Acid-base Studies

In studies of acids and bases – computational design, synthesis and, especially, acidity and basicity measurements (in non-aqueous solutions and in the gas phase) we are among the leaders in the world. Detailed list of publications with annotations can be found at our acid-base chemistry website.

Measurement and computational methods. We measure acidities and basicities of molecules in terms of their pKa values. The measurement technique is UV-Vis spectrophotometry, a reliable and accurate technique. The measurement method is relative – every compound is measured against reference compounds (with known pKa values) so that both of them are dissolved in the same solution. The directly measured quantities are the pKa differences (∆pKa values) between the measured compound and reference compound (obtained via multilinear regression data treatment of a series of spectra) from which the pKa of the investigated compound is thereafter found by subtracting the pKa of the reference compound. The relative nature of the measurement method enables decreasing or eliminating many uncertainty sources (impurities in compounds, traces of water in solvents) because they influence both compounds in a similar way. We also take good care of the experimental conditions – anhydrous solvents, dry atmosphere, etc (see e.g. recent works in pKa measurements of superbases or pKa measurements of superacids) – and measuring every compound against at least two (often three or more) reference compounds (and the average pKa value obtained is taken as the result). As a result, reliable pKa values are obtained and small differences between pKa values of compounds as a result of subtle structural differences of or differences in conditions can be uncovered.

All contemporary computational approaches are used in our group for obtaining both gas phase (G2, G2MP2, DFT approaches, QCSDT, W1, etc) and solution (COSMO-RS, SMD, CPCM, etc) data.

Results. Possibly the most important acid-base chemistry related contribution from our group to the scientific community are reliable experimental self-consistent scales of acidity and basicity (pKa scales) in different non-aqueous solvents:

A very important achievement is the first experimental realization of the concept of unified acidity scale (unified pH scale), a universal, solvent-independent approach for expressing acidity of different media.

Also, computations have led to highly useful results. In recent publications we have presented probably the most reliable aqueous pKa values for the classical strong acids (mineral acids: HCl, HBr, HI, HClO4, CF3SO3H) and the most reliable aqueous pKa values for a number of organic bases (DBU, TBD, MTBD, Phosphazenes, guanidines, etc).

Work is in progress in expanding the acidity and basicity scales and relating the pKa values to the reactivity of the compounds, their catalytic activity, behavior in MS ion sources, etc.