The pKa rule in light of local mode force constants

The prediction of cocrystal or molecular salt formation from organic acid and base constituents in solution usually relies on the pKa rule, an empirical protocol based on the relative tendencies of the isolated protonated form of the base (BH+) and the isolated acid (HA) to undergo dissociation. The underlying physical rationale for this empirical rule has been missing so far. A promising step forward is to harness local mode force constants as bond strength descriptors to gain insights into the salt/cocrystal assessment. Utilizing the water pKa probe at the & omega;B97X-D(PCM)/aug-cc-pVTZ level of theory for a test set comprising various pyridine and benzoic acid derivatives, we observe a linear relationship between pKa values and the local mode force constant ka in this proof-of-concept study for the test examples that, according to the literature, form cocrystals or molecular salts. In addition, we propose a new way of assessing those cases where & UDelta;pKa falls within the critical 0-3 range. We conclude with a future perspective on how the novel local force constant descriptor can be applied to large databases in the context of machine learning approaches.

Automated summary

The prediction of cocrystal or molecular salt formation from organic acid and base constituents in solution usually relies on the pKa rule, an empirical protocol based on the relative tendencies of the isolated protonated form of the base (BH+) and the isolated acid (HA) to undergo dissociation. However, the underlying physical rationale for this empirical rule has been missing so far. A promising step forward is to utilize local mode force constants as bond strength descriptors to gain insights into the salt/cocrystal assessment.