Excited States of Bromopyrimidines Probed by VUV Photoabsorption Spectroscopy and Theoretical Calculations.

Mónica Mendes,João Ameixa,Fábris Kossoski,João Pereira-Da-Silva,Filipe Ferreira Da Silva,Ana I Lozano,Søren V Hoffmann,Nykola C Jones,Rodrigo Rodrigues

doi:10.3390/ijms22126460

Abstract

We report absolute photoabsorption cross sections for gas-phase 2- and 5-bromopyrimidine in the 3.7–10.8 eV energy range, in a joint theoretical and experimental study. The measurements were carried out using high-resolution vacuum ultraviolet synchrotron radiation, with quantum chemical calculations performed through the nuclear ensemble approach in combination with time-dependent density functional theory, along with additional Franck–Condon Herzberg–Teller calculations for the first absorption band (3.7–4.6 eV). The cross sections of both bromopyrimidines are very similar below 7.3 eV, deviating more substantially from each other at higher energies. In the 7.3–9.0 eV range where the maximum cross-section is found, a single and broad band is observed for 5-bromopyrimidine, while more discernible features appear in the case of 2-bromopyrimidine. Several π* ← π transitions account for the most intense bands, while weaker ones are assigned to transitions involving the nitrogen and bromine lone pairs, the antibonding σ*Br orbital, and the lower-lying Rydberg states. A detailed comparison with the available photo-absorption data of bromobenzene is also reported. We have found significant differences regarding the main absorption band, which is more peaked in bromobenzene, becoming broader and shifting to higher energies in both bromopyrimidines. In addition, there is a significant suppression of vibrational structures and of Rydberg states in the pair of isomers, most noticeably for 2-bromopyrimidine.

Highlights

The absorption of ionizing radiation by living cells has long been recognized as a significant cause of long-term biological damage, leading to cellular death, mutations and/or cancer
The measurements were carried out using high-resolution vacuum ultraviolet synchrotron radiation, with quantum chemical calculations performed through the nuclear ensemble approach in combination with time-dependent density functional theory, along with additional Franck–Condon Herzberg–Teller calculations for the first absorption band (3.7–4.6 eV)
Most can be immediately associated with a corresponding canonical orbital: π*(a2) with the lowest unoccupied molecular orbital (LUMO), π*(b1) with the LUMO + 1, π(b1) with the highest occupied molecular orbital (HOMO), n+(a1) with the HOMO-3, π(a2) and πBr(b1) with either HOMO-4 or HOMO-5, depending on the isomer

Summary

Introduction

The absorption of ionizing radiation by living cells has long been recognized as a significant cause of long-term biological damage, leading to cellular death, mutations and/or cancer. The ability of radiation in inducing modifications of the cellular components is important and useful for treatment of diseases, especially in cancer therapy. In the subsequent radiotherapy stage, the damage is expected to be more localized in regions rich with radiosensitizers drugs. Through this combination, it is possible to increase tumor cell killing, with less damage to healthy tissues [4,5]

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