Theoretical characterization of the hydrogen-bond interaction of diacetamide with water and methanol

Abstract

Ab initio HF/6-31G** calculations have been carried out to determine the equilibrium structure and vibrational frequencies of dimers formed from interaction of diacetamide with water and methanol. In both water and methanol complexes, three stable structures are found, the first one being a cyclic double hydrogen-bonded structure a and the two others being open structures with hydrogen bonds formed at the carbonyl in cis (b) and in trans (c) position. The most stable structure is the cyclic structure; the stabilisation energy computed at the MP2/6-31G** level being -44 kJ mol ^-1 for the water complex and -48 kJ mol ^-1 for the methanol complex. Structure c is about 1 kJ mol ^-1 more stable than structure b, showing that the two carbonyl groups have about the same proton acceptor ability. The force constant of the non-bonded OH group of water slightly increases upon hydrogen-bond formation in agreement with the anticooperativity theory. A correlation is found between the force constant of the free and bonded OH bonds and the corresponding OH distances. Comparison of the energy of structures a and b of water and methanol complexes suggests that the acidity of the hydroxylic compound is more determinant than its proton acceptor ability.