An ab initio study of excited state molecular electrostatic potential maps and other related properties of 5-fluorouracil: a comparative study with uracil

An ab initio study of electronic transitions of 5-fluorouracil (5FU) has been carried out using ground and excited state geometry optimization and the results thus obtained are compared with those of uracil. Excited states of the molecules were generated using configuration interaction considering single electron excitations (CIS) from the filled to the unfilled molecular orbitals. The 6-31G* basis set was used for all the calculations. While uracil is unstable in its lowest singlet pi-pi* excited state, 5FU, like thymine, is stable in the corresponding pi-pi* excited state. Further, both uracil and 5FU are planar in the ground state but uracil in the lowest singlet n-pi* excited state and 5FU in the lowest singlet n-pi* and pi-pi* excited states are appreciably nonplanar. The MEP map for the lowest singlet n-pi* excited state of uracil, drawn in a plane located at a distance of about 1.8 Angstrom from the molecular plane, shows a localized reactive region above the C5C6 bond which is involved in photodimerization. However. this feature is missing in the corresponding map of 5FU. Further, there is no such feature in the S-2 (pi-pi*) excited state MEP map of uracil that can be correlated with the property of photodimerization which seems to suggest that the photodimerization reaction in uracil, occurring under ultraviolet irradiation, dominantly involves covalent interactions. (C) 2001 Elsevier Science B.V. All rights reserved.