Ab initio studies on the interactions of water with tetramethylurea and tetramethylthiourea

The interactions of a water molecule with tetramethylurea (TMU) and tetramethylthiourea (TMTU) were studied by ab initio molecular orbital calculations using the supermolecule approach at the HF and MP2 levels, The water molecule was found to be able to form nearly linear hydrogen bonds both with the oxygen and nitrogen atom of TMU. Although the HF SCF interaction energy is significantly smaller in the second case the total interaction energy decreases only by 15% due to the increase in dispersion forces. For a water molecule in the neighbourhood of the carbonyl (thionyl) group the total TMU-H2O and TMTU-H2O energies are nearly identical, whereas for the water molecule in the region of the amide group the latter interaction energy is much smaller. Hydrogen bond formation was not observed between the hydrogen atom of the water molecule and any of the nitrogen atoms of the TMTU molecule.