Study of the diacetamide-water dimer with ab initio and density functional theory methods

The hydrogen bonding of 1: 1 complexes formed between diacetamide and water molecule have been completely investigated in the present study using density functional theory and second order Moller-Plesset perturbation (MP2) method. The large basis sets 6-311++g(d,p) and 6-311++g(2d,2p) have been employed to determine the equilibrium structure of the interacting complexes. All the results reveal a planar configuration of the amide groups and a tendency of the CH3 group to eclipse the C=O bond for the geometry of the isolated diacetamide molecule. Calculations at different theoretical levels indicate that cis-trans configuration is the most stable isomer in both gas phase and solution phase. Three reasonable geometries on the potential energy hypersurface of diacetamide with water system are considered with the global minimum being a cyclic double-hydrogen bonded structure. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated and a result that two carbonyl groups have about the same proton acceptor ability has been obtained. Finally, the solution phase studies are also carried out using the onsager reaction field model at B3LYP/6-311++g** level for the isolated diacetamide molecule and the hydrogen-bonded complex of diacetamide with water. The results indicate the polarity of the solvent has played an important role on the structures and relative stabilities of different isomers. (C) 2004 Published by Elsevier B.V.