IR, Raman spectroscopic, DFT, AIM and NCI characterization of O-H‧‧‧O/π‧‧‧π bonds in dimer and trimer species as computed from MD simulations in water for Protocatechuic acid

Abstract

In the present work we report a study of the O-H‧‧‧O dimer, bifurcated dimer and O-H‧‧‧O/π‧‧‧π bonded trimer species as determined by Molecular Dynamics simulations for Protocatechuic acid. Multiple O-H‧‧‧O bond interactions are correlated to the unsymmetrical broad band structure observed in the experimental IR absorption spectrum near 3400 cm−1. To gain an understanding of the O-H‧‧‧O/π‧‧‧π bond interactions and its resultant structures referred above, we carried out MD simulations using ten monomer species in water solvent of 1000 molecules for 1 ns. We have identified different O-H‧‧‧O bonds between hydroxyl-hydroxyl, carboxyl-hydroxyl and carboxyl-carboxyl groups in the dimer and bifurcated dimer species, whereas in the trimer, there are O-H‧‧‧O bonds and π‧‧‧π bonds in ring-to-ring interactions. Computed radial distribution functions confirm that (i) the O-H‧‧‧O/π‧‧‧π bonds exist in all the dimer/trimer species; (ii) the donor-acceptor bond lengths in O-H‧‧‧O bonds are less than the sum of van der Waals radii of the H and O atoms. Lifetimes of the O-H‧‧‧O bonds at 5.0 to 13.5 ps correspond to the association/dissociation of those bonds. Stabilization energies for both intra- and inter-molecular O-H‧‧‧O bonds in the dimer and trimer lie in the range 0.93 – 11.20 kcal/mol. Stabilization energies of the six π‧‧‧π bonded trimer species lie in the range 0.13 – 0.44 kcal/mol. Geometry, vibrational modes and other electronic properties for the most stable dimer computed at B3LYP/6-311++G (d, p) level has been shown to be a satisfactory model.