An ab Initio Study of the Trimethylamine−Formic Acid and the Trimethylammonium Ion−Formate Anion Complexes, Their Monohydrates, and Continuum Solvation

High-level ab initio calculations carried out up to an effective MP4SDQ/6-311+G(3df,2p)//HF/6-31+G* level predict that the trimethylamine−formic acid complex in vacuo is favored by 7.0 kcal/mol (Δ G 298 ) relative to the trimethylammonium−formate complex. Interaction with a single water molecule is, according to calculated results, not sufficient to make the ion-pair complex the predominant one, the lowest energy monohydrated neutral complex being favored by 4.7 kcal/mol (Δ G 298 ) in comparison to the lowest energy monohydrated ion-pair. Calculations on the effect of a dielectric continuum on the binary and monohydrated ternary complexes using the Self-Consistent Isodensity Polarized Continuum Model (SCI-PCM) indicate that a strong dielectric continuum with a dielectric constant larger than ca. 9 is required to make the binary ion-pair complex predominant. However, only a relatively weak dielectric continuum with a dielectric constant in the range of 4−6 is required in order to favor the monohydrated ion-pair complex.