Molecular orbital theory of the hydrogen bond. 25. Water–uracil complexes in excited n → π states

Hydrogen bonding of uracil with water in excited n → π* states has been investigated by means of ab initio SCF -CI calculations on uracil and water–uracil complexes. Two low-energy excited states arise from n → π* transitions in uracil. The first is due to excitation of the C₄? O group, while the second is associated with excitation of the C₂? O group. In the first n → π* state, hydrogen bonds at O₄ are broken, so that the open water–uracil dimer at O₄ dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N₃? H and a cyclic structure at N₃? H and O₄ in the ground state, collapses to a different “wobble” dimer at N₃? H and O₂ in the excited state. The third dimer, a “wobble” dimer at N₁? H and O₂, remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O₂ are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N₁? H and O₂ changes to an excited open dimer at N₁? H. The “wobble” dimer at N₃? H and O₄ remains intact, and the open dimer at O₄ is further stabilized upon excitation. Dimer blue shifts of n → π* bands are nearly additive in 2:1 and 3:1 water:uracil structures. The fates of the three 2:1 water:uracil trimers and the 3:1 water:uracil tetramer in the first and second n → π* states are determined by the fates of the corresponding excited dimers in these states.