Ivar Koppel

Pentakis(trifluoromethyl)phenyl, a sterically crowded and electron-withdrawing group: synthesis and acidity of pentakis(trifluoromethyl)benzene, -toluene, -phenol, and -aniline.

A general route to functionalized pentakis(trifluoromethyl)phenyl (C6(CF3)5) derivatives, promising building blocks for designing novel stable carbenes, radical species, superacids, weakly coordinating anions and other practically and theoretically useful species, is presented. This pertrifluoromethylation route proceeds via conveniently pregenerated (trifluoromethyl)copper (CF3Cu) species in DMF, stabilized by addition of 1,3-dimethyl-2-imidazolidinone (DMI). These species react with hexaiodobenzene at ambient temperature to give the potassium pentakis(trifluoromethyl)phenoxide along with hexakis(trifluoromethyl)benzene and pentakis(trifluoromethyl)benzene in a combined yield of 80%. A possible reaction pathway explaining the formation of pentakis(trifluoromethyl)phenoxide is proposed. Pentakis(trifluoromethyl)phenol gives rise to easily functionalized pentakis(trifluoromethyl)chlorobenzene and pentakis(trifluoromethyl)aniline. Pertrifluoromethylation of pentaiodochlorobenzene and pentaiodotoluene allows straightforward access to pentakis(trifluoromethyl)chlorobenzene and pentakis(trifluoromethyl)toluene, respectively. XRD structures of several C6(CF3)5 derivatives were determined and compared with the calculated structures. Due to the steric crowding the aromatic rings in all C6(CF3)5 derivatives are significantly distorted. The gas-phase acidities (Delta Gacid) and pKa values in different solvents (acetonitrile (AN), DMSO, water) for the title compounds and a number of related compounds have been measured. The origin of the acidifying effect of the C6(CF3)5 group has been explored using the isodesmic reactions approach.

Solute-solvent and solvent-solvent interactions in binary solvent mixtures. 2. Effect of temperature on the ET(30) polarity parameter of dipolar hydrogen bond acceptor-hydrogen bond donor mixtures

The effect of temperature on the Dimroth-Reichardt ET(30) parameter of binary mixtures of dimethyl sulfoxide, acetonitrile and nitromethane with alcohols and water was studied. The ET(30) polarity parameter of many of these binary mixtures exhibits a strong synergism. Dimethyl sulfoxide, acetonitrile and nitromethane form hydrogen-bonded complexes with the alcohols more polar than themselves. The ET(30) values of the mixtures were fitted according to an earlier model, based on solvent exchange equilibria, that allows calculation of the ET(30) values of the hydrogen-bonded complexes. The variation of the ET(30) values of the pure solvents and the hydrogen-bonded complexed solvents with temperature shows that the synergism decreases as the temperature increases.

Critical test of performance of B3LYP functional for prediction of gas-phase acidities and basicities

Abstract The gas-phase acidities and basicities for 49 acids and 32 bases, calculated using B3LYP hybrid DFT method and 6-31G ∗ , 6-31+G ∗ , 6-311+G ∗∗ , and 6-311+G(3df,3pd) basis sets are compared with corresponding experimental values. The best results were obtained with 6-311+G(3df,3pd) basis set; the average absolute errors were below 2.5 kcal/mol both for basicities and acidities. Good results for both acidities and basicities (the average absolute errors were ⩽3 kcal/mol) were also obtained using the 6-311+G ∗∗ basis set and even with a moderate 6-31+G ∗ basis set (mean absolute errors were

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.

SUPERACIDITY OF NEUTRAL BRONSTED ACIDS IN GAS PHASE

Quantum chemical calculations of potentially superacidic neutral Brönsted acids were carried out using the PM3 method. It was shown that the PM3 method can be used to predict the gas phase acidities of acidic compounds only if empirical corrections are made. A strong acidifying effect is predicted for a new family of compounds in which an sp2 oxygen is substituted by an (DOUBLE BOND) NSO2CF3 group. So, for example, such replacement is expected to result in acid strengthening by 47.5 kcal/mol in the case of CH3CHO and by 22.7 kcal/mol in the case of CF3SO2OH. The acidities of such compounds are predicted to be increased further (nonadditively) by stepwise replacements of (DOUBLE BOND) O by (DOUBLE BOND) NSO2CF3. The geometries of known superacidic systems were reproduced quite well by PM3 method. The geometries of several superacidic systems were analyzed.

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.

Quantum chemical calculations of geometries and gas-phase deprotonation energies of linear polyyne chains

The molecular geometries of polyyne chains H(CC)nH with their deprotonated forms (anions) have been optimized using ab initio LCAO-SCF molecular orbital (MO) method and density functional theory at different basis set levels. The polyynes possess a series of alternating single and triple bonds. On the theoretical side the persistence of bond alternation and the effect of chain lengthening on the individual bond length in linear conjugated polyyne chains has been investigated. The common conclusion has been drawn that the bond alternation will persist and that bond length variation will be small. The triple bond length increases progressively toward the asymptotic limits as the value of n increases progressively. If the split-valence basis set was employed, the total charges obtained using the Mulliken population analysis yielded unrealistic values. Using natural bond orbital (NBO) analysis or Baders analysis, the net charges of the individual atoms converge very rapidly to their asymptotic limits, and the central atoms have almost zero charges in contrast to the Mulliken population analysis results. The reliability of deprotonation energies of neutral polyynes and their monoanionic derivatives calculated from the differences in molecular energy of the parent chains and the corresponding anions E(H(CC)n−)–E(H(CC)nH) and E(−(CC)n−)–E(H(CC)n−) was tested for different basis sets. The increase of the number of CC bonds in the chain decreases these differences asymptotically. The studied compounds are the best available building blocks in bimetallic compounds with useful properties in molecular electronics and nonlinear optics.

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.

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.