Ab Initio Study of Different Acid Molecules Interacting with H2O

TL;DR

This study uses ab initio calculations to analyze how various acid molecules interact with water, revealing differences in predicted structures and energies depending on the computational method used.

Contribution

It compares B3LYP-DFT and MP2 methods for modeling acid-water interactions, highlighting discrepancies in optimized structures and energies.

Findings

B3LYP-DFT and MP2 predict different lowest-energy isomers.

Binding energies are 10-15% higher with MP2.

Significant differences in potential energy surfaces for H2SO3-H2O.

Abstract

Using the Gaussian-03 for ab initio calculations, we have studied interaction of different acid molecules with a single water molecule. The molecular and supermolecular optimized structures were found with the Becke-3-Lee-Yang-Parr (B3LYP-hybrid potential) calculations of density-functional theory (DFT) methods as well as the Moeller-Plesset second-order perturbation theory, using the basis set of Aug-cc-pVDZ quality and the CRENBL ECP effective core potential for molecules containing heavy iodine atom. Possible isomers of studied acids and supermolecules, consisting of acid molecules coupled with a single water molecule, are shown. Energies, zero-point energies (ZPEs), thermal enthalpies and free energies, as well as the corresponding binding energies for the theoretical methods were calculated. It was found that optimized structures of supermolecular isomers with lowest energies corresponding to the global minimum on the potential energy surfaces can be different for both theories. The simplest structure acids H2S and H2Se, forming acid-water supermolecules, can give clear evidence of disagreement of the two theoretical methods concerning optimization of lowest energy structures, because the B3LYP-DFT method gives the lowest-energy structure for the first supermolecular isomer, but the MP2 method for the second possible isomer. A dramatic difference between potential energy surfaces for both theories applying to the optimized structure finding of the H2SO3-H2O supermolecular isomers was found, because MP2 supermolecular geometries cannot exist for the corresponding B3LYP-DFT ones, for which the frequency characteristics of the supermolecular isomers were also calculated. In general, the binding energies and ZPE ones for the MP2 method are 10-15% larger than those for the B3LYP-DFT method.