Analysis of N-Heterocyclic Carbenes and their Monolayers by Xray Photoelectron Spectroscopy: Peak-Fitting, Effects of Molecular Architecture and Impact of Possible Impurities

X-ray photoelectron spectroscopy (XPS) is a frequently employed technique for surface characterization, particularly useful in studying self-assembled monolayers (SAMs). Herein, we examine precursors and SAMs of N-heterocyclic carbenes (NHCs) on gold. We establish a reliable and reproducible XPS peak-fitting model for 1,3-diisopropyl-1H-benzo[d]imidazol-3-ium hydrogen carbonate and its CF₃-tagged derivative to enable accurate detection, quantification, and chemical analysis of NHCs by XPS. The effects of molecular architecture variations on the N 1s peak were investigated and found to be insignificant across a wide range of derivatives. The characteristics of the N 1s peak after deposition on a gold surface were examined. All samples show a symmetric N 1s peak centered around 400.5 eV, with a FWHM ranging from 1.2 to 1.6 eV, which is a. 0.6 eV shift compared to the starting hydrogen carbonate precursors, which ap-pear at 401.1 eV. Examination of hydrogen carbonate salts during extended irradiation under ultra-high vacuum illustrates that dehydration and eventually decarboxylation likely take place during the process of measuring the XP spectra. More substantial degradation of the precursors can be observed after 4 hours of exposure to the conditions of measurement for simple NHC precursors, with the CF₃-tagged NHC salts degrading more rapidly within 2 hours due to C–F bond cleavage. Common impurities that may result from synthetic conditions and can complicate the N 1s spectra are also presented