Vibronic fine structure in the nitrogen 1s photoelectron spectra from Franck-Condon simulations II: Indoles

TL;DR

This study combines Franck-Condon simulations and DFT to analyze vibronic effects in nitrogen 1s XPS of 17 indole molecules, revealing rules for spectral evolution and differences between amine and imine nitrogens.

Contribution

It provides a systematic analysis of vibronic coupling in indoles, including rules for spectral changes due to structural modifications and insights into nitrogen-specific effects.

Findings

Simulated spectra agree well with experiments for 4 molecules.

Distinct vibronic behaviors of amine and imine nitrogens are identified.

Structural changes influence spectral evolution and local geometries.

Abstract

The vibronic coupling effect in nitrogen 1s X-ray photoelectron spectra (XPS) was systematically studied for a family of 17 bicyclic indole molecules by combining Franck-Condon simulations (including the Duschinsky rotation effect) and density functional theory. The simulated vibrationally-resolved spectra of 4 molecules agree well with available experiments. Reliable predictions for this family further allowed us to summarize rules for spectral evolution in response to three types of common structural changes (side chain substitution, CH$\leftrightarrow$N replacement, and isomerization). Interestingly, vibronic properties of amine and imine nitrogen are clearly separated: they show negative and positive $\Delta$ZPE (zero-point vibration energy of the core-ionized with respect to the ground state), respectively, indicating flatter and steeper PESs induced by the N 1s ionization; amine N's show stronger mode mixing effects than imine N's; the 1s ionizations on two types of nitrogens led to distinct changes in local bond lengths and angles. The rules are useful for a basic understanding of vibronic coupling in this family, and the precise spectra are useful for future reference and data mining studies.