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.

Acidity of Strong Acids in Water and Dimethyl Sulfoxide.

Careful analysis and comparison of the available acidity data of HCl, HBr, HI, HClO4, and CF3SO3H in water, dimethyl sulfoxide (DMSO), and gas-phase has been carried out. The data include experimental and computational pKa and gas-phase acidity data from the literature, as well as high-level computations using different approaches (including the W1 theory) carried out in this work. As a result of the analysis, for every acid in every medium, a recommended acidity value is presented. In some cases, the currently accepted pKa values were revised by more than 10 orders of magnitude.

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.

Solution and gas-phase acidities of all-trans (all-E) retinoic acid: an experimental and computational study.

Retinoic acid is of fundamental biological importance. Its acidity was determined in the gas phase and in acetonitrile solution by means of mass spectrometry and UV/Vis spectrophotometry, respectively. The intrinsic acidity is slightly higher than that of benzoic acid. In solution, the situation is opposite. The experimental systems were described theoretically applying quantum chemical methods (wave function theory and density functional theory). This allowed the determination of the molecular structure of the acid and its conjugate base, both in vacuo and in solution, and for computational estimates of its acidity in both phases.

Superacidity of closo-dodecaborate-based Brønsted acids: a DFT study.

The structures and intrinsic gas-phase acidities (GA) of some dodecaborane acids, the derivatives of YB12H11H (Y = PF3, NH3, NF3, NMe3), B12H12H2, and B12H12H(-) (HA, H2A, and HA(-), respectively) have been computationally explored with DFT B3LYP method at the 6-311+G** level of theory as new possible directions of creating superstrong Brønsted acids. Depending on the nature and number of the substituents different protonation geometries were investigated. In general, the GA values of the neutral systems varied according to the substituents in the following order: CF3 < F < Cl and in case of anionic acids: CF3 < Cl < F. The dodecatrifluoromethyl derivative of H2A, B12(CF3)12H1H2, emerges as the strongest among the considered acids and is expected to be in the gas phase at least as strong as the undecatrifluoromethyl carborane, CB11(CF3)11H1H. The GA values of the respective monoanionic forms of the considered acids all, but the (CF3)11 derivative, remained higher than the widely used threshold of superacidity. The HA derivatives' (Y = PF3, NF3) GA's were approximately in the same range as the H2A acids'. In the case Y = NH3 or NMe3 the GA values were significantly higher. Also, the pKa values of B12H12H2, CB11H12H, and their perfluorinated derivatives in 1,2-dichloroethane (DCE) were estimated with SMD and cluster-continuum model calculations. The obtained estimates of pKa values of the perfluorinated derivatives are by around 30 units lower than that of trifluoromethylsulfonylimide, making these acids the strongest ever predicted in solution. The derivatives of B12H12H2 are as a rule not significantly weaker acids than the respective derivatives of CB11H12H. This is important for expanding practical applicability of this type of acids and their anions, as they are synthetically much more accessible than the corresponding CB11H12(-) derivatives.

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.

Relativistic effects on acidities and basicities of Brønsted acids and bases containing gold.

It is usually believed that relativistic effects as described by the Dirac-Schrödinger equation (relative to the classical or time-independent Schrödinger equation) are of little importance in chemistry. A closer look, however, reveals that some important and widely known properties (e.g., gold is yellow, mercury is liquid at room temperature) stem from relativistic effects. So far the influence of relativistic effects on the acid-base properties has been mostly ignored. Here we show that at least for compounds of gold such omission is completely erroneous and would lead to too high basicity and too low acidity values with errors in the range of 25-55 kcal mol(-1) (or 20 to 44 powers of ten in pK(a) units) in the gas-phase. These findings have important implications for the design of new superstrong acids and bases, and for the understanding of gold-catalysed reactions.

A new class of organosuperbases, N-alkyl- and N-aryl-1,3-dialkyl-4,5-dimethylimidazol-2-ylidene amines: synthesis, structure, pK(BH+) measurements, and properties.

