Theoretical studies on the hydration of formic acid by ab initio and ABEEMσπ fluctuating charge model

The interaction between formic acid (FA) and water was systemically investigated by atom-bond electronegativity equalization method fused into molecular mechanics (ABEEM sigma pi/MM) and ab initio methods. The geometries of 20 formic acid-water complexes (FA-water) were obtained using B3LYP/aug-cc-pVTZ level optimizations, and the energies were determined at the MP2/aug-cc-pVTZ level with basis set superposition error (BSSE) and zero-point vibrational energy (ZPVE) corrections. The ABEEM sigma pi potential model gives reasonable properties of these clusters when compared with the present ab initio data. For interaction energies, the root mean square deviation is 0.74 kcal/mol, and the linear coefficient reaches 0.993. Next, FA in aqueous solution was also studied. The hydrogen-bonding pattern due to the interactions with water has been analyzed in detail. Furthermore, the ABEEM sigma pi charges changed when H2O interacted with the FA molecule, especially at the sites where the hydrogen bonds form. These results show that the ABEEM sigma pi fluctuating charge model is fine giving the overall characteristic hydration properties of FA-water systems in good agreement with the high-level ab initio calculations.