The proton affinity scale, and effects of ion structure and solvation

Abstract

In the last three decades, several extensive thermochemical ladders were constructed that connect molecules over a wide range of gas-phase basicities (GBs, i.e., −Δ G° protonation) and proton affinities (PAs, i.e., −Δ H° protonation). The data include GB ladders from low pressure ion cyclotron resonance (ICR), GB and PA ladders from pulsed high pressure mass spectrometry (PHPMS), absolute reference PAs from spectroscopic measurements, and ab initio calculations. Comparison amongst the ladders identifies some systematic expansions or contractions (mostly <10%), mainly due to uncertainties in temperature measurement. After global adjustments for these effects and anchoring to accurate local standards, all the data are consistent, including the directly measured PA ladders from variable temperature PHPMS measurements. Bracketing experiments and association thermochemistry provide further independent verification of the GB and PA scales. The derived entropies of protonation ( S p) from PHPMS measurements and theoretical calculations are structurally reasonable and mutually consistent, which supports the entropy results and the consistencies of the GB and PA scales. Special structural effects such as internal hydrogen bonds in polyfunctional and biomolecules ions can also be quantified. The overall data suggest that the GB and PA scales, spanning a range of 120 kcal mol −1 for 38 reference compounds, have been established within ±0.8 kcal mol −1, and the S p values within ±1.5 cal mol −1 K −1, which may be the limits of accuracy of current methods. The gas-phase data, used in thermochemical cycles, allows calculating the solvation energies of ions. Molecular and solvation effects can be separated, and the latter decomposed into continuum and hydrogen bonding terms. Altogether, the gas-phase data allows decomposing the energetics of organic acid–base chemistry into structurally sensible and quantitatively understandable factors.