A series of stable organosuperbases, N-alkyl- and N-aryl-1,3-dialkyl-4,5-dimethylimidazol-2-ylidene amines, were efficiently synthesized from N,N'-dialkylthioureas and 3-hydroxy-2-butanone and their basicities were measured in acetonitrile. The derivatives with tert-alkyl groups on the imino nitrogen were found to be more basic than the tBuP(1) (pyrr) phosphazene base in acetonitrile. The origin of the high basicity of these compounds is discussed. In acetonitrile and in the gas phase, the basicity of the alkylimino derivatives depends on the size of the substituent at the imino group, which influences the degree of aromatization of the imidazole ring, as measured by (13)C NMR chemical shifts or by the calculated ΔNICS(1) aromaticity parameters, as well as on solvation effects. If a wider range of imino-substituents, including electron-acceptor substituents, is treated in the analysis then the influence of aromatization is less predominant and the gas-phase basicity becomes more dependent on the field-inductive effect, polarizability, and resonance effects of the substituent.

17O NMR studies of ortho-substituent effects in substituted phenyl tosylates.

(17)O NMR spectra for 35 ortho-, para-, and meta-substituted phenyl tosylates (phenyl 4-methylbenzenesulfonates), 4-CH(3)-C(6)H(4) SO(2)OC(6)H(4)-X, at natural abundance in acetonitrile at 50 °C were recorded. The (17)O NMR chemical shifts, δ((17)O), of the sulfonyl (SO(2)) and the single-bonded phenoxy (OPh) oxygens for para and meta derivatives correlated well with dual substituent parameter treatment using the Taft inductive, σ(I), and resonance, σº(R), constants. The influence of ortho substituents on the sulfonyl oxygen and the single-bonded phenoxy oxygen chemical shifts, δ((17)O), was found to be nicely described by the Charton equation: δ((17)O)(ortho) = δ((17)O)(H) + ρ(I)σ(I) + ρ(R)σ°(R) + δE(s)(B) when the data treatment was performed separately for electron-donating +R substituents and electron-attracting -R substituents. Electron-attracting meta and para substituents in the phenyl moiety caused deshielding while the electron-donating meta, para and ortho +R substituents produce shielding effects on the sulfonyl (SO(2)) and single-bonded phenoxy (OPh) oxygens. The influence of ortho inductive and resonance effects in the case of +R substituents was found to be approximately twice higher than the corresponding influence from the para position. Due to the steric effect of ortho substituents a decrease in shielding of the oxygens at the sulfonyl group (δE(s)(B) > 0, E(s)(B) < 0) was detected.

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.

Calculations of pKa of superacids in 1,2-dichloroethane.

Acidity calculations for some CH and NH superacids in 1,2-dichloroethane (DCE) were carried out using SMD and COSMO-RS continuum solvation models. After comparing the results of calculations with respective experimental pK(a) values it was found that the performance of SMD/M05-2X/6-31G* method is characterized by the mean unsigned error (MUE) of 0.5 pK(a) units and the slope of regression line of 0.915. The similar SMD/B3LYP/6-31G* approach was slightly less successful. The strong correlation over entire data set is confirmed by R(2) values of 0.990 and 0.984 for M05-2X and B3LYP functionals, respectively. The COSMO-RS method, while providing the value of the linear regression line slope similar to the corresponding values from SMD approach, characterized by rather loose correlation (R(2) = 0.823, MUE = 1.7 pK(a) units) between calculated and experimental pK(a) values in DCE solution.

Acidity of anilines: calculations vs experiment.

The gas-phase acidities of ca. 60 monosubstituted anilines (with acidity span of almost 50 kcal mol(-1)) have been calculated using density functional theory (DFT) at the B3LYP/6-311+G** level. At this relatively simple level of theory the calculated (ΔG(calc)) and available experimental (ΔG(exp)) acidities are in reasonable quantitative correlation according to the following equation: ΔG(obs) = a + bΔG(calc), where a=20.79, b=0.942, n=27, R(2)=0.990, and s=0.78 kcal·mol(-1). The slope is not far from its ideal value. Substituent effects on the acidities were dissected separately into those operating in the neutral acid molecule and in its conjugated anion using the isodesmic homodesmotic reactions. All in all, both forms, neutral and anionic, are contributing in combination to make up the gross acidity of anilines. However, the contributions of the anions into the gross substituent effects are much larger than the substituent effects in the neutral anilines. Some of the systems were used in testing a relatively new theoretical model, COSMO-RS (conductor-like screening model for real solvents), using it for the prediction of pK(a) values in DMSO. The method proved to be rather accurate for showing pK(a) trends (R(2)=0.980 in DMSO). However, the predicted absolute pK(a) values were all somewhat lower (rmsd=2.49 kcal·mol(-1)) than the respective experimental values.

