Theoretical studies on the hydrogen bonding interaction of complexes of formic acid with water

The hydrogen bonding of the complexes formed between formic acid and water molecules (with up two water) has been completely investigated in the present study using density functional theory (DFT) and second-order Moller-Plesset perturbation (MP2) method; the large basis sets 6-311++g(d,p) and 6-311++g(2d,2p) have been employed to determine the equilibrium structure and vibrational frequencies of the interacting complexes. Twelve reasonable geometries on the potential energy hypersurface of the formic acid and water system are considered; six are with one water molecule, and six are with two water molecules. For the complexes with one water molecule, three are with the (T)-formic acid and three are with (C)-formic acid, and the most stable structure is a cyclic double-hydrogen-bonded structure. For the complexes with two water molecules, we calculated six structures, the global minimum being a cyclic double-hydrogen-bonded structure. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated. The infrared spectrum frequencies, IR intensities, and vibrational frequency shifts are reported. Finally the solution phase studies are also carried out using the Onsager reaction field model in water solvent at B3LYP/6-311++g(d,p) level.