The effect of hydrogen bonding on the structures of uracil and some methylderivatives studied by experiment and theory

The equilibrium structures of uracil, thymine (5-methyluracil), 1-methylura cil and 1,3-dimethyluracil have been determined from ab initio MO calculati ons at the HF, MP2 and DFT/6-31G(d) levels. Thymine has also been studied i n the solid state by X-ray diffraction. A systematic investigation of the e ffects of intermolecular association on the structure of uracil and its met hyl derivatives has then been carried out by theoretical calculations. Base d on information from the solid state, a dimer for 1-methyluracil, a trimer for thymine and a hexamer for uracil are used as theoretical models (at HF and DFT levels) to simulate the neighboring intermolecular interactions fo und in the crystal. The geometrical perturbations observed in the uracilic moiety, on going from an isolated molecule to the crystal phase, are mainly due to self-association and confirm the fundamental role of conjugative st abilization of the intermolecular hydrogen bonding in such DNA bases.