Polytrifluoromethylation versus polyfluorination of the isomers of Kekulé benzene and phenol: a theoretical study.

The study of the polyfluorination and polytrifluoromethylation effects on electronic structure and intrinsic acidities has been performed using DFT B3LYP and NBO calculations for the valence isomers of benzene and phenol, i.e., the structures of prismane, benzvalene, Kekulé, and Dewar systems. Also the isodesmic reaction analysis approach to estimate the effects of the substituents on the acidity of the compounds has been used. Although in some systems the additivity of fluorine substituents was more than 100%, the acidifying effects of the fluorine substituents were comparable to that of the CF(3) in only one case. Isodesmic reaction analysis of substituent effects shows that steric effects of poly-CF(3) substitution in the alicyclic cage compounds are significantly smaller. The relative thermodynamic stabilities of the valence isomers of Kekulé benzene were significantly lower than that of the aromatic cycle. The introduction of fluorine substituents often destabilized the compounds even further. Out of the fluorinated hydroxy derivatives of prismane, Dewar benzene, and benzvalene, not all are predicted to be stable enough to be able to undergo a reversible protonation-deprotonation process. In the case of several hydroxy derivatives deprotonation is accompanied by the rupture of a C(α)-C(ß) bond and in some cases by the rearrangement to the corresponding phenolate anion. The isomerization of benzene derivatives resulted in more cases where the acidities increased compared with the respective phenol derivatives. The only hydroxy compound with significantly higher acidity than the respective phenol was pentafluorinated 2-OH-Dewar benzene, which was expected to rival the gas-phase acidity of perfluoro-1-adamantanol, currently the most acidic experimentally measured perfluorinated alcohol.

Gas-phase basicities around and below water revisited.

This work employs Fourier transform ion cyclotron resonance (FT-ICR) and the Gaussian quantum chemistry composite methods W1 and G2 to experimentally and computationally analyze gas-phase basicities (GB) for a series of weak bases in the basicity region around and below water. The study aims to clarify the long-standing discrepancy between reported GB values for weak bases obtained via high-pressure mass spectrometry (HPMS) and ICR; the ICR scale is observed to be more than 2 times contracted compared to the HPMS scale. The computational results of this work support published HPMS data. This agreement improves with increasing sophistication of the computational method and is excellent at the W1 level. Several equilibria were also re-examined experimentally using FT-ICR. In the experiments with some polyfluorinated weak bases (hexafluoro-2-propanol and nonafluoro-2-methyl-2-propanol), it was found that two protonation processes compete in the gas phase: protonation on oxygen and protonation on fluorine. In these species, protonation on fluorine proceeds faster and is statistically favored over protonation on oxygen but leads to cations that are thermodynamically less stable than oxygen-protonated cations. The process may also lead to the irreversible loss of HF. The rearrangement of fluorine-protonated cations to oxygen-protonated cations is very slow and is further suppressed by the process of HF abstraction. These results at least partially explain the discrepancy between published HPMS data and earlier FT-ICR findings and call for the utmost care in using FT-ICR for gas-phase basicity measurements of heavily fluorinated compounds. The narrower dynamic range of ICR necessitates the measurement of several problematic bases and produces some differences between the ICR results in the present work and the published HPMS data; the wider dynamic range allows HPMS to overcome these difficulties in connecting the ladder.

Boratabenzene Anions C5B(CN)6(-) and C5B(CF3)6(-) and the superacidic properties of their conjugate acids.

