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

✍ Scribed by Janet E. Del Bene

• Publisher: John Wiley and Sons
• Year: 1981
• Tongue: English
• Weight: 602 KB
• Volume: 2
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.540020210

✦ Synopsis

Hydrogen bonding of uracil with water in excited nx* 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 -T* transitions in uracil. The first is due to excitation of the C4-O group, while the second is associated with excitation of the (22-0 group. In the first nx* state, hydrogen bonds at 0 4 are broken, so that the open water-uracil dimer a t 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 0 4 in the ground state, collapses to a different "wobble" dimer at N3-H and 0 2 in the excited state. The third dimer, a "wobble" dimer a t N1-H and 0 2 , remains intact, but is destabilized relative to the ground state. Although hydrogen bonds a t 0 2 are broken in the second nx* state, the three water-uracil dimers remain bound. The "wobble" dimer at N1-H and 0 2 changes to an excited open dimer at N1-H. The "wobble" dimer at Ns-H and O4 remains intact, and the open dimer a t O4 is further stabilized upon excitation. Dimer blue shifts of nx* bands are nearly additive in 2:l and 3:l water:uracil structures. The fates of the three 2:l water:uracil trimers and the 3:l water: uracil tetramer in the first and second n -K * states are determined by the fates of the corresponding excited dimers in these states.