Theoretical insight into the interactions of TMA-benzene and TMA-pyrrole with B3LYP density-functional theory (DFT) and ab initio second order Moller-Plesset perturbation theory (MP2) calculations

A detailed theoretical investigation of the tetramethylammonium(TMA)-benzen e and TMA-pyrrole complexes has been performed to obtain the interaction pr operties of TMA with aromatics. Diffuse :functions have been found to be im portant in the computational studies of these noncovalent complexes. Adding diffuse functions to the basis set decreases the binding energy by about 1 0% for the TMA-aromatic systems. Dispersion interactions in the TMA-aromati c systems are very important. They enhance the binding interactions between the TMA and the aromatic ring systems by about 0.5 kcal . mol(-1) per inte racting atomic pair, which is in agreement with the estimates of Rappe and Bernstein.(1) Also, for the TMA-pyrrole complex, the presence of the disper sion interaction leads to a dramatic change in the optimized structure. Bec ause B3LYP cannot handle properly the dispersion in the calculation, use of the Moller-Plesset second-order perturbation or other sophisticated method s should be considered in computational studies of cation-pi interactions i n systems containing nonsymmetric dispersion interacted atomic pairs. The o rbital interaction is unimportant in the TMA-aromatic interaction according to the detailed analysis of the molecular orbitals. The TMA-aromatic inter actions basically come from the typical cation-pi interaction and the dispe rsion interaction. Because the electron density in the II56 aromatic system of pyrrole is larger than that in the II66 system of benzene, the pi elect ron cloud on pyrrole is more easily polarized under the influence of cation s, which may lead to a relatively stronger cation-pi interaction in the TMA -pyrrole complex than in the TMA-benzene complex.