DFT & PM3 calculated charges on atoms in heterocyclic molecules and XPS

The charges on the nitrogen and oxygen atoms computed by quantum chemical methods using Density Functional Theory (DFT) and semi-empirical Parameterized Model number 3 (PM3) for certain heterocyclic compounds used as corrosion inhibitors are compared for the first time with the data of X-ray photoelectron spectroscopy (XPS) studies. A contradiction between the results of DFT calculations and XPS data is found that leads to a somewhat critical approach to conclusions made on the basis of quantum chemical calculations as applied to the corrosion processes and mechanism of corrosion inhibition.