confirm that I and II are superbases in solution.

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.

Fluoro‐ and Perfluoralkylsulfonylpentafluoroanilides: Synthesis and Characterization of NH Acids for Weakly Coordinating Anions and Their Gas‐Phase and Solution Acidities

Fluoro- and perfluoralkylsulfonyl pentafluoroanilides [HN(C6F5)(SO2X); X = F, CF3, C4F9, C8F17] are a class of imides with two different strongly electron-withdrawing substituents attached to a nitrogen atom. They are NH acids, the unsymmetrical hybrids of the well-known symmetrical bissulfonylimides and bispentafluorophenylamine. The syntheses, the structures of these perfluoroanilides, their solvates, and some selected lithium salts give rise to a structural variety beyond the symmetrical parent compounds. The acidities of representative subsets of these novel NH acids have been investigated experimentally and quantum-chemically and their gas-phase acidities (GAs) are reported, as well as the pKa values of these compounds in acetonitrile (MeCN) and DMSO solution. In quantum chemical investigations with the vertical and relaxed COSMO cluster-continuum models (vCCC/rCCC), the unusual situation is encountered that the DMSO-solvated acid Me2SO-H-N(SO2CF3)2, optimized in the gas phase (vCCC model), dissociates to Me2SO-H(+)-N(SO2CF3)2(-) during structural relaxation and full optimization with the solvation model turned on (rCCC model). This proton transfer underlines the extremely high acidity of HN(SO2CF3)2. The importance of this effect is studied computationally in DMSO and MeCN solution. Usually this effect is less pronounced in MeCN and is of higher importance in the more basic solvent DMSO. Nevertheless, the neglect of the structural relaxation upon solvation causes typical changes in the computational pKa values of 1 to 4 orders of magnitude (4-20 kJ mol(-1)). The results provide evidence that the published experimental DMSO pKa value of HN(SO2CF3)2 should rather be interpreted as the pKa of a Me2SO-H(+)-N(SO2CF3)2(-) contact ion pair.

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 pKα (estimate of pKa) of 35 and spanning more than 30 pKa units, has been compiled. Basicities of 47 compounds (around half of which are newly synthesized) are included. The solution basicity of the well-known t-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 (pKa > 40) basicities in acetonitrile, which cannot be directly measured. Analysis of structure-basicity trends is presented.

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

The syntheses of the perfluorinated alcohols (F(5)C(6))(F(3)C)(2)COH (1) and (F(5)C(6))(C(5)F(10))COH (2) are described. Both compounds were prepared in reasonable yields (1: 65%, 2: 85%) by reacting the corresponding ketone with C(6)F(5)MgBr, 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 (19)F 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...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...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).

Alternative Eluent Composition for LC-MS Analysis of Perfluoroalkyl Acids in Raw Fish Samples

A wide range of anthropogenic pollutants that possess serious environmental and health risks are known. One type of these harmful substances that have become a focus of interest during the past decade are perfluoroalkyl acids (PFAAs), which are extensively used in industry for different purposes. Due to the harmful effects that these compounds might cause in living organisms, EFSA and EU CONTAM panel have issued a monitoring program for PFAAs in foodstuffs. This has given rise to intense research dedicated to the analysis of PFAAs over the past few years. This work focuses on chromatographic analysis of three PFAAs in fish. The analytes, perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), and perfluorooctanesulfonic acid (PFOS), are commonly associated with the production of fluoropolymers. Fluorinated alcohols are used as eluent components, and their possible advantages as eluent modifiers in LC-MS analysis of PFAAs, alternative retention mechanism and enhanced ionization efficiency, are examined. The analyzed fish samples originating from Estonian fresh and marine waters had low contents of PFAAs.