Molecular orbital theory of the hydrogen bond. 24. Ground‐state water–uracil complexes

Abstract

AbstractAb initio SCF calculations with the STO‐3G basis set have been performed to investigate the structural, energetic, and electronic properties of mixed water–uracil dimers formed at the six hydrogen‐bonding sites in the uracil molecular plane. Hydrogen‐bond formation at three of the carbonyl oxygen sites leads to cyclic structures in which a water molecule bridges N1H and O2, N3H and O2, and N3H and O4. Open structures form at O4, N1H, and N3H. The two most stable structures, with energies of 9.9 and 9.7 kcal/mole, respectively, are the open structure at N1H and the cyclic one at N1H and O2. These two are easily interconverted, and may be regarded as corresponding to just one “wobble” dimer. At 1 kcal/mole higher in energy is another “wobble” dimer consisting of an open structure at N3H and a cyclic structure at N3H and O4. The third cyclic structure at N3H and O2 collapses to the “wobble” dimer at N3H and O4. The two “wobble” dimers are significantly more stable than the open dimer formed at O4, which has a stabilization energy of 5.4 kcal/mole. Uracil is a stronger proton donor to water through N1H than N3H, owing to a more favorable molecular dipole moment alignment when association occurs through H1. Hydration of uracil by additional water molecules has also been investigated. Dimer stabilization energies and hydrogen‐bond energies are nearly additive in most 2:1 water:uracil structures. There are three stable “wobble” trimers, which have stabilization energies that vary from 7 to 9 kcal/mole per water molecule. Hydrogen‐bond strengths are slightly enhanced in 3:1 water:uracil structures, but the cooperative effect in hydrogen bonding is still relatively small. The single stable water–uracil tetramer is a “wobble” tetramer, with two water molecules which are relatively free to move between adjacent hydrogen‐bonding sites, and a stabilization energy of approximately 8 kcal/mole per water molecule. Within the rigid dimer approximation, successive hydration of uracil is limited to the addition of one, two, or three water molecules.