Ab initio study of acid molecules interacting with H2O

TL;DR

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

Contribution

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

Findings

Different theoretical methods predict different lowest-energy structures.

Significant discrepancies exist between B3LYP-DFT and MP2 results.

Potential energy surfaces vary notably between methods.

Abstract

Using the Gaussian-03 for ab initio calculations, interactions of various acid molecules with a single water molecule were studied. 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 M{\o}ller-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 thermal free energies, as well as the corresponding binding energies for the theoretical methods were calculated. It was also 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 most simply structured acids H 2 S and H 2 Se, 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 was also found between potential energy surfaces for both theories applying to the finding of the optimized structures of the H 2 SO 3 -H 2 O supermolecular isomers, because MP2 supermolecular geometries cannot exist for the corresponding B3LYP-DFT ones, for which the frequency characteristics of the supermolecular isomers were also calculated. ......