Electron impact ionization of 5-and 6-chlorouracil:: appearance energies

Electron impact ionization of the gas phase modified DNA/RNA bases 5- and 6-ClU was studied using a crossed electron/neutral beams technique in combination with a quadrupole mass spectrometer. 5- and 6-ClU belong to the class of halouracils which are used in radiation therapy to increase the effect of ionizing radiation to tumours, when they are incorporated into cancer tissue. Besides determining the mass spectra for both molecules at the electron energy of 70 eV, the ionization efficiency curves for each parent ion and the most abundant fragment ions were measured near the threshold and the corresponding appearance energies (AEs) were derived using an iterative, non-linear least square fitting procedure using the Marquart-Levenberg algorithm based on the Wannier threshold law. The most abundant cations observed in mass spectra have a threshold value of AE ((C3H2ClNO)+/5-ClU) = 11.12 +/- 0.03 eV and AE ((C3H2NO)(+)/6-ClU) = 12.06 +/- 0.03 eV The present AE value for the parent ion of 5-ClU AE((5-ClU)(+)/5-ClU) = 9.38 +/- 0.05 eV is in fair agreement with previous calculations at the B3LYP level of theory. The AE((6-ClU)(+)/6-ClU) = 9.71 +/- 0.05 eV is 0.33 eV higher than that for 5-ClU. (C) 2003 Elsevier B.V. All rights reserved.