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 MP4SD Q/6-311+G(3df,2p)//HF/6-31+G level predict that the trimethylamine-fo rmic acid complex in vacuo is favored by 7.0 kcal/mol (Delta G(298)) r elative to the trimethylammonium-formate complex. Interaction with a s ingle water molecule is, according to calculated results, not sufficie nt to make the ion-pair complex the predominant one, the lowest energy monohydrated neutral complex being favored by 4.7 kcal/mol (Delta G(2 98)) in comparison to the lowest energy monohydrated ion-pair. Calcula tions on the effect of a dielectric continuum on the binary and monohy drated ternary complexes using the Self-Consistent Isodensity Polarize d Continuum Model (SCI-PCM) indicate that a strong dielectric continuu m 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.