Electron attachment to 5-chloro uracil

Abstract

Electron attachment (EA) and dissociative electron attachment (DEA) to 5-chloro uracil (5-ClU) was studied in the gas phase using a crossed electron/molecule beams technique. Besides production of a parent anion via a zero energy resonance, ion yields of nine different negative ions were observed in the electron energy range from about 0 to 14 eV. In the electron energy range from about zero to 5 eV, the formation of a transient negative ion was induced by electron attachment to the π* resonances located at about 0.24, 1.5, and 3.6 eV leading subsequently by unimolecular decay to various negative fragment ions. Absolute partial cross sections for EA and DEA to 5-ClU were obtained from the measured ion yields using a simple calibrating method. The dominant negative ion observed in the present experiment was (C4H2N2O2)− (corresponding to 5-ClU minus HCl) with a mass to charge ratio of 110, followed by Cl− ion (mass to charge ratios 35 and 37), the partial cross sections being σ(0.23 eV)=5×10−18 m2 and σ(0.23 eV)=3×10−18 m2, respectively. The parent anion produced (5-ClU)− has only a cross section value of σ(0 eV)=3×10−20 m2. The energetic thresholds for the formation of particular negative ions from 5-ClU in the gas phase were calculated at the G2(MP2) level of theory and compared with the experimental results. On the basis of these calculations structure and relative stability of some of the fragment ions was predicted. In the case of the two most abundant ions, their formation was observed well below the calculated electron energy threshold. The formation of these negative ions below the thermodynamic threshold is explained in terms of the high vibrational energy of the 5-ClU prior to the EA process.

Bond Breaking and Temporary Anion States in Uracil and Halouracils: Implications for the DNA Bases

“…1 to note the existence of structures near 0.31 and 0.54 eV in the ET spectra of the halouracils. They match quite closely peaks in the DEA cross sections of chlorouracil reported at 0.24 and 0.57 eV [5] and at 0.28 eV in bromouracil [4]. As supported by our VOE calculations, these features cannot be associated with the lowest valence anion states, and thus we must seek another source.…”

“…Surprisingly, there is no sign of the major DEA feature at 1.01 eV, and we return to discuss this later. We note that uracil, thymine, cytosine, and chlorouracil all display peaks in DEA, at 1.01, 1.03, 1.11, and 0.99 eV, respectively [2][3][4][5]. Taken as a whole, the DNA bases and halouracils exhibit sharp features grouped in the ranges 0.24 -0.34, 0.54 -0.74, and 0.99-1.1 eV.…”

“…We expect as well that coupling of the dipole anion with that associated with temporary occupation of C 5 -Cl will excite VFRs at the C 5 -Cl stretching frequency. We propose that this accounts for the structure near 0.06 eV in the DEA of chlorouracil [5], but without knowledge of the dipole binding energy we cannot be sure of the vibrational quantum number.…”

“…This work, in part, has served to motivate further gas phase studies of electron collisions with the DNA bases, the RNA base uracil, and the halouracils in an effort to understand the fundamental mechanisms involved. Most recently, Hanel et al [2] (uracil), Denifl et al [3] (uracil, thymine and cytosine), Abouaf et al [4] (thymine and 5-bromouracil), and Denifl et al [5] (5-chlorouracil) have examined the electron energy dependence of the production of various anion fragments and made estimates of the magnitudes of the dissociative electron attachment (DEA) cross sections. The radiological significance of the halouracils has been discussed in several of these references and will not be repeated here.…”

Dissociative electron attachment to uracil deuterated at the N1 and N3 positions

“…In an earlier work [1], sharp structure appearing in the dissociative electron attachment (DEA) cross-section of uracil [2] and the halo-uracils [3] at energies below 4 eV was attributed in part to vibrational Feshbach resonances (VFRs) [4] associated with the dipole bound anion state (DBS) of uracil. In brief, it was argued that mixing of this diffuse electronic state with the lowest lying valence anion state of 2 Σ symmetry predissociates the dipole bound state, allowing those vibrational levels lying above the threshold energy for the DEA process to contribute to bond breaking.…”