A quantum chemical study of aniline/ammonia clusters. Thermodynamic properties and frequency analysis

The potential energy surface of the aniline/ammonia cluster is analyzed by correlated ab initio molecular orbital methods: MP2 and DFT (B3-LYP functio nal) with the 6-31 + G* basis set. Five minima were located at the B3-LYP/6 -31 + G* level, and four minima at the MP2/6-31 + G* level, The most stable conformation is a consequence of the hydrogen bond between the NH bond of the aniline molecule and the lone pair of the nitrogen atom of ammonia. Oth er local minima result from the interaction between the aromatic ring of an iline and the hydrogen atoms of ammonia, or from the hydrogen bond through the hydrogen atoms of the aromatic ring and the nitrogen atom of ammonia. S ingle-point calculations at different levels have been done. From the obtai ned results the predicted energy for the global minimum is about -4.50 kcal /mol. A comparison between experimental and calculated frequencies suggests that, aside from the most stable dimer, some other minima could be present . Some of the thermodynamic properties of the cluster were calculated from its harmonic frequencies. The analysis of the obtained values seems to conf irm the existence of several cluster conformations which affects the experi mental results. (C) 2000 Elsevier Science B.V. All rights reserved.