Sascha K. Goll

Anchor points for the unified Brønsted acidity scale: the rCCC model for the calculation of standard Gibbs energies of proton solvation in eleven representative liquid media.

The COSMO cluster-continuum (CCC) solvation model is introduced for the calculation of standard Gibbs solvation energies of protons. The solvation sphere of the proton is divided into an inner proton-solvent cluster with covalent interactions and an outer solvation sphere that interacts electrostatically with the cluster. Thus, the solvation of the proton is divided into two steps that are calculated separately: 1) The interaction of the proton with one or more solvent molecules is calculated in the gas phase with high-level quantum-chemical methods (modified G3 method). 2) The Gibbs solvation energy of the proton-solvent cluster is calculated by using the conductor-like screening model (COSMO). For every solvent, the solvation of the proton in at least two (and up to 11) proton-solvent clusters was calculated. The resulting Gibbs solvation energies of the proton were weighted by using Boltzmann statistics. The model was evaluated for the calculation of Gibbs solvation energies by using experimental data of water, MeCN, and DMSO as a reference. Allowing structural relaxation of the proton-solvent clusters and the use of structurally relaxed Gibbs solvation energies improved the accordance with experimental data especially for larger clusters. This variation is denoted as the relaxed COSMO cluster-continuum (rCCC) model, for which we estimate a 1σ error bar of 10 kJ mol(-1) . Gibbs solvation energies of protons in the following representative solvents were calculated: Water, acetonitrile, sulfur dioxide, dimethyl sulfoxide, benzene, diethyl ether, methylene chloride, 1,2-dichloroethane, sulfuric acid, fluorosulfonic acid, and hydrogen fluoride. The obtained values are absolute chemical standard potentials of the proton (pH=0 in this solvent). They are used to anchor the individual solvent specific acidity (pH) scales to our recently introduced absolute acidity scale.

Absolute Brønsted Acidities and pH Scales in Ionic Liquids

Although receiving large interest over the last years, some fundamental aspects of Brønsted acidity in ionic liquids (ILs) have up to now been insufficiently highlighted. In this work, standard states, activity, and activity coefficient definitions for IL solvent systems were developed from general thermodynamic considerations and then extended to a general mixed solvent standard state. By using the bromide/bromoaluminate systems as representative ILs, formulae for thermodynamically consistent pH scales for ILs with simple (Br(-) ) and complex ([Aln Br3n+1 ](-) ) anions were derived on the basis of the chemical potential of the proton. Supported by quantum chemical [ccsd(t)/MP2/DFT/COSMO-RS] calculations, Gibbs solvation energies of the proton were calculated, which allowed the ILs to be ranked in absolute acidity, that is, pHabs or μabs (H(+) , IL), and additionally allowed their acidity to be compared with molecular Brønsted acid systems. It was shown that bromoaluminate ILs are suited for reaching superacidic conditions. The complexity of autoprotolysis processes in C6 MIM(+) [AlBr4 ](-) (C6 MIM=1-hexyl-3-methylimidazolium) with or without the addition of basic (i.e. Br(-) ) or acidic (AlBr3 and/or HBr) solutes was examined in detail by model calculations, and they indicated a large thermodynamic influence of small deviations from the exact stoichiometric composition.

Bulk Gas‐Phase Acidity

The capability of a gaseous Brønsted acid HB to deliver protons to a base is usually described by the gas-phase acidity (GA) value of the acid. However, GA values are standard Gibbs energy differences and refer to individual gas pressures of 1 bar for acid HB, base B(-), and proton H(+). We show that the GA value is not suited to describe the bulk acidity of a gaseous acid. Here the pressure dependence of the activities of HB, H(HB)(n)(+), and B(HB)(m)(-) that result from gaseous autoprotolysis have to be considered. In this work, the pressure-dependent absolute chemical potential of the proton in the representative gaseous proton acids CH(4), NH(3), H(2)O, HF, and HCl was worked out and the general theory to describe bulk gas phase acidity--that can directly be compared with solution acidity--was developed.

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.

Applying the unified pH scale: absolute acidities in the gas phase and anchor points for eleven representative liquid media

The investigations on our recently introduced unified acidity scale [1] based on the absolute chemical potential of the proton pointed out the inadequateness of the established GA scale. Earlier it was inter alia found that, when trying to correlate pKa with GA values, in several cases the correlation was broken without any sufficient explanation [2,3]. However, the GA does not take into account the pressure dependent speciation in the gas phase. In this contribution we systematically extend the theory of acidity in the gas phase from standard GAs and GBs to the real existing bulk phases [4]. Furthermore we present the rCCC (relaxed COSMO cluster-continuum) model [5], a quantum chemical solvation model for the calculation of Gibbs solvation energies of the proton with good accuracy. The rCCC values can be used to anchor individual pH scales in different solvents to our universal scale.

Quantum chemical calculations on a unified pH scale for all phases

One for all – a unified Bronsted acidity scale...! On the basis of the absolute chemical potential of the proton a unified absolute pH scale universally applicable in the gas phase, in solution and the solid state is introduced [1]. This scale allows to directly compare acidities in different media and to give a thermodynamically meaningful definition of superacidity and can be used in all areas where proton activity changes during use, e.g. in proton-induced catalytic reactions, hydrocarbon processing, fuel cells, in the biological proton pump, and others. For calculating acidities we developed and applied several models based on quantum chemistry (modified G3, MP2/def2-QZVPP, MP2-extrapolated CCSD(T)/aug’-cc-pVDZ®QZ, COSMO@BP86/defTZVP, etc.). Our investigations also point out the inadequateness of the established GA scale, which in contrast to our unified acidity scale does not take into account the pressure dependent speciation in the gas phase. Below the accessible absolute Bronsted acidities given by their μabs(H+) or so-called pHabs values in different media as expressed by the width of their protochemical window (= pKAP) are illustrated (Figure 1). Author details Institute for Inorganic and Analytical Chemistry and Freiburger Materialforschungszentrum FMF, Albert-Ludwigs-University of Freiburg, 79104 Freiburg, Germany. Institute of Chemistry, University of Tartu, Tartu, 50411, Estonia.