Electron Attachment to 5-Fluorouracil: The Role of Hydrogen Fluoride in Dissociation Chemistry

Abstract

We investigate dissociative electron attachment to 5-fluorouracil (5-FU) employing a crossed electron-molecular beam experiment and quantum chemical calculations. Upon the formation of the 5-FU^- anion, 12 different fragmentation products are observed, the most probable dissociation channel being H loss. The parent anion, 5-FU^-, is not stable on the experimental timescale (~140 µs), most probably due to the low electron affinity of FU; simple HF loss and F^- formation are seen only with a rather weak abundance. The initial dynamics upon electron attachment seems to be governed by hydrogen atom pre-dissociation followed by either its full dissociation or roaming in the vicinity of the molecule, recombining eventually into the HF molecule. When the HF molecule is formed, the released energy might be used for various ring cleavage reactions. Our results show that higher yields of the fluorine anion are most probably prevented through both faster dissociation of an H atom and recombination of F^- with a proton to form HF. Resonance calculations indicate that F^- is formed upon shape as well as core-excited resonances.

Keywords: 5-fluorouracil; anion; electron attachment; hydrogen fluoride.

Figure 1

(a) Local minimum structure of 5-FU as optimized at the B3LYP/aug-cc-pVDZ level. (b) Singly-occupied Hartree-Fock orbitals in the lowest-lying valence state (VS) and the dipole bound state (DBS) within CCSD(T)/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ(C,N,O,F)TZ(H)+ calculations, respectively. Wavefunction phases are shown in purple and salmon-pink. Color code: carbon—brown; nitrogen—blue; fluorine—yellow; oxygen—red; hydrogen—grey.

Figure 2

Anion efficiency curves of fragments formed with masses (a) 129 u, (b) 110 u, (c) 86 u, and (d) 82 u upon electron attachment to 5-FU. The red line represents the cumulative peak from the Gaussian fit.

Figure 3

Simplified reaction pathways after electron attachment to FU, forming FU⁻ (the central anion at −0.23 eV). Ion mass is included for FU⁻ and experimentally observed ions. The energy is given in eV with respect to the neutral FU molecule as calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/aug-cc-pVDZ level. For the H pre-dissociation pathway, small numerical discrepancies between energies of local minima and transition states or dissociation limits are induced through CCSD(T) recalculation of B3LYP-optimized structures; therefore, both CCSD(T) and B3LYP values are given. Color code: carbon—brown; nitrogen—blue; fluorine—yellow; oxygen—red; hydrogen—grey.

Figure 4

Simplified reaction pathways after formation of HF molecule within FU⁻. Ion mass is included for experimentally observed ions. The energy is given in eV with respect to the neutral FU molecule as calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/aug-cc-pVDZ level. Color code: carbon—brown; nitrogen—blue; fluorine—yellow; oxygen—red; hydrogen—grey.

Figure 5

Anion efficiency curves of fragments formed with masses (a) 66 u, (b) 62 u, (c) 59 u, and (d) 58 u upon electron attachment to 5-FU. The red line represents the cumulative peak from the Gaussian fit.

Figure 6

Anion efficiency curves of fragments formed with masses (a) 42 u, (b) 39 u, (c) 26 u, and (d) 19 u upon electron attachment to 5-FU. The red line represents the cumulative peak from the Gaussian fit.

Figure 7

Binding energy, D, plotted against the stabilizing charge, Δq, values for which the anion is electronically stable and extrapolated to Δq → 0. SOMO orbital visualized with the contour value of 0.05 (a₀)^−f3/2 for Δq = 0.7 (a) and for Δq = 2.4 (b). E_R stands for the calculated position of the resonance related to the release of the fluoride anion.