Designing superacids: A computational study of protonated boratabenzenes and the gas-phase acidity of their conjugate acids is presented. Conjugate acids of boratabenzenes substituted with CN or CF(3) groups (see figure) are highly acidic species; the protonated hexacyanoboratabenzene and hexakis(trifluoromethyl)boratabenzene have computational gas-phase acidities of 250.5 and 276.8 kcal mol(-1), respectively.

Gas-phase Brønsted superacidity of some derivatives of monocarba-closo-borates: a computational study.

The structures and gas-phase acidities (GA) of several CB(11)H(12)H-based carborane acid derivatives (HA) have been calculated with DFT B3LYP method using 6-311+G**, 6-311++G** basis sets. In order to verify the obtained GA values, several systems were also studied at G3(MP2) level of theory. Inserted substituents (CF(3), F, Cl, Br, I, CN, CH(3), etc.) followed the "belts" of the monocarborane cage starting from the boron antipodal to the carbon. In general, the predicted intrinsic gas-phase acidities of the systems varied according to the substituents in the following order of decreasing strength: CF(3) > F > Cl > Br > I > CN > CH(3). Nevertheless, some inconsistencies occurred. F and CN derivatives with lower degree of substitution had weaker intrinsic acidities than the respective Cl derivatives, but the situation was reversed in the case of a larger number of substituents. To obtain better understanding how the substituents influence the basicity of the carborane anion, three hypothetical reaction series were investigated, in which the protonation center was fixed on the boron atom (B(12)), antipodal to the carbon (C(1)), and a single substituent replaced the hydrogens at the vertexes of the three remaining positions (C(1), B(2), and B(7)). The intrinsic gas-phase acidities in these series of neutral carborane-based acids CB(11)X(1)H(11)H are found to clearly depend on the field-inductive and resonance effects of the substituent X. Some influence of the polarizability of X on the reaction center (B(12)) could be detected only in the alpha position (B(7)).

IEF-PCM calculations of absolute pKa for substituted phenols in dimethyl sulfoxide and acetonitrile solutions.

Absolute (nonrelative) pKa calculations for substituted phenols were carried out in nonaqueous media, demonstrating the predictive power of the integral equation formalism PCM method with a mean unsigned error of 0.6 pKa units for DMSO and 0.7 pKa units for MeCN at the B3LYP/6-31+G** level of theory combined with the scaled B3LYP/6-311+G** gas-phase data. At the same time, the correlation between the calculated and experimental pKa values yielded the value of the linear regression slope very close to unity for both DMSO and MeCN. Computation of pKa of neutral acids in nonaqueous solutions with a reasonable precision obviously depends on carefully tuned cavities, optimized for nonaqueous solutions. The ability of continuum solvation model to compensate charge escape from the cavity, which is prominent in the case of anions, is also required. And finally, good quality gas-phase data is essential to achieve required pKa precision.

Hydrogen-bonding interactions of (CF3)3CH and (CF3)3C- in the gas phase. An experimental (FT-ICR) and computational study.

Hydrogen-bonding interactions involving 2-(trifluoromethyl)-1,1,1,3,3,3-hexafluoropropane (1H) and 1(-) have been quantitatively studied by means of Fourier transform ion cyclotron resonance spectrometry. The existence of the species (1HCl)(-) and (1H1)(-) was demonstrated, and their thermodynamic stabilities were determined experimentally and computationally. In addition, some of their structural features were analyzed.

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.

A unified view to Brønsted acidity scales: do we need solvated protons?

The most comprehensive solvent acidity scale spanning 28 orders of magnitude of acidity was measured in the low-polarity solvent 1,2-dichloroethane (DCE). Its experimental core is linked to the unified acidity scale (pHabs) in an unprecedented and generalized approach only based on experimental values. This enables future measurements of acid strengths and acidity adjustments in low polarity solvents. The scale was cross-validated computationally. The purely experimental and computational data agree very well. The DCE scale includes 87 buffer systems with values between -13.0 and +15.4, i.e. similar to water at hypothetical and extreme pH values of -13.0 to +15.4. Unusually, such high acidities in DCE are not realized via solvated protons, but rather through strongly acidic molecules able to directly donate their proton, even to weak bases dissolved in the solution. Thus, in all examined cases, not a single solvated proton is present in one liter of DCE.