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Molecular orbital theory of the hydrogen bond. 25. Water–uracil complexes in excited n → π states
Author(s) -
Del Bene Janet E.
Publication year - 1981
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540020210
Subject(s) - dimer , excited state , chemistry , uracil , hydrogen bond , crystallography , water dimer , molecule , wobble base pair , ground state , atomic physics , physics , dna , transfer rna , rna , biochemistry , organic chemistry , gene
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 C 4 O group, while the second is associated with excitation of the C 2 O group. In the first n → π* state, hydrogen bonds at O 4 are broken, so that the open water–uracil dimer at O 4 dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N 3 H and a cyclic structure at N 3 H and O 4 in the ground state, collapses to a different “wobble” dimer at N 3 H and O 2 in the excited state. The third dimer, a “wobble” dimer at N 1 H and O 2 , remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O 2 are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N 1 H and O 2 changes to an excited open dimer at N 1 H. The “wobble” dimer at N 3 H and O 4 remains intact, and the open dimer at O 4 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.