Molecular orbital theory of the hydrogen bond. 27. Substituent effects in water: 4‐R‐pyrimidine complexes

Abstract

AbstractAb initio SCF calculations with the STO‐3G basis set have been performed to investigate substituent effects on the structures and stabilization energies of water:4‐R‐pyrimidine complexes, with R including CH3, NH2, OH, F, C2H3, CHO, and CN. Except for the cyclic water:4‐aminopyrimidine complex hydrogen bonded at N3, these complexes have open structures stabilized by a nearly linear hydrogen bond formed through a nitrogen lone pair of electrons. When hydrogen bonding occurs at N3, the complexes may have planar or perpendicular conformations depending on the substituent, but when hydrogen bonding occurs at N1, the perpendicular is generally slightly preferred, and there is essentially free rotation of the 4‐R‐pyrimidine. Primary substituent effects alter the electronic environment at the nitrogens, and tend to make N3 a poorer site for hydrogen bonding than N1, primarily because of a stronger π electron‐withdrawing effect at N3. However, the relative stabilities of complexes hydrogen bonded at N1 and N3 are also influenced by secondary substituent effects, which may be significant in stabilizing complexes bonded at N3. Substitutent effects on the structures and stabilization energies of the water:4‐R‐pyrimidine complexes are similar to substitutent effects in water:2‐R‐pyridine and water:4‐R‐pyrimidine complexes are similar to substitutent effects in water:2‐R‐pyridine and water:4‐R‐pyridine complexes. Configuration interaction calculations indicate that although absorption of energy by the pyrimidine ring destabilizes the water:4‐R‐pyrimidine complexes, these may still remain bound in the excited n → π* state. This is in contrast to the fate of open water:2‐R‐pyridine and water:4‐R‐pyridine complexes, which dissociate in this state.