Ultraviolet photoelectron studies of 5-trifluoromethyluracil: Electronic susceptibility of substituted uracils to nucleophilic attack

Abstract

UV photoelectron spectroscopy has been employed to investigate the electronic structure of 5-trifluoromethyluracil. Band assignments have been made by comparison with photoelectron results from uracil, thymine, 5-fluorouracil, and 1-methyl-5-trifluoromethyluracil. Spectroscopically measured ionization potentials have also been compared with energy levels predicted by INDO molecular orbital (MO) calculations. The calculations predict the ordering and the spacing of the four uppermost Orbitals in 5-trifluoromethyluracil with good accuracy. They also give a good representation of the shifts observed in the ionization potentials of these orbitals when spectra of uracil, thymine, 5-fluorouracil, 5-trifluoromethyluracil, and 1-methyl-5-trifluoromethyluracil are compared. Descriptions of electronic structure provided by the photoelectron results and the INDO calculations have been examined in conjunction with results from kinetic and equilibrium studies of sulfite attack at the 6 position of uracil and substituted uracils. Results of the comparison indicate that while resonance effects cause 5-fluorouracil to exhibit greater electron density at the 6 position than thymine or uracil, the order of reactivity is 5-fluorouracil ≥ uracil > thymine. The enhanced reactivity of 5-fluorouracil over that expected from considerations of electron density may be ascribed in part to inductive electron withdrawing effects of the F atom, which oppose resonance effects and which stabilize the transition state involved in the addition of an SO2−3 ion.