Practical estimation of XPS binding energies using widely available quantum chemistry software

Chemical shifts observed in high-resolution X-ray photoelectron spectroscopy (XPS) spectra are normally used to determine the chemical state of the elements of interest. Often, these shifts are small, or an element is present in several oxidation states in the same sample, so that interpretation of the spectra is difficult without good reference data on binding energies of the likely constituents. In many cases, reference spectra taken from pure reference samples of the chemical components can aid the peak fitting procedure. However, reference materials are not always available, so that it becomes necessary to estimate the binding energies of likely components through quantum chemical calculations. In principle, such calculations have become much easier than in the past, due to the availability of powerful personal computers and excellent software. In practice, though, care needs to be taken in the approximations, assumptions, and settings used in applying such software to calculate binding energies. In this work, we present a general summary of the methods for the calculation of the core electron binding energies and compare the use of 2 of these methods using the popular GAUSSIAN software package. Furthermore, a series of results for molecules, containing elements of the second and the third row of the periodic table, are presented and compared with experimental results, in order to establish the quality and fitness-for-purpose of the quantum chemical-based predictions.