Calibration of computationally predicted N 1s binding energies by comparison with X-ray photoelectron spectroscopy measurements

Density functional theory (DFT) predictions of the N 1s binding energies based on Koopmans' theorem are calibrated and compared with experimental X-ray photoelectron spectroscopy (XPS) measurements of gas phase molecules and surface adducts. A convenient and effective method for comparing these predicted energies to experimental measurements, using para-nitroaniline (p-NO2C6H4NH2) as a spectroscopic 'ruler', is presented. The experimental difference between the N 1s binding energies of the nitrogen atoms from the -NH2 and -NO2 groups of para-nitroaniline is compared to the difference predicted by DFT, using the B3LYP functional with multiple basis sets. Gas phase and molecularly intact condensed multilayer experimental spectra of para-nitroaniline are used to calibrate the predicted binding energies to gas phase and condensed phase (surface adducts) measurements, respectively. A method for improving the agreement with experiment by using the predicted partial charge of nitrogen is also presented.