Acid-base Studies

Contact: Prof. Ivo Leito, ivo.leito@ut.ee

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. See the list of publications for an overview.

Measurement and computational methods: molecules. We measure acidities and basicities of molecules in terms of their pK_a 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 pK_a values) so that both of them are dissolved in the same solution. The directly measured quantities are the pK_a differences (∆pK_a values) between the measured compound and reference compound (obtained via multilinear regression data treatment of a series of spectra) from which the pK_a of the investigated compound is thereafter found by subtracting the pK_a 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 pK_a measurements of superbases or pK_a measurements of superacids) – and measuring every compound against at least two (often three or more) reference compounds (and the average pK_a value obtained is taken as the result). As a result, reliable pK_a values are obtained and small differences between pK_a 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.

Measurement methods: solutions. We also measure acidities of solutions in terms of the unified pH values (pH_abs values). The differential potentiometric pH_abs measurement method, originally developed in our group has now been set up in a number of laboratories in Europe in the framework of the H2020 EMPIR 17FUN09 UnipHied project.

 

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 (pK_a and pH_abs scales) in different non-aqueous solvents:

• Acidity scale in acetonitrile spanning from pK_a 2.8 to 32.6
• Basicity scale in acetonitrile spanning from pK_a 4 to 32
• Acidity scale of superacids in 1,2-dichloroethane spanning 16 orders of magnitude
• Basicity scale of superbases in THF spanning more than 30 orders of magnitude
• pHabs Scale of liquid chromatography mobile phases made in different aqueous-organic solvent mixtures, spanning 7 orders of magnitude

For full list of publications, together with annotations, see this Table of publications related to acid-base pK_a values in non-aqueous solvents.

A very important achievement is the experimental realization of the concept of unified acidity scale (unified pH scale, pH_abs 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 pK_a values for the classical strong acids (mineral acids: HCl, HBr, HI, HClO₄, CF₃SO₃H) and the most reliable aqueous pK_a 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 pK_a values to the reactivity of the compounds, their catalytic activity, behavior in MS ion sources, etc.