Valence photoelectron spectra of thiouracils in the gas phase

TL;DR

This study combines experimental and theoretical methods to analyze the vibrationally resolved valence photoelectron spectra of thiouracils, revealing lower ionization potentials and the importance of nonadiabatic couplings at higher energies.

Contribution

It provides the first detailed vibrationally resolved spectra of thiouracils using advanced coupled cluster theory and compares them with experimental data.

Findings

First ionization potential between 8.4 and 8.7 eV, lower than uracil.

Strong vibrational progressions up to 12 eV.

Nonadiabatic couplings are significant at higher binding energies.

Abstract

We present a combined experimental and theoretical study of the vibrationally resolved valence photoelectron spectra of the complete series of thiouracils (2-thiouracil, 4-thiouracil and 2,4-dithiouracil) for binding energies between 8 and 17 eV. The theoretical spectra were calculated using equation-of-motion coupled cluster theory for ionization potential (EOM-IP-CCSD) combined with the time-independent double-harmonic adiabatic Hessian approach. For all three thiouracils, the first ionization potential is found between 8.4 and 8.7 eV, which is 1 eV lower than for the canonical nucleobase uracil. Ionization bands up to 12 eV show strong vibrational progressions and are well reproduced by the calculations. These bands are attributed to the ionization of (primarily) sulfur- and oxygen-localized valence molecular orbitals. For higher binding energies, the calculations indicate that nonadiabatic couplings are important for the interpretation of the photoelectron spectra.