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Unifiying Concepts for Acid-Base- and Redox-Chemistry: Development, Validation and Application of Absolute pH and pe Scales Culminating in the Protoelectric Potential Map PPM

Final Report Summary - UNICHEM (Unifiying Concepts for Acid-Base- and Redox-Chemistry: Development, Validation and Application of Absolute pH and pe Scales Culminating in the Protoelectric Potential Map PPM)

The UniChem project concerns the Protoelectric Potential Map (PPM), its dissemination to the public and the collection of data to generate and supply a comprehensive PPM as possible. Moreover, the synthesis of unusual cationic species is on target to present spectacular prototype problems to elucidate the frontiers of chemistry. The PPM is the two-dimensional plot of absolute reducity (reduction power) peabs against absolute Brønsted acidity (protonation power) pHabs. Both values are defined in a novel manner based on the absolute chemical potential of the electron and the proton, resp. Thereby, the comparison of redox potentials and acidity of redox and/or acid-base systems in different media is possible unlike as with the conventional definitions of redox potentials and pH values.
The concept of absolute acidity was already adopted from other authors for the publication in a textbook (Riedel, Moderne Anorganische Chemie, 4. Aufl., De Gruyter, 2012). Furthermore, the PPM concept is open to the public on a web site ( which embraces the explanation of the PPM concept and a tool for calculating pHabs and peabs values which meets the demands of the user.
In our workgroup we could establish a structure where theoretical framework, predictions by means of quantum chemical calculations, synthesis of novel species and subsequent validation by different analytical methods go hand in hand. This structure provides quick feedback to improve each of the parts successively. This holds for the generation of unified standard redox potential and acidities as anchor points, these are the basic quantities for the PPM, as well as for the stability and structure of novel chemical species. For example, we calculated the free energies of solvation of the proton – the mentioned anchor points – with quantum chemical methods which we validated by electrochemical measurements, indeed not on the absolute scale but on the relative scale within one distinct solvent. However, this approach gives hints on the accuracy of our calculations. With this procedure for the first time anchor points determining absolute acidities in ionic liquids (IL) could be generated.
This also applies to the stability of carbocations. The crystal structure of the novel norbornyl cation salt gave the final proof that the norbornyl cation has a “non classical” structure with a 3 center 2 electron bond. Also the benzo[cd]pyrenium cation and the tert-butyl cation could be synthesized and stabilized. The novel cation [P9]+ – which is the first condensed-phase homopolyatomic phosphorus cation – has a structure which consists of two P5 cages linked by a phosphonium atom.
Important extensions of the absolute Brønsted acidity concept were elaborated for gaseous acids and ionic liquids including – due to the ionic character of the solvent – novel as well as fundamental pH definitions in IL. In this context, benzene and other aromatics could be protonated and stabilized as IL. Acidity calculations were performed to evaluate the accuracy of this very novel pH value definitions in IL.
A perfluorinated derivative of sulfuric acid could be synthesized in kg scale and due to its considerably high acidity may be used to design novel superacidic media. Its anion is currently used for the synthesis of new ILs which may also serve as media for acid chemistry and electrochemistry. To sum up, we are content with the dissemination of the PPM concept conceivably leading to entries in monographs and, thought one step further, also in textbooks. We could calculate and in part experimentally validate new anchor points of IL for the PPM. Also, decisive definitions of pH values in IL and for gaseous acids were formulated which are completely novel. The synthesis of a substantial number of unusual cations, both by protonation and oxidation, succeeded. We could embed them in our theoretical framework of absolute acidity and absolute reducity by means of quantum chemical calculations as well as by experimental findings.