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

Abstract The potential energy surface of the aniline/ammonia cluster is analyzed by correlated ab initio molecular orbital methods: MP2 and DFT (B3-LYP functional) with the 6-31+ G ∗ basis set. Five minima were located at the B 3- LYP /6-31+ G ∗ level, and four minima at the MP 2/6-31+ 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. Other local minima result from the interaction between the aromatic ring of aniline 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. Single-point calculations at different levels have been done. From the obtained 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 confirm the existence of several cluster conformations which affects the experimental results.