Ab initio calculations on the hydrogen bond interaction between diacetamide and ammonia

Abstract

Ab initio HF/6–31G∗∗ calculations have been carried out to determine the equilibrium structure, energies and vibrational properties of dimers formed from the interaction of diacetamide (DIAC) and ammonia. Three stable structures are found, the first one being a cyclic double hydrogen bonded structure ( a) and the two other ones 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 stabilization energy computed at the MP2/6–31G∗∗ level being − 42 kJ mol −1. The energies of structures b and c are −11 and −10.5 kJ mol −1, respectively, and the values show that the proton acceptor abilities of the two non-equivalent carbonyl groups are about the same. The computed relative frequency shifts of the NH stretching vibration and the elongations of the NH bond are correlated. Comparison of the energies of the hydrogen bonds in DIAC complexed with ammonia, water and methanol suggests that the acidity of the guest molecule is a more determinant factor than its proton acceptor ability.