Vibronic fine structure in the nitrogen 1s photoelectron spectra from Franck-Condon simulations. III. Rules for amine/imine N atoms in small N-heterocycles

Abstract

Vibronic coupling plays a crucial role in X-ray photoelectron spectra (XPS) of molecules. In a series of three papers, we present a comprehensive exploration of the N-heterocycles family, known for their diverse structures, to summarize the general rules of vibronic coupling in high-resolution vibrationally-resolved XPS spectra at the N1s edge. Building upon our previous studies on six-membered monocyclic azines [Phys. Rev. A 106, 022811 (2022)] and fused bicyclic compounds indoles with five and six members [Phys. Rev. A 108, 022816 (2023)], in this study, we focus on investigating a series of 12 five-membered N-heterocycles using Franck-Condon simulations, incorporating Duschinsky rotation effects and density functional theory. Our calculations reveal distinct spectral characteristics of amine and imine within these 12 systems in binding energies, spectral characteristics, structural changes, vibrational coupling strengths, and effects of hydrogenation. Furthermore, we expand our analysis to encompass all 35 N-heterocycles discussed in the three papers and consolidate these findings into the general rules. we find that 1s ionization in amine nitrogen induces more substantial geometrical changes, resulting in larger vibronic coupling strength compared to imine nitrogens. The spectra of imine nitrogens exhibit two distinct characteristic peaks originating from the 0-0 and 0-1 transitions, whereas the spectra of amine nitrogens are characterized by a broad peak with numerous weak fingerprints due to significant mixing of various 0-$n$ transitions. We observe that amine (imine) nitrogens generally cause a negative (positive) change in zero-point vibrational energy. This study provides valuable insights into vibronic coupling in N-heterocycles, shedding light on the distinguishing features and behavior of amine and imine nitrogens in vibrationally-resolved XPS spectra.