Electron impact ionisation cross sections of fluoro-substituted nucleosides

Abstract

We report calculated electron-impact ionisation cross sections (EICSs) for 2′-deoxycytidine (Cyt), 2′-deoxy-5-fluorocytidine (fCyt) and 2′,2′-difluorocytidine (gemcitabine, Gem) from threshold to 10 keV. We compare the Deutsch-Märk (DM) and the binary-encounter-Bethe (BEB) methods used to obtain these cross sections. The methods yield excellent agreement with each other, within 3–4% at the cross section maxima. In particular, the DM cross sections for Cyt, fCyt and Gem yield maxima of 29.88 Å2 at 79 eV, 28.96 Å2 at 82.2 eV and 29.51 Å2 at 83.4 eV, respectively, whereas the BEB cross sections yield maxima of 28.89 Å2 at 87.6 eV, 27.97 Å2 at 91.6 eV and 29.02 Å2 at 93.4 eV, respectively. In addition, we compute EICSs for small sequences built from the considered nucleosides, i.e. for the sequences Cyt-Cyt, fCyt-Cyt, Cyt-fCyt, Gem-Cyt and Cyt-Gem. We find that the resulting EICSs differ only slightly between different sequences of the same constituents. Moreover, they can be approximated with an accuracy within 6% by simply adding the EICSs of individual molecular subsystems. Finally, we find that alterations in the ionisation energy due to the presence of an aqueous solvent can be substantial and may hence also considerably affect the resulting EICSs especially at low energies close to the ionisation threshold.Graphical abstract