Electronic structure of uracil and uridine derivatives studied by photoelectron spectroscopy

The gas phase He(I) photoelectron spectra of uracil (I) and its derivatives, 5-fluorouracil (II), ftorafur (tegafur, 5-fluoro-1-(tetrahydro-2-furyl)uracil) (III), 2',3'-dideoxyuridine (IV) and 2',3'-dideoxy-5-fluorouridine (V), have been studied by means of MNDO calculations. The first four photoelectron spectral bands of I and II are π₁, n₁; π₂ and n₂ from the top, respectively. In contrast to these cases it is extremely difficult to assign the photoelectron spectral bands n₁, π₂ and n₂ of III because of their complete overlapping with the oxygen atom n orbital band of the tetrahydrofuryl group. In order to solve this problem the difference spectrum method has been used, and the exact peak positions have been successfully detected. The photoelectron spectral band shifts were reasonably analyzed in terms of the conjugative, the short-range inductive and the long-range molecular polarization effects. Although for I, II and III the keto forms are stable, it has been revealed that in the case of IV or V both a keto and an enol form (at least) coexist predominantly in the gas phase. The π₁ ionization energy of the predominant enol form of IV is completely indifferent to the 5-fluorination, as is the case of the keto form of I. This unusual finding can be explained as follows: the molecular polarization and the electron-withdrawing inductive effects of the 5-fluorine atom are almost cancelled by the π conjugative effect.